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Aspects of the ecology of two species of Cenocorixa (Corixidae:Hemiptera) in allopatry and sympatry Reynolds, Julian Douglas 1974

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ASPECTS OF THE ECOLOGY CF TWO SPECIES OF CEJSOCQRIXA (CORIXIDAE: HEMIPTERA) IN ALLOPATRY AND SYMPATRY by JULIAN DOUGLAS REYNOLDS B.A., U n i v e r s i t y of D u b l i n , I r e l a n d , 1964 M.Sc., U n i v e r s i t y of Dublin, I r e l a n d , 1968 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Zoology We accept t h i s t h e s i s as conforming to the r e g u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA SEPTEMBER, 1974 In presenting th is thesis in par t ia l fu l f i lment of the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree t h a t the L ibrary shal l make it f ree ly ava i lab le for reference and studv. I fur ther agree that permission for extensive copying of th is thesis for scho lar ly purposes may be granted by the Head of my Department, or by his representat ives . It is understood that copying or pub l ica t ion of th is thes is for f inanc ia l gain sha l l not be allowed without my wri t ten permission. Department of The Univers i ty of B r i t i s h Columbia Vancouver 8, Canada Date D c H W i ABSTRACT Facets of the b i o l o g y of the s i b l i n g s p e c i e s Cenocorixa b i f i d a and C. e x p l e t a (Corixidae) were compared i n d i f f e r e n t h a b i t a t s , c o n s t i t u t i n g both sympatric and a l l o p a t r i c p o p u l a t i o n s . The study aimed to i d e n t i f y reasons f o r the d i f f e r i n g d i s t r i b u t i o n of the two s p e c i e s . In p a r t i c u l a r , i t sought to (a) e x p l a i n why and how the s p e c i e s c o e x i s t e d over part of t h e i r range i n l a k e s of moderate s a l i n i t y , and (b) deduce why C. e x p l e t a i s absent from the more freshwater l a k e s . The t h e s i s i s d i v i d e d i n t o s i x s e c t i o n s . An i n t r o d u c t i o n , reviewing competition theory and- summarising the problem, i s f o l l o w e d by s t u d i e s on the environments i n v e s t i g a t e d , i n Chapter I I . Chapter I I I d i s c u s s e s d i s t r i b u t i o n , abundance and breeding p a t t e r n of the c o r i x i d s p e c i e s encountered, to o b t a i n data r e l e v a n t to c e r t a i n i n d i r e c t measures of c o m p e t i t i o n , and Chapters IV and V d e s c r i b e f e e d i n g experiments and s e r o l o g i c a l gut analyses of f i e l d - c o l l e c t e d c o r i x i d s . The f e e d i n g experiments aimed to o b t a i n one measure of the fundamental niche, while the gut enalyses were c a r r i e d out f e r a measure of the r e a l i s e d niche. The f i n a l s e c t i o n d i s c u s s e s the f i n d i n g s i n terms of p o t e n t i a l s p e c i e s i n t e r a c t i o n . The study area comprised s i x l a k e s on Becher's P r a i r i e , i n the C h i l c o t i n area of B.C., and another near Kamloops; a l l l a k e s were s i m i l a r i n general morphology. Although temperature p a t t e r n s were s i m i l a r between l a k e s , they v a r i e d c o n s i d e r a b l y i n c o n d u c t i v i t y . Temperatures and c o n d u c t i v i t y data c o r r o b o r a t e i i e a r l i e r f i n d i n g s , suggesting that the s t u d i e d years were not a t y p i c a l . Oxygen l e v e l s and phytoplankton primary p r o d u c t i o n were g e n e r a l l y higher i n the more freshwater l a k e s , but the c o n t r i b u t i o n of a q u a t i c macrophytes and b e n t h i c algae there was not measured. Phytoplankon p r o d u c t i o n was r e s t r a i n e d . P l a n k t e r s were abundant i n a l l l a k e s , e s p e c i a l l y i n s h o r e , but the l e v e l s f l u c t u a t e d most i n the t h r e e freshwater l a k e s , being very low there before mid-June. Diaptomid copepods and f a i r y shrimps were con f i n e d to the higher s a l i n i t y l a k e s , while chaoborids and amphipods were r e s t r i c t e d to the three freshwater l a k e s ; other i n v e r t e b r a t e s were widely d i s t r i b u t e d . Both biomass and d i v e r s i t y of organisms were h i g h e s t i n the freshwater l a k e s , but they were by no means low i n the s a l i n e lakes s t u d i e d . The data suggest t h a t there was an abundance of food i n a l l lakes at a l l times. £• b i f i d a bred i n a l l l a k e s i n v e s t i g a t e d , although excluded a p p a r e n t l y through i n c r e a s i n g s a l i n i t y from l a k e LB 2 a f t e r s p r i n g . C. e x p l e t a bred s y m p a t r i c a l l y with C. b i f i d a i n waters of over 6000 umhos cm - 1 c o n d u c t i v i t y , whereas Cy,matia americana, M§sperocorixa l a e v i g a t a , C a l l i c o r i x a audeni and Sicjara sp. bred only i n the three most freshwater l a k e s . C. e x p l e t a produced three g e n e r a t i o n s a year i n the h i g h e s t s a l i n i t i e s (LB 2 and Barnes Lakes) and two i n other high s a l i n i t i e s . C. b i f i d a produced two g e n e r a t i o n s a year i n the higher s a l i n i t i e s , but l i k e a l l the other c o r i x i d s , i n the three most freshwater l a k e s often produced only one g e n e r a t i o n a year. C o r i x i d s showed no i i i d e f i n i t e t r e n d s o f d i f f e r e n t a b u n d a n c e w i t h a l k a l i n i t y , and t h e two s p e c i e s o f C e n o c o r i x a d i d n o t show d e p r e s s e d p o p u l a t i o n l e v e l s i n s y m p a t r y . F u r t h e r , t h e p h e n o l o g y was e s s e n t i a l l y c o n t e m p o r a r y i n s y m p a t r y . C . e x p l e t a when a l l o p a t r i c o c c u r r e d i n b o t h m a r g i n a l and m i d - l a k e a r e a s , w h e r e a s C . b i f i d a when a l l o p a t r i c was more c o n f i n e d t o t h e l i t t o r a l a r e a s . In b o t h s p e c i e s , h a b i t a t p r e f e r e n c e s o f i n s t a r s v a r i e d w i t h s e a s o n , b u t i n g e n e r a l t h e l a r g e s t i n d i v i d u a l s o f e a c h s p e c i e s p r e f e r r e d t h e most c o m p l e x e n v i r o n m e n t s . O v e r a l l , C . e x p l e t a i n s y m p a t r y was r e l a t i v e l y commanest o v e r d e e p e r w a t e r s and o v e r p l a i n mud o r s i l t , w h e r e a s C . b i f i d a p r e f e r r e d r o c k s , l o g s and r e e d s . H o w e v e r , t h e s e g r e g a t i o n was n o t m a r k e d . B o t h C . b i f i d a and C . e x p l e t a t o o k a wide v a r i e t y o f p r e y s o f f e r e d i n e x p e r i m e n t s . C . e x p l e t a a d u l t s a c c e p t e d more o f t h e p r e y s o f f e r e d t h a n d i d C . b i f i d a . J u v e n i l e s showed l e s s marked s p e c i e s d i f f e r e n c e s t h a n d i d a d u l t s , and a c c e p t e d p r e y more o f t e n t h a n t h e m . B o t h s p e c i e s a c c e p t e d b o t h l i v e and d e a d o r g a n i s m s . J u v e n i l e s o f b o t h s p e c i e s p r e f e r r e d d e a d p r e y t o l i v e ; h o w e v e r , o n l y f o r C . e x p l e t a was t h i s t r e n d s i g n i f i c a n t o v e r a l l . When o f f e r e d l i v e o r dead m i x e d p l a n k t o n , C . b i f i d a t o o k c h i e f l y c e r i o d a p h n i i d s , w h e r e a s C . e x p l e t a t o o k b o t h d i a p t o m i d s and c e r i o d a p h n i i d s i n t h e p r o p o r t i o n s o f f e r e d . I n o t h e r c h o i c e s i t u a t i o n s , b o t h s p e c i e s f e d more o f t e n on c h i r o n o m i d s t h a n d a p h n i i d s , t a k i n g z y g o p t e r a n s l e a s t . H o w e v e r , u n s u c c e s s f u l a t t a c k s were f a r more f r e q u e n t on d a p h n i i d s t h a n on t h e b e n t h i c i v and l i t t o r a l organisms. In summary, the experimental f e e d i n g r e s u l t s suggest t h a t the fundamental niches of the two s p e c i e s of Cenocorixa are s i m i l a r , but not i d e n t i c a l . Guts of f i e l d - c o l l e c t e d c o r i x i d s were analysed by s e r o l o g i c a l techniques using 10 a c t i v e a n t i s e r a . Mcst r e a c t i o n s i n d i c a t e d a c l e a r l y c a r n i v o r o u s h a b i t , l e s s than 1% being p o s i t i v e f o r algae. R e s u l t s suggest that foods eaten varied dynamically between i n s t a r s , l a k e s and seasons. In C. b i f i d a d a p h n i i d s and zygopterans were more f r e q u e n t l y recorded i n guts from the sympatric l a k e s than from the freshwater l a k e s . Ephemeropterans and amphipods were i d e n t i f i e d c h i e f l y from summer-collected c o r i x i d s , and were c h i e f l y r e s t r i c t e d to the freshwater l a k e s , p a r a l l e l i n g t h e i r d i s t r i b u t i o n . However, some r e a c t i o n s were a l s o seen i n guts from these l a k e s with diaptomid antiserum, perhaps a r e a c t i o n t o other copepod s p e c i e s . In C. exp_leta most p o s i t i v e r e s u l t s were seen f o r daphniids i n s p r i n g and f a l l c o r i x i d samples. More male than female C. exjaleta took ephemeropterans, and more male than female C. b i f i d a took zygopterans. S p e c i f i c d i f f e r e n c e s between s a l i n e and freshwater l a k e s seemed to r e l a t e to prey d i s t r i b u t i o n , but i n Lake LB 2 C. e x g l e t a took s i g n i f i c a n t l y more diaptomids i n f a l l than i n summer. In sympatry, most j u v e n i l e s of both s p e c i e s took an i n c r e e a s i n g l y d i v e r s e d i e t with i n c r e a s i n g i n s t a r s i z e , although the p r e f e r r e d order chironomids, then zygopterans, ephemeropterans and diaptomids, remained r e l a t i v e l y c o n s t a n t . Second i n s t a r C. e x ^ l e t a reached a p l a t e a u l e v e l of f e e d i n g i n t e n s i t y only reached by t h i r d i n s t a r C. b i f i d a ; t h i s seems V r e l a t e d to i n s t a r s i z e . In l a r g e r i n s t a r s , C. b i f i d a showed more s e r o l o g i c a l r e a c t i o n s with most f o o d s t u f f s than e. e x p l e t a . O v e r a l l , C. e x p l e t a showed more p o s i t i v e r e a c t i o n s than C. b i f i d a with chironomids and d a p h n i i d s , but l e s s with diaptomids and zygopterans. In sympatry, C. b i f i d a fed more on d a p h n i i d s and ephemeropterans than i n a l l o p a t r y , which tends to c o n t r a d i c t ideas of competition f o r food. The s e r o l o g i c a l data suggest that the r e a l i s e d niche with r e s p e c t t o f e e d i n g i s not markedly more r e s t r i c t e d than the fundamental niche. Other c o r i x i d s p e c i e s had s i m i l a r l y c a r n i v o r o u s d i e t s , C. audeni showing more r e a c t i o n s f o r amphipods and chironomids than H. l a e v i g a t a , but l e s s f o r chaoborids, suggesting more be n t h i c o r i e n t a t i o n f o r the former. In f i e l d - c o l l e c t e d c o r i x i d s , red guts u s u a l l y contained diaptomids, while brown guts r e a c t e d with a v a r i e t y o f a n t i s e r a . In s e v e r a l brown and o l i v e guts there was no s e r o l o g i c a l r e a c t i o n , i n d i c a t i n g t h a t the a n t i s e r a d i d not cover the t o t a l d i e t a r y range. O c c a s i o n a l guts were b r i g h t blue-green, suggesting cyanophyte m a t e r i a l . Despite, a wide g e o g r a p h i c a l o v e r l a p between the s p e c i e s , C. b i f i d a and C. e x p l e t a possess somewhat d i f f e r e n t osmotic and i o n i c r e g u l a t o r y c a p a c i t i e s , and show s l i g h t l y d i f f e r e n t h a b i t a t and food p r e f e r e n c e s . T h e r e f o r e they cannot be considered as t r u e e c o l o g i c a l homologues, and hence competition need not n e c e s s a r i l y r e s u l t i n e x c l u s i o n of one or other. With regard to h a b i t a t , some of the d i f f e r e n c e s observed may be r e l a t e d to v i v a r i a t i o n i n t h e e n v i r o n m e n t s be tween l a k e s s t u d i e d , but o t h e r d i f f e r e n c e s show t h e s p e c i e s n o t t o be i d e n t i c a l . C . e x £ l e t a seems t o have a r a t h e r w i d e r f u n d a m e n t a l n i c h e i n t e r m s o f h a b i t a t t h a n C . b i f i d a , but n e i t h e r show marked r e s t r i c t i o n i n s y mpa t r y . The s e r o l o g i c a l f e e d i n g r e s u l t s s u g g e s t t h a t i n s y m p a t r y , e a c h s p e c i e s t o o k d i f f e r e n t p r o p o r t i o n s o f t h e same f o o d s t u f f s . T h e r e a l i s e d n i c h e s a p p e a r somewhat r e s t r i c t e d f r o m t h e f u n d a m e n t a l n i c h e d a t a o b t a i n e d f r o m f e e d i n g e x p e r i m e n t s . C . e x p l e t a seemed somewhat more s p e c i a l i s e d i n f e e d i n g t h a n C . b i f i d a , t h u s f u l f i l l i n g t h e r e q u i r e m e n t s c f an i n c l u d e d n i c h e s p e c i e s ; a g a i n a p o i n t u s u a l l y c o n s i d e r e d t o p e r m i t c o e x i s t e n c e . H o w e v e r , d i e t and h a b i t a t p r e f e r e n c e s o f s p e c i e s and i n s t a r s v a r i e d w i t h s e a s o n and l a k e , s u g g e s t i n g t h a t p o o l i n g d a t a o b s c u r e s much o f t h e r e a l v a r i a t i o n . In g e n e r a l , t h e e v i d e n c e shows t h a t t h e two s p e c i e s o f £i£2£2Ei2S^ a r e n o t e x a c t e c o l o g i c a l h o m o l o g u e s , and c o m p e t i t i o n b e t w e e n them i n s y m p a t r y was n o t e v i d e n t . The r e a s o n f o r t h e a b s e n c e o f C . e x £ l e t a f r o m t h e more f r e s h w a t e r l a k e s , t o w h i c h i t a p p e a r s p h y s i o l o g i c a l l y s u i t e d , s t i l l r e m a i n s o b s c u r e . TABLE OF CONTENTS v i i EPAGE ABSTRACT i LIST OF FIGURES x i LIST OF TABLES x i i i ACKNOWLEDGEMENTS x v i CHAPTER I. INTRODUCTION 1 THE CONCEPTS OF COMPETITION, COMPETITIVE INTERACTION AND COMPETITIVE EXCLUSION 3 THE CORIXIDAE: THE ANIMALS TO BE STUDIED 10 CHAPTER I I . THE ENVIRONMENTAL BACKGROUND 14 INTRODUCTION: THE STUDY AREA 14 MATERIALS AND METHODS 19 L i m n o l o g i c a l Parameters 20 (i) Temperatures 20 ( i i ) C o n d u c t i v i t y 30 Primary P r o d u c t i v i t y 21 Aquatic P l a n t s 24 Plankton 24 Other Organisms 25 RESULTS 26 L i m n o l o g i c a l Parameters 26 (i) Temperatures 26 ( i i ) C o n d u c t i v i t y 29 v i i i Primary P r o d u c t i v i t y . . 29 Aquatic Macrophytes 33 Plankton 37 Other Organisms 45 DISCUSSION AND SUMMARY OF FINDINGS 52 CHAPTER I I I . THE CORIXIDAE 58 INTRODUCTION 58 MATERIALS AND METHODS 58 RESULTS 59 C o r i x i d D i s t r i b u t i o n and Abundance 59 Habitat Survey 71 DISCUSSION 83 CHAPTER IV. FEEDING EXPERIMENTS 94 INTRODUCTION 94 PRELIMINARY EXPERIMENTS 95 MATERIALS AND METHODS 97 RESULTS 105 S t r u c t u r e and Behaviour 105 S i n g l e Prey Experiments 112 (i) Feeding p a t t e r n s of a d u l t Cenocorixa c o l l e c t e d i n d i f f e r e n t lakes 116 ( i i ) Feeding p a t t e r n s of a d u l t Cenocorixa c o l l e c t e d a t d i f f e r e n t seasons 118 ( i i i ) Feeding p a t t e r n s of Cenocorixa males and females 120 (iv) Feeding p a t t e r n s of t e n e r a l and o l d a d u l t s of Cenocorixa 120 (v) General f e e d i n g p a t t e r n of C e n c c c r i x a a d u l t s 123 i > (vi) g e n e r a l feeding p a t t e r n of Cenocorixa j u v e n i l e s 126 ( v i i ) Summary of Cenocorixa f e e d i n g p a t t e r n . 130 Experiments with Choice of Prey 131 Foods Accepted by Other Species 135 DISCUSSION 135 CHAPTER V. SEROLOGICAL INVESTIGATION CF GUT CONTENTS 150 INTRODUCTION 150 METHODS 153 P r e p a r a t i o n of S l i d e s and Templates 153 Pr e p a r a t i o n of Antigens 157 P r e p a r a t i o n of A n t i s e r a 159 The S e r o l o g i c a l T e s t s 162 Test s o f the S e r o l o g i c a l Method 170 (i) S p e c i f i c i t y of the a n t i s e r a 170 ( i i ) Strength of the a n t i s e r a 173 RESULTS . . . 176 Feeding P a t t e r n of C. b i f i d a 179 Feeding P a t t e r n of C. exp l e t a 190 D i f f e r e n c e s i n Feeding P a t t e r n of Cenccorixa Males and Females 200 Feeding P a t t e r n of Cenocorixa from D i f f e r e n t Lakes 77777. .7.7. 203 Feeding P a t t e r n of Cenocorixa C o l l e c t e d at D i f f e r e n t Seasons - 205 D i f f e r e n c e s between Feeding P a t t e r n s cf Cenccorixa i n Sympatry 206 Summary of Cenccorixa Feeding P a t t e r n s ....... 214 X Gut Colour and S e r o l o g i c a l l y Determined Gut Contents 218 D i e t of Other C o r i x i d Species , 219 DISCUSSION 224 CHAPTER VI. DISCUSSION 235 COMPETITION IN FLUCTUATING ENVIRONMENTS 240 SUMMARY AND DISC0SSION OF THE FINDINGS FOR C. BIFIDA AND C. EXPLETA 243 REFERENCES 265 x i L I S T OF FIGURES FIGURE PAGF 1 Sketch map of the study areas, with i n s e t shewing t h e i r p o s i t i o n i n B.C 16 2 D a i l y temperature ranges i n f i v e Becher's P r a i r i e l akes recorded a t two day i n t e r v a l s i n 1972 ...... 28 3 Range of c o n d u c t i v i t i e s measured i n seven study la k e s i n C e n t r a l B.C. i n 1972 31 4 S e c c h i d i s c water transparency measured at 4 day i n t e r v a l s i n s i x Becher's P r a i r i e l a k e s i n 1972 .. 35 5 Plankton volumes i n r e p l i c a t e inshore 2 l i t r e samples taken every f o u r days i n the Eecher's P r a i r i e l a k e s i n 1972 40 6 Numbers of p l a n k t e r s per 2 l i t r e s at d i f f e r e n t times of year i n Becher's P r a i r i e lakes 44 7 S p a t i a l and temporal d i s t r i b u t i o n of major i n v e r t e b r a t e groups i n s i x Becher's P r a i r i e lakes i n 1972 48 8 Temporal d i s t r i b u t i o n of stages of C. b i f i d a and C. e x ^ l e t a i n s i x l a k e s on Becher's P r a i r i e i n 1972 ..7 63 9 Temporal d i s t r i b u t i o n of stages of c o r i x i d s sympatric with C. b i f i d a i n three freshwater l a k e s on Becher's p r a i r i e 66 10 Numbers of c o r i x i d s per ten sweeps i n each lake on Becher's P r a i r i e sampled i n 1972 69 11 Map of i s o p e r c e n t i l e l i n e s f o r the percentage frequency of C. exjsleta i n Barnes Lake i n d i f f e r e n t h a b i t a t s and at four dates 76 12 D i s t r i b u t i o n of Cenocorixa s p e c i e s and i n s t a r s i n d i f f e r e n t h a b i t a t s i n Barnes Lake 79 13 Percentages of C. b i f i d a and C. e x ^ l e t a i n samples from d i f f e r e n t lake h a b i t a t s 82 14 Percentages of C. b i f i d a and C. exp_leta i n samples from d i f f e r e n t lake h a b i t a t s 86 x i i 15 Percentage acceptance of food items o f f e r e d to ad u l t C. b i f i d a and C. e x p l e t a . 125 16 Percentage acceptance o f food items o f f e r e d to j u v e n i l e C. b i f i d a and C. e x p l e t a 128 17 D e t a i l s of s e r o l o g i c a l technique 156 18 R e s u l t s of some s e r o l o g i c a l experiments, showing t y p i c a l band p a t t e r n s 167 19 R e s u l t s of some s e r o l o g i c a l experiments, showing t y p i c a l band p a t t e r n s 169 20 Summary of occurrence of food items i n the f i e l d , and percentage of guts c o n t a i n i n g s e r c l o g i c a l evidence of these items 256 x i i i LIST OF TABLES TABLE PAGE I Some s e l e c t e d p h y s i c a l and chemical f e a t u r e s cf the l a k e s s t u d i e d 18 I I Primary p r o d u c t i o n estimates and s u r f a c e oxygen readings i n s i x Becher's P r a i r i e l a k e s 32 I I I D i s t r i b u t i o n of a q u a t i c macrophytes i n the s t u d i e d lakes 36 IV Occurrence of p l a n k t o n i c crustaceans i n the s t u d i e d lakes 38 V Seasonal means of plankton volumes i n d u p l i c a t e 2 l i t r e samples 41 VI Occurrence of major i n v e r t e b r a t e groups i n the st u d i e d lakes 46 VII Chironomid l a r v a l abundance and biomass i n the Becher's P r a i r i e l a k e s 49 VIII Volumetric abundance data f o r f a u n a l components of sweep-net samples 51 IX Summary of the d i s t r i b u t i o n of c o n t r a s t i n g c o n d i t i o n s i n the studied l a k e s 53 X Summary of the d i s t r i b u t i o n of c o r i x i d s i n the s t u d i e d lakes 60 XI Sex r a t i o s of C. b i f i d a i n samples 70 XII Hidlake and inshore records of c o r i x i d s i n three l a k e s on Eecher's P r a i r i e 73 XI I I Sex r a t i o s of a d u l t Cenocorixa i n d i f f e r e n t h a b i t a t s of Barnes Lake 84 XIV D e t a i l s of prey groups used i n feeding experiments 99 XV Feeding response of C. b i f i d a i n d i f f e r e n t types of t e s t c o n t a i n e r 10 1 XVI I n t e n s i t y of c o r i x i d a c t i v i t y when o f f e r e d dead and l i v e preys 106 x i v XVII C o r i x i d gut c o l o u r a f t e r f e e d i n g known prey ...... 111 XVIII Numbers of r e p l i c a t e f e e d i n g experiments with each f o o d s t u f f f o r each s p e c i e s and i n s t a r of £§S2£2£ii^§ • ^ 3 XIX Summary of r e s u l t s of f e e d i n g experiments 114 XX Comparison of r e s u l t s of f e e d i n g experiments using a d u l t Cenocorixa c o l l e c t e d from d i f f e r e n t lake groups 117 XXI Comparison of r e s u l t s of fe e d i n g experiments using a d u l t Cenocorixa c o l l e c t e d at d i f f e r e n t seasons 119 XXII D i f f e r e n c e s i n f e e d i n g p a t t e r n between the sexes of Cenocorixa 121 XXIII Feeding i n t e n s i t y of t e n e r a l s versus o l d e r a d u l t s of Cenocorixa 122 XXIV Feeding success of C. b i f i d a and C. exjaleta when o f f e r e d n a t u r a l mixed plankton 132 XXV f e e d i n g success of C. b i f i d a and C. e x D l e t a with mixed preys . * 133 XXVI P o t e n t i a l prey organisms used to produce a n t i s e r a 158 XXVII Summary of s e r o l o g i c a l data f o r s i x c o r i x i d s p e c i e s : Numbers of times each antiserum was t e s t e d 177 XXVIII Summary of s e r o l o g i c a l data f o r s i x c o r i x i d s p e c i e s : Percentage of p o s i t i v e r e a c t i o n s 178 XXIX Feeding p a t t e r n of f i r s t i n s t a r C. b i f i d a 181 XXX Feeding p a t t e r n of second i n s t a r C. b i f i d a 182 XXXI Feeding p a t t e r n of t h i r d i n s t a r C. b i f i d a ........ 183 XXXIi Feeding p a t t e r n of f o u r t h i n s t a r C. b i f i d a 185 XXXIII Feeding p a t t e r n of f i f t h i n s t a r C. b i f i d a 186 XXXIV Feeding p a t t e r n cf a d u l t female C. b i f i d a 188 XXXV Feeding p a t t e r n of a d u l t male C. b i f i d a 189 XV XXXVI Feeding p a t t e r n of f i r s t i n s t a r C. e x p l e t a 191 XXXVII Feeding p a t t e r n of second i n s t a r C. e x p l e t a ...... 192 XXXVIII Feeding p a t t e r n of t h i r d i n s t a r C. ex p l e t a 193 XXXIX Feeding p a t t e r n of f o u r t h i n s t a r C. e x p l e t a ...... 195 XL Feeding p a t t e r n of f i f t h i n s t a r C. e x p l e t a 196 XLI Feeding p a t t e r n of a d u l t female C. e x p l e t a ....... 198 XLII Feeding p a t t e r n of a d u l t male C. e x p l e t a 199 XLIII D i f f e r e n c e s i n f e e d i n g p a t t e r n of male and female C. b i f i d a and C. e x p l e t a 201 XLIV D i f f e r e n c e s i n f e e d i n g p a t t e r n of male and female C. b i f i d a and C. e x p l e t a : p r o b a b i l i t i e s 202 XLV Feeding p a t t e r n s of separate i n s t a r s of C. b i f i d a and C. e x p l e t a 208 XLVI D i f f e r e n c e s i n f e e d i n g p a t t e r n between the i n s t a r s of C. b i f i d a and C. e x p l e t a i n symppatry . 210 XLVII Food p r e f e r e n c e s of sympatric C. b i f i d a and C. e x p l e t a at d i f f e r e n t seasons 211 XLVIII Seasonal pr e f e r e n c e s of C. b i f i d a and C. exp l e t a f o r d i f f e r e n t f o o d s t u f f s 213 XLIX D i f f e r e n c e s i n feed i n g p a t t e r n between C. b i f i d a and C. e x p l e t a 215 L Percentages of p o s i t i v e r e a c t i o n s to a n t i s e r a prepared f o r s p e c i e s common across the range cf la k e s s t u d i e d 217 LI Gut c o l o u r s i n r e l a t i o n to s e r o l o g i c a l l y determined gut contents 220 LII Comparison of f e e d i n g data f o r d i f f e r e n t c o r i x i d s p e c i e s 223 L I I I Feeding p r e f e r e n c e s of Cenocorixa s p e c i e s i n experiments and from s e r o l o g i c a l d eterminations .. 253 x v i ACKNOWLEDGEMENTS Th i s study was supported by a grant from the N a t i o n a l Research C o u n c i l of Canada t o Dr. G. G. Scudder i n a i d of r e s e a r c h , and a postgraduate s c h o l a r s h i p to me. I would l i k e to express my g r a t i t u d e t o Dr. Scudder f o r h i s c o n t i n u a l guidance i n the study. My thanks are due to Mr. S. Borden, Ms. Eolores L a u r i e n t e and Dr. R. Peterman f o r t h e i r a s s i s t a n c e i n computer o p e r a t i o n s , to Ms. Esta Sharpe, Ms. S h e i l a Brooke and Ms. Daphne Hard f o r advice on aspects o f the s e r o l o g i c a l t e c h r i g u e , and to Dr. Nordan and the s t a f f of the U.B.C. Vivarium f o r p r o v i d i n g f a c i l i t i e s and f o r maintaining my r a b b i t s . Mr. P. E l l i c k s o n , Dr. C. V. Finnegan, Mr. B r i a n Mclean and Dr. Kasinsky k i n d l y provided equipment. I am g r a t e f u l t o Drs. T. Northccte and C. Walters f o r t h e i r p a i n s t a k i n g advice and c r i t i c i s m , and to Drs. M. S w i f t and G. Jamieson and Messrs. R. Cannings and R. Shotton f o r s t i m u l a t i n g d i s c u s s i o n s . I would p a r t i c u l a r l y l i k e to acknowledge the c o n t i n u a l help of S y l v i a C. P. Reynolds, f o r a s s i s t a n c e both i n the f i e l d and the vivarium, and f o r encouragement a t every stage of the work. 1 CHAPTER I . INTRODUCTION Natural systems are dynamic i n nature. Fauna and f l o r a are c o n t i n u a l l y changing i n abundance and d i s t r i b u t i o n . The d i s t r i b u t i o n and abundance of a s p e c i e s may thus a l t e r front year to year as w e l l as with seasonal environmental changes, and the r e s u l t s of f i e l d sampling u s u a l l y r e p r e s e n t only a moment i n a dynamic process. Many s t u d i e s o f s p e c i e s i n t e r a c t i o n s assume that they are concerned with end-pcint s i t u a t i o n s ( vide EeEach, 1966); the r e s u l t of i n v a s i o n c f a new h a b i t a t , or the outcome, i n the form of e l i m i n a t i o n of one s p e c i e s , of competition between r e l a t e d forms. However, i t i s e g u a l l y l i k e l y that an intermediate stage i n e i t h e r process would be encountered over the time span of most s t u d i e s . Competition between s p e c i e s i s fundamental t o many ecosystems, even at e q u i l i b r i u m (here defined as shewing minor o s c i l l a t i o n s which do not a f f e c t ecosystem s t r u c t u r e or f u n c t i o n ) . Where a n e c e s s i t y of l i f e , commcnly feed cr s h e l t e r , i s l i m i t i n g , s u p e r i o r l i f e forms w i l l e v e n t u a l l y crowd out those l e s s well adapted, whether c o n s p e c i f i c s or members cf a r e l a t e d s p e c i e s . R e s u l t s i n the f i e l d are most obvious i n the l a t t e r case, end-point s i t u a t i o n s being represented by absence of one f a u n a l element from an area where i t apparently could s u r v i v e . During s t u d i e s by Scudder (1969b) of the fauna c f a s e r i e s of s a l i n e l a k e s of d i f f e r i n g a l k a l i n i t y i n c e n t r a l E r i t i s h Columbia, two s p e c i e s of the water bug genus C e n c c c r i x a were 2 observed to c o e x i s t i n c e r t a i n water bodies but net i n e t h e r s . T h e i r breeding ranges appeared to f o l l o w the s a l i n i t y g r a d i e n t , C. exjaleta (Uhler) being r e s t r i c t e d to waters of higher s a l i n i t y , and C. b i f i d a h u n g e r f o r d i Lansbury tc the more freshwater l a k e s , where i t c o e x i s t e d with other breeding c o r i x i d s p e c i e s . Because of t h e i r pronounced d i s p e r s a l at c e r t a i n times of year both s p e c i e s o c c a s i o n a l l y occur i n lakes c u t s i d e t h e i r breeding range, but sympatry i n breeding p o p u l a t i o n s was only seen i n l a k e s of from 6000 to 12000 umhos cm - 1 average c o n d u c t i v i t y (Scudder, 1966). The d i s t r i b u t i o n and osmotic and i o n i c balance i n the two s p e c i e s of Cenocorixa were examined by Scudder (1969a) and Scudder e t a l . (1972), and a l l evidence to date i n d i c a t e s that both are o s m o t i c a l l y freshwater s p e c i e s . However, although C. exjaleta i s p h y s i o l o g i c a l l y capable of l i v i n g i n the lower s a l i n i t y l a k e s , i t does not occur i n them n a t u r a l l y . Thus, Scudder et a l . (1972) suggest t h a t t h i s absence of C. exjaleta from v a r i o u s freshwater lakes i s not l i k e l y due to an i n a b i l i t y to s u r v i v e i n those waters. I t i s suggested t h a t e c o l o g i c a l f a c t o r s , such as predation, a v a i l a b i l i t y of s u i t a b l e food, and i n t e r s p e c i f i c c o m p e t i t i o n among o t h e r s , may be important i n e x p l a i n i n g the absence of £• §£Ei§£§. from the lower s a l i n i t y l a k e s . However, t c date t h i s has not been demonstrated, although Jansson (1971, 1972b) has suggested a p o s s i b l e d i f f e r e n c e i n h a b i t a t p r e f e r e n c e of the s p e c i e s . T h i s t h e s i s i s thus concerned with an a p p r a i s a l of some of 3 the e c o l o g i c a l f a c t o r s t h at have been suggested as perhaps important i n the apparent e x c l u s i o n of C. e x p l e t a from the lower s a l i n i t y l a k e s . In p a r t i c u l a r , a v a i l a b i l i t y of s u i t a b l e food, f e e d i n g preference and f e e d i n g i n t e r a c t i o n i n these two s p e c i e s of Cenocorixa are c o n s i d e r e d , with data obtained both i n the f i e l d and the l a b o r a t o r y . In a d d i t i o n , the r e l a t i v e abundance of the two s p e c i e s i n a l l o p a t r y and sympatry and t h e i r phenology was s t u d i e d f o r an assessment of c o m p e t i t i v e i n t e r a c t i o n . The fundamental and r e a l i s e d niches i n the two s p e c i e s are both assessed and d i s c u s s e d . THE CONCEPTS OF COMPETITION, COMPETITIVE INTERACTION AND COMPETITIVE EXCLUSION Competition has been adequately reviewed s e v e r a l times, perhaps best by Crombie (1947), Hardin (1960), EeEach (1966), M i l l e r (1967) and D a r l i n g t o n (1972). The concept of c o m p e t i t i v e e x c l u s i o n has been inherent i n b i o l o g i c a l t h i n k i n g s i n c e Darwin h i n t e d at i t as a c o r o l l a r y to the s t r u g g l e of e x i s t e n c e (1864). G r i n n e l l (1904) among other e a r l y authors (vide Udvardy, 1959) s t a t e d t h a t "...two s p e c i e s of approximately the same food h a b i t s are not l i k e l y to remain long enough evenly balanced i n numbers i n the same r e g i o n ; one w i l l crowd out the other...". In the 1920's the ideas were f o r m a l l y demonstrated by the use of s e t theory and c a l c u l u s (Lotka, 1925; V o l t e r r a , 1926). Gause (1934, 1935), a f t e r whom the p r i n c i p l e of c o m p e t i t i v e e x c l u s i o n has sometimes been named, produced the f i r s t experimental 4 evidence i n support of i t , using simultaneous equations to v e r i f y h i s f i n d i n g s mathematically. Since then t h e r e have been many attempts t c t e s t the p r i n c i p l e . For example, MacArthur and l e v i n s (1967) v e r i f i e d the mathematics of V o l t e r r a (1926), but i t would appear t h a t the p r i n c i p l e i s not amenable to d i s p r o o f , s i n c e while evidence can be adduced to support i t ( i . e . Crombie, 1947), seemingly c o n t r a d i c t o r y r e s u l t s only i n v i t e a more c a r e f u l study of the c o n d i t i o n s of sympatry and o v e r l a p p i n g niches i n the f i e l d or l a b o r a t o r y . Thus, i n one sense, the p r i n c i p l e remains a t r i v i a l , s e l f - e v i d e n t axiom (Ayala, 1970). However, i t r e l a t e s c l o s e l y to the ideas of the s t r u g g l e f o r e x i s t e n c e i n another sense; c o m p e t i t i o n i s l i k e l y t o be most i n t e n s e between c o n s p e c i f i c s , r e s u l t i n g i n the s u r v i v a l of the f i t t e s t i n d i v i d u a l , whereas i n l e s s c l o s e l y r e l a t e d groups one whole s p e c i e s , race cr other taxon may succumb c o m p e t i t i v e l y to another i n a p a r t i c u l a r h a b i t a t , and the r e s u l t s w i l l be demonstrable above the l e v e l of the i n d i v i d u a l . T h i s has been d i s c u s s e d by LeEach (1966), amongst others. The concept of co m p e t i t i o n i t s e l f has been r e v i s e d r e p e a t e d l y . M i l l e r (1967) reviews the progress made i n these r e d e f i n i t i o n s and refinements, and po i n t s out the two major components of c o m p e t i t i o n ; e x p l o i t a t i o n , which i s the e f f e c t i v e u t i l i s a t i o n of a r e s o u r c e , alone or shared; and i n t e r f e r e n c e , where resource u t i l i s a t i o n i s l e s s e f f i c i e n t under c o m p e t i t i v e s i t u a t i o n s than by s p e c i e s alone, i n a l l o p a t r y . I n t e r f e r e n c e may be d i r e c t (behavioural t e r r i t o r i a l i t y i n p a s s e r i n e s i s an 5 obvious example), or i n d i r e c t , where one s p e c i e s degrades the environment or food source through i t s a c t i v i t i e s , making i t l e s s a c c e p t a b l e to the other. The g e n e r a l l y accepted d e f i n i t i o n of competition was s t a t e d by Crombie (1947) as "...the demand, t y p i c a l l y a t the same time, of more than one organism f o r the same resources cf the environment, i n excess of immediate supply...", and other authors ( i . e . L a r k i n , 1956; Weatherley, 1963) i n g e n e r a l , accept t h a t s p e c i e s i n t e r a c t i o n , i f i t i s to be l a b e l l e d as c o m p e t i t i o n , must share a common resource. An opposite s c h o o l , s t a r t i n g with N i c h o l s o n (1933), f e e l t h a t only the c r i t e r i o n t h a t s u r v i v a l decreases as d e n s i t y i n c r e a s e s , i s needed to j u s t i f y use of the term •competition'. T h i s 'extended c o m p e t i t i o n ' i n c l u d e s p a r a s i t i s m and p r e d a t i o n , and any i n t e r a c t i o n between organisms, no matter how complex or i n d i r e c t , t h a t i s , or may be disadvantageous t c any of them, must be i n c l u d e d ( D a r l i n g t o n , 1972). Problems i n the d e f i n i t i o n of the 'niche' are summarised by E l t o n and M i l l e r (1952) and by Weatherley (1963). Foll o w i n g E l t o n (1927), who used the term i n the sense of a ' p r o f e s s i o n ' such as 'scavenger', Weatherley (1963) and many o t h e r s de s c r i b e d the niche as based on the n u t r i t i o n a l r o l e of the animal i n the ecosystem, reasoning t h a t food i s the most l i k e l y f a c e t of the environment to be i n s h o r t supply. On the other hand, G r i n n e l l (1917) and others saw the niche as e s s e n t i a l l y an 'address'; a d i v i s i o n of the h a b i t a t which i s a s m a l l e r component c f , but not fundamentally d i f f e r e n t from, an ecosystem. T h i s r a i s e s the 6 qu e s t i o n of the term 'sympatric', which i s u s u a l l y used i n a geographic, l a r g e - s c a l e sense but which could l o g i c a l l y be used to connote c o e x i s t e n c e i n any environmental d i v i s i o n from the m i c r o - e c o l o g i c a l s c a l e up to the geographic. The 'address* concept of the niche i s unsuited to com p e t i t i o n s t u d i e s , s i n c e i t i s h i g h l y probable that many s p e c i e s w i l l c o e x i s t i n the s m a l l e s t measurable environmental d i v i s i o n . Thus we r e q u i r e a conceptual model which w i l l "...accomodate both the f u n c t i o n a l and d i s t r i b u t i o n a l concepts..." ( M i l l e r , 1967). Hutchinson came c l o s e to t h i s i n d e s c r i b i n g the niche i n terms of s e t theory as an ' n-dimensicnal hyperspace' (Green, 1971), and Mi l s t e a d (1972), i n an attempt to q u a n t i f y the niche f o r comparative purposes assessed the c o n t r i b u t i o n s and importance of r e p r o d u c t i v e s t r a t e g y , p h y s i o l o g i c a l and b i o l o g i c a l responses to adverse c c n d i t i o n s , foods and f e e d i n g , p r e d a t i o n , a d a p t a t i o n and so f o r t h . Together these comprise the t o t a l n a t u r a l h i s t o r y of each organism. A f u r t h e r c o m p l i c a t i o n i s the d i s t i n c t i o n between 'fundamental 1 niches - those d e f i n e d by o b s e r v a t i o n s of the s p e c i e s i n a l l o p a t r y and i n l a b o r a t o r y surveys - and ' r e a l i s e d ' n i c h e s ; those observed i n nature i n each i n s t a n c e . Where the r e a l i s e d niche i s s m a l l e r or more r e s t r i c t e d than the fundamental, some form of i n t e r f e r e n c e or e x p l o i t a t i o n c o m p e t i t i o n must be invoked as an e x p l a n a t i o n . T h i s provided the s t a r t i n g p o i n t i n the present study, s i n c e the fundamental niche f o r the Cenocorixa s p e c i e s i n terms of s a l i n i t y t o l e r a n c e was l a r g e r than t h a t observed i n the B r i t i s h Cclumbia lakes s t u d i e d . 7 To c o n f i r m i n t e r s p e c i f i c c o m p e t i t i o n , one must shew s i g n i f i c a n t o v e r l a p i n fundamental niches, i n c l u d i n g showing that i n sympatry the r e a l i s e d niche i s s m a l l e r than i n a l l o p a t r y ( M i l l e r , 1967). P a r a l l e l i n g the h i s t o r i c a l development o u t l i n e d above, there has been much r e d e f i n i t i o n of the p r i n c i p l e cf c o m p e t i t i v e e x c l u s i o n , or, as DeBach (1966) would p r e f e r , the " p r i n c i p l e of c o m p e t i t i v e displacement". Hutchinson (1965) r e s t a t e d the p r i n c i p l e as " . . . c l o s e l y a l l i e d s p e c i e s l i v i n g together p r a c t i c a l l y always occupy s l i g h t l y d i f f e r e n t n i c h e s , or, i n other words, have d i f f e r e n t t o l e r a n c e s and optima". This i s c e r t a i n l y a masterpiece of e v a s i o n . Even a f r e q u e n t l y accepted summary of the p r i n c i p l e , "complete competitors cannct c o e x i s t " , i s l i k e w i s e t o t a l l y ambiguous (Hardin, 1960) and f u r t h e r l a c k s a c l a u s e d e a l i n g with i n e q u a l i t i e s i n r e p r o d u c t i o n . C l e a r l y each term needs p r e c i s i o n . Ayala (1969, 1970) i n v e s t i g a t e d two Drosophila s p e c i e s which reached a steady s t a t e c o e x i s t e n c e although one s p e c i e s was more s u c c e s s f u l i n the l a r v a l stage, and the other as a d u l t s . He c o n s i d e r e d t h a t h i s r e s u l t s e x p e r i m e n t a l l y i n v a l i d a t e d the p r i n c i p l e , but the resource •niche* i n v o l v e d was not adequately d e f i n e d and G i l p i n :and J u s t i c e (1972) decided t h a t h i s f i n d i n g s were i n harmony with the t h e o r e t i c a l work i n Gause and Witt (1935). Gause (1970) a l s o f e l t that Ayala's experiment r e p l i c a t e d one of h i s own 1935 experiments, where two a p p a r e n t l y competing Paramecium s p e c i e s were i n f a c t e x p l o i t i n g l a r g e l y d i f f e r e n t n i c h e s , one c h i e f l y i n the l i q u i d medium of a 8 c u l t u r e feeding on b a c t e r i a and the other predominantly e a t i n g y e a s t s at the bottom. Gause himself r e s t a t e d the p r i n c i p l e i n 1970 to read "two s p e c i e s competing f o r l i m i t e d resources can only c o e x i s t i f they i n h i b i t the growth of the competing s p e c i e s l e s s than t h e i r own". A l l authors admit the term 'niche' needs c l a r i f i c a t i o n . A f u r t h e r problem with c o e x i s t e n c e i s the p o s s i b i l i t y that i n t e r s p e c i f i c a s s o c i a t i o n may be a f u n c t i o n of other f a c t o r s (assessed by Cole, 1949, 1954) such as mutual a t t r a c t i o n ; one s p e c i e s seeking out the other as prey or f o r other reasons; or both being a t t r a c t e d or r e p e l l e d e i t h e r by another s p e c i e s or by an environmental s i t u a t i o n . D a r l i n g t o n (1972) has r e s t a t e d Gause's p r i n c i p l e i n a form t h a t i s v a l i d without e x c e p t i o n , as "...two p o p u l a t i o n s or s p e c i e s cannot long c o e x i s t i f they compete f o r a v i t a l r e s o u r c e , l i m i t a t i o n of which i s the d i r e c t and c n l y f a c t o r l i m i t i n g both p o p u l a t i o n s " . Where resource competition does not lead to e x t i n c t i o n of one party, p o p u l a t i o n s are l i m i t e d by s e p a r a t e density-dependent f a c t o r s , so that the resource i s not f u l l y u t i l i s e d . D a r l i n g t o n p o i n t s out that although many p l a n t s compete f o r l i g h t , they c o e x i s t , and there i s s u r p r i s i n g l y l i t t l e a daptive r a d i a t i o n t o u t i l i s e d i f f e r e n t wavelengths. Thus, although even l i g h t f a l l i n g at the e a r t h ' s s u r f a c e i s t h e o r e t i c a l l y a l i m i t e d resource, other f a c t o r s are a c t u a l l y l i m i t i n g and so are competed f o r f i r s t . S i m i l a r l y , S t a n l e y ' s (1973) cropping p r i n c i p l e , which s t a t e s t h a t p r e d a t i o n a l l o w s g r e a t e r d i v e r s i f i c a t i o n of prey 9 s p e c i e s , i s thus net an i n v a l i d a t i o n of the p r i n c i p l e of c o m p e t i t i v e e x c l u s i o n , s i n c e p r e d a t i o n by reducing numters of a f o r m e r l y v i c t o r i o u s s p e c i e s can l e a d to u n d e r - u t i l i s a t i o n of a r e s o u r c e , and hence competitors can c o e x i s t . S t a t e d as above by D a r l i n g t o n , the p r i n c i p l e of c o m p e t i t i v e e x c l u s i o n needs no f u r t h e r t e s t i n g . However, the time element i s now e x p l i c i t l y mentioned, and the r e a l i n t e r e s t i n and s i g n i f i c a n c e cf the p r i n c i p l e now becomes more obvious, i n that i t i n d i c a t e s t h a t under c e r t a i n c o n d i t i o n s , c o e x i s t e n c e can occur i n many h a b i t a t s . Recorded examples of such c o e x i s t e n c e are surveyed by M i l l e r (1967) and the f a c t o r s p e r m i t t i n g them are analysed. MacArthur and Levins (1964) looked a t a h y p o t h e t i c a l patchy environment and p o i n t e d out t h a t r e l a t e d s p e c i e s avoided c o m p e t i t i v e e l i m i n a t i o n through s i z e or h a b i t a t d i f f e r e n c e s ; pure s p e c i a l i s t (coarse-grained) s p e c i e s , such as predators t a k i n g l a r g e prey, do not u s u a l l y i n t e r a c t or a f f e c t each other and so may c o e x i s t ; whereas s p e c i e s s p e c i a l i s i n g i n a p a r t i c u l a r p r o p o r t i o n of two or more resources w i l l be e l i m i n a t e d i f t h a t p r o p o r t i o n should change s i g n i f i c a n t l y . Most mixed-resource s p e c i e s are found i n s t a b l e communities. H a i r s t o n (1959) a l s o observed that i n a m u l t i - s p e c i e s community the l e s s s u c c e s s f u l s p e c i e s are only found i n very s u i t a b l e , r e s t r i c t e d areas, thus t h e i r d i s t r i b u t i o n appears clumped. On the other hand, s u c c e s s f u l s p e c i e s are more numerous and l e s s clumped, and appear c l o s e r to a random d i s t r i b u t i o n . T h i s i s r e l a t e d to M i l l e r ' s (1967) ideas of 'included n i c h e s ' where two competitors have o v e r l a p p i n g n i c h e s , one i n s i d e the o t h e r ' s . 10 MacArthur (1958) ana Slobodkin (1962) conclude that most of today's communities are near e q u i l i b r i u m ; the e c o l o g i c a l p r i n c i p l e s d i s c u s s e d above apply to these but not n e c e s s a r i l y to a l t e r e d or e a r l y s u c c e s s i o n a l stages. However, t h i s statement might need q u a l i f i c a t i o n . In a recent study, Grenney et a l . (1973) used a t h e o r e t i c a l model of a phytoplankton p o p u l a t i o n to e x p l a i n the observed c o e x i s t e n c e of many a l g a l s p e c i e s i n an a p p a r e n t l y homogeneous environment, i n terms cf a c o n t i n u a l v a r i a t i o n i n environmental c o n d i t i o n s with time. Thus, the p o p u l a t i o n never r e a l l y a t t a i n e d c o m p e t i t i v e e q u i l i b r i u m , although tending towards d i f f e r e n t e q u i l i b r i a at d i f f e r e n t times. S i m i l a r l y , Stewart and L e v i n (1973) showed mathematically t h a t i n a s e a s o n a l l y changing environment, c o n d i t i o n s f o r s t a b l e c o e x i s t e n c e may be met when two or more s p e c i e s compete f o r one or more r e s o u r c e s , thus c o n t r a d i c t i n g c e r t a i n forms of the c o m p e t i t i v e e x c l u s i o n p r i n c i p l e . I t seems t h a t e c o l o g i c a l c o n d i t i o n s of c o m p e t i t i o n i n ' s t a b l e ' and ' f l u c t u a t i n g ' environments may need se p a r a t e t h e o r e t i c a l a n a l y s i s . THE CORIXIDAE : THE ANIMALS TO BE STUDIED C o r i x i d s as a group have been the s u b j e c t of a l a r g e body of r e s e a r c h , i n p a r t r e l a t e d to t h e i r occurrence i n a wide range of h a b i t a t s , t h e i r abundance, and t h e i r d i s p e r s a l p a t t e r n s . S e v e r a l authors, such as Hungerford (1919), Macan (1938, 1962) and I s t o c k (1973) c o n s i d e r them to be p a r t i c u l a r l y s u i t e d to s t u d i e s of s p e c i e s i n t e r a c t i o n s , c h i e f l y f o r these reasons of 11 d i s t r i b u t i o n and abundance. Much work has been done on c o r i x i d d i s t r i b u t i o n p a t t e r n s i n d i f f e r e n t h a b i t a t s , o f t e n i n r e l a t i o n to the water chemistry, h a b i t a t f a c i e s and other s p e c i e s present ( i . e . Brocks and K e l t o n , 1967; Brown, 1948; Knowles and W i l l i a m s , 1973; Lansbury, 1960; Macan, 1938, 1949, 1954a, 1954b, 1962; Martin, 1970; Pajunen, 1970a; Popham, 1952; S a i l e r , 1948; Savage, 1971; Scudder et a l . , 1972). O l o f f s and Scudder (1966) d i s c u s s e d an aspect of t h i s ; the i r r e v e r s i b l e e f f e c t of high temperature, such as may be found i n shallow or temporary ponds, upon c u t i c u l a r wax and waterproofing. The permanence and s u i t a b i l i t y of the h a b i t a t has been r e l a t e d to d i s p e r s a l and migration tendencies i n many s t u d i e s , notably those of Brown (1951) and Pajunen (1970b), and many authors have d i s c u s s e d i t s r e l a t i o n s h i p with the i n c i d e n c e of a l a r y polymorphism and consequent f l i g h t l e s s n e s s ( i . e . Leston, 1953; Johnson, 1960, 1963, 1966; Macan, 1939; Eajunen and Jansson, 1969b; Popham, 1943, 1952, 1959, 1964; Pcpham and Lansbury, 1960; R i c h a r d , 1958, 1966; Southwood, 1961; Southwood and Johnson, 1957; Scudder, 1964, 1971; Scudder and Meredith, 1972; and Simpson, 1968). Johnson (1963, 1966) considered the e f f e c t of p h y s i o l o g i c a l f a c t o r s such as the q u a l i t y and l e n g t h of the p r e - o v i p o s i t i o n p e r i o d on the r e g u l a t i o n cf migratory f l i g h t . Other s t u d i e s have d e a l t with c o r i x i d l i f e h i s t o r y phenomena, such as p o p u l a t i o n s t r u c t u r e and annual production ( i . e . Ancona, 1933; C r i s p , 1962a; Ist o c k , 1973; Pcpham, 1952; 12 Jansson and Scudder, 1974) while the last-named authors and C r i s p (1962b), Johnson (1963), Martin (1970, 1972), Pajunen (1970a, 1970c), Savage (1971) and many others d i s c u s s e d aspects of c o r i x i d development, o v i p o s i t i o n and o v a r i a n a r r e s t , i n r e l a t i o n to environmental f a c t o r s such as food and temperature. F i n a l l y , although c e r t a i n s t u d i e s of c o r i x i d f e e d i n g h a b i t s have i n d i c a t e d evidence of p r e d a t i o n on a g u a t i c i n v e r t e b r a t e s , many authors s t a t e t h a t c o r i x i d s as a group are p r i m a r i l y microphagous and d e t r i t u s - f e e d e r s . The s u b j e c t i s reviewed below, i n Chapter IV. In a d d i t i o n to d i s c u s s i n g the feeding p a t t e r n , c o r i x i d morphology has o f t e n been r e l a t e d to p o s s i b l e f e e d i n g behaviour, notably by Hale (1922), Hungerfcrd (1919), J a r i a l et a l . , (1969), Marks (1957), Parsons . (1957b, 1966), Poisson (1923), Slack (1947) and Sutton (1947a, 1951). Most evidence f o r competition comes from f e e d i n g behaviour. T h e r e f o r e , the d i e t of Cenocorixa i s l i k e l y t c shew evidence of any c o m p e t i t i v e displacement or e x c l u s i o n i n the f i e l d , i n some l a k e s . Information on feeding i n the genus Cenoccrixa i s given by Jansson (1971) and Jansson and Scudder (1971, 1974) , c h i e f l y i n the context of s u c c e s s f u l r e a r i n g of both s p e c i e s through s e v e r a l generations on a d i e t of f r o z e n b r i n e shrimp. There was no d i r e c t s u b s t a n t i a t i o n of e i t h e r p r e d a t i o n on l i v i n g prey, or pla n t f e e d i n g , although Scudder and Jansson (1971) s p e c u l a t e t h a t f i e l d - c o l l e c t e d c o r i x i d s with red guts may have been fe e d i n g on diaptomids. The present study sought t o look f u r t h e r i n t o the c o n d i t i o n s of c o e x i s t e n c e of the two s p e c i e s of Cenocorixa i n a 13 s e r i e s of smal l l a k e s of d i f f e r i n g s a l i n i t y i n c e n t r a l B r i t i s h Columbia. The major study was an i n v e s t i g a t i o n of c o r i x i d f e e d i n g p a t t e r n s , as these were most l i k e l y to provide evidence of c o m p e t i t i o n . F o l l o w i n g s t u d i e s of the environment and of the c o r i x i d s i n v o l v e d (chapters I I and I I I ) , experiments f e e d i n g d i f f e r e n t p o t e n t i a l prey organisms to c o r i x i d s under l a b o r a t o r y c o n d i t i o n s (chapter IV) were supplemented by gut examination of f i e l d - c o l l e c t e d c o r i x i d s from each h a b i t a t , using a s e r o l o g i c a l technique to i d e n t i f y the l a r g e l y f l u i d gut contents (chapter V). F i n a l l y , i n the l a s t chapter the data are assessed to see i f co m p e t i t i v e e x c l u s i o n p r o v i d e s a p l a u s i b l e e x p l a n a t i o n cf the d i s t r i b u t i o n p a t t e r n of Cenocorixa s p e c i e s i n the f i e l d , i n the l i g h t of those f a c e t s of t h e i r b i o l o g y so f a r s t u d i e d . 14 CHAPTER I I . THE ENVIRONMENTAL BACKGROUND INTRODUCTION : THE STUDY AREA The main area chosen f o r study l i e s i n the Fraser P l a t e a u r e g i o n of c e n t r a l B r i t i s h Columbia and i n c l u d e s lakes that have alr e a d y been i n v e s t i g a t e d i n some d e t a i l by Scudder (1969b), Topping (1970) and Cannings (1972). The Becher's P r a i r i e study area i s on the C h i l c o t i n P l a t e a u near Riske Creek, west of the F r a s e r River near Williams Lake. The t e r r a i n i s gently u n d u l a t i n g with a v e g e t a t i o n of Carex, P o t e n t i l l a , A r t e m i s i a and other dry parkland p l a n t s ( B e i l , 1970), i n t e r r u p t e d at i n t e r v a l s by b e l t s of Douglas f i r , j a c k p i n e and aspen, the l a t t e r c h i e f l y marking damper areas. U n d e r l y i n g r o c k s are Permian, T r i a s s i c and T e r t i a r y p lateau beds. Mean p r e c i p i t a t i o n i s 35 cm y r _ 1 , and mean monthly temperatures - 10 C i n January and + 25 C i n J u l y . Of about 75 lakes and sloughs on Becher's P r a i r i e , s i x of the l a r g e r were s e l e c t e d f o r s p e c i a l study, namely Rcund-up Lake (Phalerope) , Lake Lye (Box 20-21), Barnes Lake (Eox 4), Lake Greer (Box 89) , East Lake (Racetrack) and Reck (Reck) Lake (Names used by Scudder (1969b) i n parentheses). These lakes were s e l e c t e d f o r eq u i v a l e n c e of s i z e , permanence and to encompass the s a l i n i t y range of l a k e s i n the area. There are nc permanent i n f l o w or outflow streams l i n k i n g any of these l a k e s . F i g u r e 1 shows something of the morphometry cf each l a k e . The maximum depth of the lakes i s about 6 m, although Rock L. and L. Greer are about h a l f t h i s depth. The areas of the 15 F i g u r e 1. Sketch-map of the study areas, with i n s e r t s showing Lake LB 2, and the p o s i t i o n of the study areas i n B . C . 17 l a k e s are a l s o f a i r l y s i m i l a r , and the dominant i c n s are the same (Table I ) . At a s i m i l a r a l t i t u d e to the Becher's P r a i r i e l a k e s (around 900 m), Lake LB 2 (Scudder, 1969), near Lac du Bois i n the h i l l s north of Kamloops, was sampled p e r i o d i c a l l y to o b t a i n c o r i x i d s and p o t e n t i a l prey organisms. LB 2 has dimensions s i m i l a r to the lakes on Becher's P r a i r i e (Table I ) , and has a mean recorded c o n d u c t i v i t y of 20,000 umhos cm - 1, with s i m i l a r dominant i o n s except f o r the presence of sulphates. The morphoedaphic index l i s t e d i n Table I f o r each lake was c a l c u l a t e d using t o t a l d i s s o l v e d s o l i d s measurements (Topping, 1969) and mean depths. T h i s index has proved a u s e f u l guide to r e l a t i v e p o t e n t i a l p r o d u c t i o n , s p e c i f i c a l l y of f i s h , of water bodies i n r e s t r i c t e d areas of simila*!. geography and c l i m a t e ; i t was developed from ideas of florthcote and L a r k i n (1956) and Rawson (1952), and supercedes e a r l i e r measures cf p o t e n t i a l p r o d u c t i v i t y which used depth and area f a c t o r s (Ryder et a l . , 1974) . C o l l e c t i o n s of some p o t e n t i a l prey organisms were made a t Marion Lake i n the U.B.C. Research F o r e s t ( H a l l and Hyatt, 1974). Samples of c o r i x i d s and other fauna were a l s o taken i n s m a l l temporary ponds by Wesbrook Crescent and 16th Avenue on the U.B.C. Endowment Lands i n Vancouver. Lake Elevation Area Depth (m) (m) (ha) Mean Max. Conductivity ( mhos/cm at 20°C) Range Mean pH Major Major Range cation anions Morpho-Edaphic Index LB 2 Barnes Round-up Lye Greer 899 945 945 945 945 3.1 1.1 2.5 2720-20580 16514 9.2- 9.7 Na CC>3, SO^ 17.2 2.0 4.5 3000-20000 11820 9.3- 9.7 Na COg, HCC<3 30.8 2.6 6.2 2820- 9000 6890 9.2- 9.3 Na 46.5 2.8 5.4 1100-12000 6548 9.1- 9.6 Na 15.2 1.0 2.3 1400- 2200 1602 8.4- 9.5 Na C0 3, HC03 C0 3, HC03 HCO, 15013 4893 2062 1765 1221 Rock East 945 945 34.6 1.1 2.5 1435- 2600 1496 8.6-10.5 Na HCO, 27.0 1.9 6.5 400- 600 600 7.8- 9.7 Na HCO„ 1027 196 Table I. Some selected physical and chemical features of the lakes studied (modified i n part after Scudder, 1969; Topping, 1969; and Cannings, 1973). 19 MATERIALS AND METHODS Since c o r i x i d s do not feed below 5 C (Jansson and Scudder, 1972) no winter sampling was done. J u v e n i l e s of Cenocorixa f i r s t appear i n the study l a k e s about mid-May (Jansscn and Scudder, 1974). I n t e n s i v e f i e l d sampling was t h e r e f o r e c a r r i e d out from mid-May to mid-October, 1972, at the Becher's F r a i r i e l a k e s , and l e s s r e g u l a r l y at Lake LB 2. In both areas, r e g u l a r samples were a l s o made i n 1973. Samples were a l s o c o l l e c t e d when necessary at the U.B.C. and Marion Lake s i t e s from 1971 to 1973. The f o l l o w i n g d e s c r i b e s methods and m a t e r i a l s f o r the main study area l a k e s i n the 1972 f i e l d s t u d i e s . A sampling area was chosen f o r each l a k e , reasonably c l e a r of weeds, and as f a r as p o s s i b l e with s i m i l a r tcpcgraphy and shore development. The area was s u f f i c i e n t l y l a r g e to allow sampling a l l summer with minimal d i s r u p t i o n of the h a b i t a t . Midlake samples were taken a t the deepest p o i n t s of each la k e , as determined from depth c h a r t s i n Topping (1970). 20 Parameters (i) Temperatures Surface temperatures were measured at the i n s h c r e sampling s t a t i o n over 10 cm of water, and midlake, every four days i n each of the s i x l a k e s on Becher's p r a i r i e using a F i s h e r p a r t i a l immersion thermometer graduated from -10 C to +110 C. Automatic monthly c h a r t r e c o r d e r s (Ryan Model D-30, 0 to +30 C, Chart 380-4) were i n s t a l l e d i n f i v e of the l a k e s on Eecher's P r a i r i e (except Rock Lake) and recorded temperature from mid-May to mid-October 1972. They were placed i n areas of a p p r e c i a b l e s l o p e , 1.5 to 2.0 m from shore and 20-30 cm deep; the only marked v a r i a b l e being the presence or absence of weeds. ( i i ) C o n d u c t i v i t y Water was c o l l e c t e d at approximately monthly i n t e r v a l s from the s i x Becher's P r a i r i e l a k e s , f i l t e r e d through phytcplanktcn n e t t i n g (meshes 54u) and s t o r e d i n a 1 l i t r e p o l y e t h y l e n e b o t t l e f o r c o n d u c t i v i t y measurements. S p e c i f i c conductance was estimated using a Copenhagen Radiometer c o n d u c t i v i t y meter, type CDM 2d, and r e s u l t s were c o r r e c t e d to a standard 25 C temperature. 21 Primary P r o d u c t i v i t y Three e s t i m a t i o n s were made of the p h o t o s y n t h e t i c a c t i v i t y of p l a n k t o n i c algae during the summer of 1972, tc obtain an i n d i c a t i o n of the s i z e of that p o r t i o n of the a l g a l base of the food web extending e v e n t u a l l y to the c o r i x i d s under i n v e s t i g a t i o n . Water l i g h t t r a n s m i s s i o n was measured with a white and black quartered 20 cm diameter Secchi d i s c . Readings were made every f o u r days a t the midlake s t a t i o n i n each lake, f o l l o w i n g T y l e r (1968). Weather parameters (wind, degree of c l o u d i n e s s , p r e c i p i t a t i o n ) were recorded on each o c c a s i o n . Rodhe (1958) estimated the maximum p h o t o s y n t h e t i c r a t e to occur at a l e v e l of 20 - 25 percent of t o t a l l i g h t e x t i n c t i o n recorded with a p h o t o c e l l . Since S e c c h i d i s c e x t i n c t i o n depth r e p r e s e n t s approximately 15 percent of subsurface i l l u m i n a t i o n measured with a p h o t o c e l l (Steeman N i e l s e n , 1958) the maximum p h o t o s y n t h e t i c depth was d e r i v e d from the s i m p l i f i e d photosynthesis-depth and l i g h t e x t i n c t i o n - d e p t h curves i n Vollenweider (1969) to l i e a t about one t h i r d of the S e c c h i d i s c e x t i n c t i o n depth. The M i l l e r (1914) method of oxygen determination, e l a b o r a t e d by walker et a l . (1970) was used because of i t s a p p r o p r i a t e n e s s f o r s a l i n e water s i t u a t i o n s (Thomas, 1953; Eayly and W i l l i a m s , 1966). L i g h t and dark b o t t l e oxygen changes (de s c r i b e d i n Patten et a l . , 1964) were used i n s t e a d of r a d i o -22 i s o t o p e t r a c e r techniques (e.g. Steeman N i e l s e n and Cushing, 1958) a f t e r a c o n s i d e r a t i o n of f i e l d c o n d i t i o n s and the experimental s i t u a t i o n . Four cords were attached 40 cm apart to a 2 m long spar with other dimensions 5 by 2.5 cm to hold p r o d u c t i v i t y b o t t l e s . These were standard 300 ml reagent b o t t l e s (Wheaton No-Sol-Vit No. 2) with ground-glass s t o p p e r s ; 'dark 1 b o t t l e s and t h e i r s t o p p e r s were covered i n heavy-duty black polythene s h e e t i n g secured with black masking tape. Each b o t t l e had a simple c r a d l e of twine l e a d i n g to a c u r t a i n r i n g ; t h i s c o u l d be attached to the cord so that the b o t t l e would hang h o r i z o n t a l l y (Rodhe, 1958) at the d e s i r e d depth. At the midlake sampling p o i n t the spar was anchored and a S e c c h i d i s c transparency reading taken. Four r e p l i c a t e l i g h t and one dark b o t t l e s were f i l l e d , one b o t t l e per l i t r e to a l l o w ample f l u s h i n g , from a two l i t r e Van Corn b o t t l e f i l l e d 10 cm below the s u r f a c e . Stoppers were wired i n place and b l a c k polythene taped over the neck and stopper of the *dark* b o t t l e as a f u r t h e r p r e c a u t i o n a g a i n s t l i g h t p e n e t r a t i o n . B o t t l e s were then hung h o r i z o n t a l l y a t one t h i r d of the S e c c h i d i s c e x t i n c t i o n depth, which approximated the r e g i o n of maximum ph o t o s y n t h e t i c r a t e . The experiment was l e f t to run f o r s i x hours from midday, with the spar a c t i n g as f l o a t and marker but not i n t e r f e r i n g with l i g h t t r a n s m i s s i o n . Hidlake water temperature was measured at the s t a r t and end of the experiment at the chosen depth. The t e s t p e r i o d chosen (6 h) was a compromise between i n a c c u r a c i e s i n s h o r t experiments r e s u l t i n g 23 from d i s t u r b a n c e e f f e c t s and from delays during t i t r a t i o n s , and i n c r e a s i n g i n a c c u r a c i e s r e s u l t i n g from longer experiments; these may i n c l u d e depression of photosynthesis (Vollenweider, 1969) and problems of b a c t e r i a l growth (Steeman N i e l s e n , 1958). One dark and three l i g h t b o t t l e s were used e x p e r i m e n t a l l y ; the f o u r t h f i l l e d b o t t l e was immediately used to o b t a i n i n i t i a l oxygen readings i n the same manner as the o t h e r s at the end of the experiment. Each b o t t l e was sampled to o b t a i n r e p l i c a t e oxygen de t e r m i n a t i o n s , f o l l o w i n g the M i l l e r method which measures o x i d a t i o n of f e r r o u s ions i n an a l k a l i n e medium. A 50 ml sample was placed i n a 100 ml c o n i c a l f l a s k and 5 ml a l k a l i n e t a r t r a t e reagent p l u s methylene blue i n d i c a t o r were added. T h i s was t i t r a t e d with a f e r r o u s s o l u t i o n to the disappearance cf the blue c o l o u r , using a 20 ml Corning d i s p o s a b l e p i p e t t e . To minimise a d d i t i o n of atmospheric oxygen the f l a s k was g e n t l y s w i r l e d and s t i r r e d with the submerged t i p of the p i p e t t e . The t i t r e i s approximately equal to the oxygen content i n ml, converted to mg l - 1 by m u l t i p l y i n g by 1.43 (Thomas, 1953). If the assumption, not always c o r r e c t (Bunt, 1965), i s made that r e s p i r a t i o n r a t e s are s i m i l a r i n dark and l i g h t , the dark b o t t l e v alues s u b t r a c t e d from those i n the f i n a l l i g h t b o t t l e s give a measure of gross p h o t o s y n t h e s i s . Net photosynthesis was obtained by s u b t r a c t i n g i n i t i a l from f i n a l l i g h t b o t t l e measurements; t h i s , doubled, gives an t approximate d a i l y p h o t o s y n t h e t i c r a t e a t optimum depth (Steeman N i e l s e n , 1958) , which i s converted to d i e l r a t e by s u b t r a c t i n g four times the net r e s p i r a t i o n rate from the doubled gross 24 photo-synthetic r a t e . The problems of such c o n v e r s i o n s from an experimental to a d a i l y r a t e are s t r e s s e d by Vollenweider (1969) but remain e s e n t i a l l y unsolved. From the i n f o r m a t i o n , a F i g u r e f o r d a i l y primary production was obtained, expressed as mg C f i x e d per m2 per day, using 1.9 mg 02 = 1 mg C (Steeman N i e l s e n and Cushing, 1958). Aquatic p l a n t s Macrophyte samples were c o l l e c t e d monthly, i n 1970 by Scudder i n a l l s i x Becher's P r a i r i e l a k e s . In 1972 the s h o r e l i n e of each was t r a v e r s e d monthly and herbarium specimens c o l l e c t e d and i d e n t i f i e d , with a r e c o r d of abundance, f l o w e r i n g and f r u i t i n g s t a t u s , and p o s i t i o n r e l a t i v e to the water's edge, by S y l v i a C. P. Reynolds. Plankton During 1970 monthly q u a n t i t a t i v e (2 l i t r e ) s u r f a c e plankton samples were c o l l e c t e d inshore i n d u p l i c a t e by Scudder. A l l those f o r the s i x lakes under study were examined and the p l a n k t e r s i d e n t i f i e d and counted. Diaptomid and c y c l o p o i d copepods were i d e n t i f i e d f o l l o w i n g m i c r o d i s s e c t i o n and mounting i n P o l y v i n y l Lactophenol with l i g n i n pink; most i d e n t i f i c a t i o n s were confirmed by Dr. K. A. P a t a l a s . 25 In 1972 plankton i n a l l s i x lakes was sampled q u a n t i t a t i v e l y i n d u p l i c a t e inshore and midlake with a 2 - l i t r e Van Dorn water b o t t l e . The c o l l e c t e d water sample was f i l t e r e d through a c o n i c a l zooplankton net of 417^ mesh s i z e , which r e t a i n e d a l l a d u l t s and a good p r o p o r t i o n of n a u p l i i . The o u t s i d e of the net was washed to concentrate the contents and samples were run i n t o 6 dram g l a s s v i a l s c o n t a i n i n g atout 0.5 dram formaldehyde s o l u t i o n . Any p l a n k t e r s remaining cn the net were added using a f i n e p aintbrush. A l l 2 - l i t r e samples were l a t e r f i l t e r e d and the wet volume of plankton measured by displacement to the nearest 0.05 ml. Midlake g u a n t i t a t i v e samples were supplemented by a v e r t i c a l haul made with the same net, weighted to allow r a p i d s i n k i n g . Other Organisms S e m i - q u a n t i t a t i v e b e n t h i c sweep-net samples were c o l l e c t e d bi-monthly i n 1970 by Scudder and these were examined to o b t a i n some measure of the a v a i l a b i l i t y and abundance of p o t e n t i a l food organisms. T o t a l displacement volume of whole samples was measured and the volume and t o t a l numbers of s e l e c t e d major groups were a l s o determined. For s i x weeks from mid-May of 1972 a l l i n v e r t e b r a t e s taken i n sweep-net samples were preserved; a f t e r t h i s date these were not c o l l e c t e d q u a n t i t a t i v e l y . Occurrence of c e r t a i n b e n t h i c organisms i n 1972 was a l s o obtained from the i n s h o r e 2 l i t r e 26 plankton samples. RESULTS L i 2 3 2 l o g i c a l Parameters (i) Temperatures Temperature maxima and minima at 30 cm depth read from temperature c h a r t s were p l o t t e d f o r each second day f o r the Becher's P r a i r i e lakes to i n d i c a t e the extent of d a i l y and seasonal temperature f l u c t u a t i o n s (Figure 2). Inshore temperatures were g e n e r a l l y higher than those midlake during the day, and i n shallower lakes both inshore and midlake temperatures showed more extreme d a i l y f l u c t u a t i o n s than i n deeper l a k e s . Mixing seemed to occur throughout the season. In ge n e r a l temperature p a t t e r n s were s i m i l a r between la k e s , r e f l e c t i n g t h e i r p r o x i m i t y and comparable s i z e and depth f e a t u r e s . F o l l o w i n g i c e - m e l t about the end of A p r i l , 1972, warming was r a p i d and temperatures reached a f l u c t u a t i n g ' p l a t e a u ' l e v e l by mid-May, when sampling began; t h i s l e v e l held u n t i l the end of August. Temperatures then f e l l o f f slowly but f a i r l y s t e a d i l y u n t i l i c e s t a r t e d t o form i n e a r l y October. Maximum recorded temperatures f o r 1972 were s i m i l a r i n a l l l a k e s , the highest being 26.7 C f o r Round-up Lake i n August. The temperature data 27 F i g u r e 2. D a i l y t e m p e r a t u r e r a n g e s i n f i v e B e c h e r ' s P r a i r i e l a k e s , r e c o r d e d a t two day i n t e r v a l s f r o m May t o O c t o b e r , 197 2. 29 i n d i c a t e t h a t i n 1972 t h i s parameter remained s i m i l a r to that recorded i n previous seasons (Cannings, 1973; J a r s s c n and Scudder, 1974). ( i i ) C o n d u c t i v i t y C o n d u c t i v i t y was measured on f o u r o c c a s i o n s during 1972. C o n d u c t i v i t i e s i n c r e a s e d as the season progressed, reaching h i g h e s t l e v e l s i n October; t h i s was due c h i e f l y to the gradual mixing of a s u r f a c e l a y e r of l o w - s a l i n i t y meltwater with deeper, more s a l i n e waters. The c o n d u c t i v i t y values recorded (Figure 3) f a l l c o n s i s t e n t l y around the high end of the ranges given by Scudder (1969b), i n d i c a t i n g no s i g n i f i c a n t departure from the p r e v i o u s c o n d i t i o n s recorded i n these water bodies. Primary P r o d u c t i v i t y Oxygen det e r m i n a t i o n s and p r e l i m i n a r y p r o d u c t i v i t y e s t i m a t e s i n d i c a t e that i n the three more freshwater lakes on Becher's P r a i r i e oxygen l e v e l s were g e n e r a l l y higher than i n the more a l k a l i n e waters and production by phytoplankton was at times high (Table I I ) . However, the data a l s o suggest that p r o d u c t i o n by p l a n k t o n i c algae i s not important i n r e p l e n i s h i n g water oxygen i n the t h r e e most a l k a l i n e l a k e s . In f a l l p r o d u c t i o n was low, even i n the more freshwater l a k e s . Secch i d i s c water transparency data c o l l e c t e d tc assess 3 0 F i g u r e 3. Range of c o n d u c t i v i t i e s measured i n seven study l a k e s i n 1972. V e r t i c a l bars i n d i c a t e ranges recorded i n the l i t e r a t u r e . i^MHO 20,000-16.000H 32 Table I I . Primary production estimates and s u r f a c e cxygen readings i n s i x lakes cn Becher's P r a i r i e ( F c i d e t a i l s of c a l c u l a t i o n s see t e x t ) . r — —r - — T | LAK E | P ARAM ET ER | 18 MAY |27-11 MAY |7-9 AUG 120-21 SEP | Barnes |depth» | 10 |50 | 53 | 63 | 11 e m p 2 | 16.0 |18.5 | 24. 4 I 10.0 | loxygen3 | 9. 15 |7. 15 | 6.68 |8.44 | Iphotosyn *| |0.50 1-1.28 |0.36 1 | productn 5 | | 95 I neg. | 68 | Round-up |depth* | 10 | 50 | 55 | | | temp2 J 11.6 117.6 | 24.0 | | loxygen3 | 9.65 17.51 | 7.05 | | |photcsyn*| I (-1.48) | 1.24 | 1 |productn3 | I neg. | 236 I i y e Idepthi | 10 | 60 I 73 | 70 1 |temp 2 | 12.1 | 17.2 | 23. 2 |11.0 1 loxygen3 | 8.91 | 7 .87 | 6.86 18.58 1 |photosyn *| | 2. 22 12.14 |0.92 1 | productn 5 | 1422 | 407 | 175 | Greer Idepth* | 10 |30 I 65 I 57 | |temp2 | 14.0 120.8 I24.5 |9.6 | (oxygen3 | 10.01 | 14.47 I 6.44 I8.44 | | photosyn*| |9.18 |0.64 | 1.04 |productn 5 | | 1744 | 121 | 198 | Bock |depth» | 10 | | 38 | | |temp2 | 14.5 | | 23. 4 | j loxygen3 | 11.08 | | 8.39 | | |photcsyn*| | |0.26 | • | productn5 | J | 49 | | last Idepthi | 10 I 10 | 28 | 26 I |temp 2 | 14.0 | 19.5 | 27. 4 110.9 | loxygen3 | 10.08 I22.52 | 13.59 I 5.62 I | photosyn*| | 2.36 | 4.72 |0.72 j | productn 5 | |448 | 897 | 137 i i 1 : t e s t depth (cm) 2 : temperature (C) 3 : s u r f a c e oxygen (mg l - 1 ) * : net d i e l photosynthesis (mg 1 _ 1 ) 5 : primary production (mg C rc2 24h). 33 l i g h t p e n e t r a t i o n i n c o n j u n c t i o n with the plankton and p r o d u c t i v i t y study are recorded, with o b s e r v a t i o n s cf a l g a l blooms, i n F i g u r e 4. Harked f l u c t u a t i o n s i n transparency were noted and no doubt r e l a t e to blooms, to atmospheric c c n d i t i o n s and t o s i l t washed i n a f t e r heavy r a i n s . There was no c o n s i s t e n t p a t t e r n a c r o s s the l a k e s , although i n some, transparency d e c l i n e d i n f a l l . A g u a t ic Macrop_hytes Although one s p e c i e s of Najadaceae occurred i n g u a n t i t y i n Lake LB 2 which has a mean c o n d u c t i v i t y of 20,000 umhos cm - 1, no p l a n t s were found i n the t h r e e most a l k a l i n e l a k e s cn Becher's P r a i r i e . However, i n the ether l a k e s ( c o n d u c t i v i t y below 2,600 jimhos cm - 1) Myriophyllum and najadaceous p l a n t s were o f t e n abundant (Table I I I ) and formed e x t e n s i v e i n s h o r e f r i n g e s up to a depth of about 1.5 m. Nacrophyte growth was r a p i d once warming of the l a k e s s t a r t e d , forming meadows by the end cf May, with d i s t i n c t z o n a t i o n with depth. F r i n g i n g bands u s u a l l y had Z a n i c h e l l i a i n s h o r e , then Potamogeton, Myriojahyllum and f i n a l l y Rupjoia i n the deepest water. P l a n t s s t a r t e d to d i e back i n e a r l y September as temperatures f e l l , and by the approach of freeze-up most macrophytes had again disappeared. Juncus and Scirjsus s p e c i e s grew on the damp s h o r e l i n e s of a l l l a k e s , o c c a s i o n a l l y e n t e r i n g the water i n the three most * a l k a l i n e l a k e s and i n Rock Lake. They thus provided almost the 34 F i g u r e 4. S e c c h i d i s c water transparency i n s i x Becher's P r a i r i e l a k e s , measured at 4 day i n t e r v a l s i n 1972. Shaded areas ('b*) i n d i c a t e presence of strong a l g a l blooms. 36 Table I I I . D i s t r i b u t i o n of a q u a t i c macrcphjtes cf the Eecher's P r a i r i e l a k e s and Lake IE 2. i T T 1 1 1 r 1 1 | | LE 2 | Barnes Round. Lye | Greer| Reck | East | |E MER GENTS : | | | | | | | I I S c i r ^ u s | | | | | | | | | v a l i d us | | x | x | x | x | x I I |Juncus I I I I I I I I | b a l t i c u s | x | x | x | x | x | x | x I |Carex spec i e s | I x | x | I I I I |S UBM ERGEC: I I I I I I I I I Myric^hjllum I I I I I I I I I S£i£SiJJS I I I I I x | x | x | 1Ceratcflhylluro | I I I I I I I I dejersum | | | | | | I x | I Pcta fflccje ten I I I I I I I I I jgectinatus | I I I | x | x | x | IJ?« P_usillus | | | | | | | | I o r f o l i o s u s | | | | | | | x | 12 S i 1J2.£.9£.££2 I I I I I I I I | species | x | I I I I I I JRujDjaia I I I I I I I I | maritima | | | | I x J x | I I Z a n n i c h e l l i a I I I I I I I I I £alustris | | | | | x | x | I |P£iYacnum I I I I I I I I I §!li}ibium I I I I | x | | x | 37 only cover f o r organisms i n Barnes and Round-up Lake and Lake Lye. Plankton Although the plankton determinations were c h i e f l y made from 1970 samples, v e r t i c a l haul samples from 1972 i n d i c a t e d a s i m i l a r d i s t r i b u t i o n of s p e c i e s i n the l a k e s s t u d i e d . Thus, plankton data f o r 1970 c o u l d be used to supplement 1972 i n f o r m a t i o n . Diaptomid copepods were present only i n the three most a l k a l i n e l a k e s on Becher's P r a i r i e and i n LB 2; none were ever recorded from the three f r e s h e r water l a k e s (Table IV). C y c l o p o i d copepods and daphniids were found i n a l l l a k e s , but the s p e c i e s composition v a r i e d with a l k a l i n i t y . Some s p e c i e s were found i n a l l or almost a l l l a k e s , while others appear to have q u i t e a r e s t r i c t e d d i s t r i b u t i o n . In Lakes LE 2 and Earnes l a r g e zooplankton s p e c i e s such as Diaptomus nevadensis and £S£l!Iii3 SPP« were l e s s apparent than i n other l a k e s , the plankton being dominated by s m a l l diaptomid s p e c i e s and by c e r i o d a p h n i i d s . T h i s p a t t e r n i s seen i n the absence cf f i s h or other v e r t e b r a t e predators i n these l a k e s . Mean volumes of p l a n k t e r s over the summer of 1972 are shown i n F i g u r e 5, and seasonal means f o r midlake and i n s h o r e samples are l i s t e d i n Table V. Inshore values were always higher than those obtained f o r midlake samples. The magnitude of the mean 38 Table IV. Occurrence of z o o p l a n k t o n i c crustaceans i n the study l a k e s . Parentheses i n d i c a t e e i t h e r a s i n g l e o b s e r v a t i o n , or l i t e r a t u r e r e f e r e n c e s not a u t h e n t i c a t e d i n t h i s study. P l a n k t e r s : piatotomus s i c i l i s D. nevadensis Eucyclops a g i l i s Cyclops v e r n a l i s C. navus Cyclops sp. P.arac.Y.clops a l b i d u s Daphnia pulex D. s i m i l i s D. s c h o d l e r i D. ma^na Ceriodaphnia SSadrangulata ^ • r e t i c u l a t a Ceriodaphnia s p e c i e s Moina E u t c h i n s o n i Moina E ^ c t i r o s t r i s SilE2£ephalus v e t u l u s Soapholeberis j c i n ^ i i Chyjlorus S E ^ a e r i c u s Number of pl a n k t e r taxa LB 2 Barnes Bound| Lye | Greer| Bock | East | x 1 x 1 X ! | (x) 1 X | X I I 1 1 x 1 X 1 x | 1 x | ! 1 X 1 x | x 1 x 1 I 1 1 x | ' x 1 I1 I1 1 1 ! ! ! ! x 1 ! ! ! ! 1 x 1 x I x 1 X 1 x | x 1 x 1 X | x 1 X i x | x 1 x 1 x 1 X 1 x | x 1 x 1 1 1 i i x 1 x 1 i i X 1 x | 1 1 1 1 1 1 x 1 x 1 1 1 1 x I x 1 1 i ! 1 1 X | i ! ! 1 X | 1 x | x i x 1 ! ! 1 x | x 1 x 1 !_ !_ _{ {. {_ x 1 7 1 3 1 8 1 9 | 9 1 10 | 39 F i g u r e 5. Plankton volumes (ml l-»)in r e p l i c a t e d inshore 2 l i t r e samples taken at 4 day i n t e r v a l s i n the Becher's P r a i r i e l akes i n 1972. 41 Table V. Seasonal means of plankton volumes i n m l . l - 1 , from d u p l i c a t e 2 l i t r e samples c o l l e c t e d at 4-day i n t e r v a l s . LAKE INSHORE MIDLAKE : RATIO IN:MID ROCK LYE BARNES GREER 0.1218 (37) 0.0379 (36) 0.0290 (38) 0.0202 (38) ROUND-UP 0.0182 (35) EAST 0.0113 (37) 0.0036 (34) 0.0054 (33) 0.0063 (35) 0.0033 (36) 0.0042 (34) 0.0036 (35) 33.8 7.0 4.6 6. 1 4.3 3. 1 1.0 1.0 1.0 1.0 1.0 1.0 42 s e a s o n a l d i f f e r e n c e ranged from 3.1 to 7.0 times more inshore than midlake, with an extreme value of 33.8 times more i n s h o r e than midlake i n Rock Lake. Inshore plankton i n the three more a l k a l i n e l a k e s r a r e l y f e l l below 0.1 ml per 2 l i t r e sample. However i n the more freshwater l a k e s , i n s h o r e plankton was i n general very scarce i n May and e a r l y June, and again i n August and the f i r s t h a l f of September, and was abundant i n the l i t t o r a l of these l a k e s only from mid-June to August. Moinids ( i n Barnes Lake) and daphniids peaked e a r l y i n the a l k a l i n e l a k e s i n 1970, then decreased as diaptomids rose ( F i g u r e 6) . The daphniid peak i n the more freshwater l a k e s occurred contemporaneously with t h a t f o r diaptomids i n the more s a l i n e l a k e s . C y c l o p o i d copepods dominated the plankton i n the freshwater l a k e s i n s p r i n g , before daphniid p o p u l a t i o n s had become w e l l e s t a b l i s h e d . O v e r a l l , plankton abundance was at a f a i r l y high l e v e l i n a l l l a k e s , with a s p r i n g dominance of diaptomids i n the more a l k a l i n e l a k e s and a high p o p u l a t i o n of daphniid p l a n k t e r s i n the l i t t o r a l of the freshwater l a k e s i n midsummer and again i n l a t e f a l l . 43 F i g u r e 6. Numbers of some p l a n k t e r s per 2 l i t r e s a t d i f f e r e n t times of year i n l a k e s on Becher's P r a i r i e , from 1970 samples. Graph points represent monthly samples, May to September. A: A l l Daphnia species. a: Greer, b: East, c: Lye Lakes. B: Diaptomids and Moina. D. s i c i l i s ; a: L. Lye, b: Round-up, c: Barnes L. Moina sp.; d: Barnes L. C: Cyclopoids. a: Rock, b: Greer, c: East, d: Lye Lakes. 45 Other Organisms Table VI l i s t s the occurrence of the major benthic f a u n a l components i n each l a k e , from sweep-net samplings and l i t e r a t u r e r e c o r d s (Scudder, 1969b; Topping, 1968; Cannings, 1973). While few macroscopic organisms were seen i n lake LE 2, there i s a wide range of forms present i n the other l a k e s ; a few taxa such as Chaoborus and amphipods are c o n f i n e d to the freshwater l a k e s , and f a i r y shrimps (Branchinecta) to the most a l k a l i n e waters. Data from 2 l i t r e plankton and sweep-net samples p l o t t e d at four day i n t e r v a l s (Figure 7) show that chircncmid and c o l e o p t e r a n l a r v a e and zygopteran and ephemeropteran naiads are present i n a l l l a k e s over most of the summer. Zygcpteran nymphs occur r e d i n a l l l a k e s , but most abundantly among submerged p l a n t s . Young nymphs were conspicuous i n the l i t t e r a l fauna over most of the summer. Ephemeropteran nymphs were a l s o common a l l year, with s m a l l forms seen c h i e f l y a f t e r mid-summer. Chironomid l a r v a e were present i n a l l l a k e s i n the mud and among weeds, emerging at d i f f e r e n t times throughout the summer season (Cannings, 1973). Sweep-netting and plankton sampling g r e a t l y underestimated t h e i r biomass, as only i n d i v i d u a l s i n the water column were captured, so data from Cannings (1973 and unpublished) have been presented as Table VII. In the freshwater l a k e s chaoborid l a r v a e and amphipods were a l s o present throughout the year. Branchinecta were abundant i n Lake LB 2 throughout the summer, but were seen only v e r n a l l y and Table VI. Occurrence of major c o n s t i t u e n t s cf the i n v e r t e b r a t e fauna i n the lakes s t u d i e d . Parentheses i n d i c a t e e i t h e r a s i n g l e o b s e r v a t i o n , or l i t e r a t u r e r e f e r e n c e s net a u t h e n t i c a t e d i n t h i s s t u dy. T LE 2 T" ORGANISMS : Chaobcrus 1. Aedes 1. Chironomid 1. Nctonecta sc. Enallagma sp. C a l l i b a e t i s s T r i c h o p t e r a 1 Coleoptera H ydracarina Ostracoda lJESH2]liliecta "spT Amphipoda S n a i l s Leeches Number of groups Barnes Round. Lye (x) X X X X X X X X X X X X X X I X X X X X X X (X) Greer X X X X X X X X X Reck x X X X X X X X X 4 _L 11 East x x x x X X X X X X 12 47 F i g u r e 7. S p a t i a l and temporal d i s t r i b u t i o n of major i n v e r t e b r a t e s i n s i x Becher's P r a i r i e lakes sampled by sweep-net at 4 day i n t e r v a l s i n 1972. 1= Barnes, 2= Lye, 3= Round-up, 4= Greer, 5= Rock, 6= Fast Lakes. &= chironomids, B= c h a o b o r i d s , C= zygopterans, D= ephemeropterans, E= amphipods, F= b e e t l e l a r v a e . Table VII. Chironomid l a r v a l abundance and biomass i n s i x Becher's P r a i r i e l a k e s ( a f t e r Cannings, 1973 and unpublished data) LAKE L a r v a l biomass Maximum numbers Minimum numbers (ml per m2) per m2 (May-Oct) per m2 (May-Oct) (means of 2 samples) BARNES 21 .0 11631 (Aug) 774 (June) ROUND-UP 1.5 3827 (July) 150 (June) LYE 10.0 2515 (Aug) 150 (June) GREER 40.0 14188 (Aug) 3311 (May) ROCK 20.0 72706 (Aug) 537 (May) EAST 35.0 14104 (July) 2128 (May) 50 i n s m a l l numbers on Becher's P r a i r i e , being absent from Lakes Lye and Round-up a f t e r the end of May, and from Barnes Lake, where they were r a t h e r more f r e q u e n t , by the end of the f i r s t week i n June, 1972; a p a t t e r n a l s o f o l l o w e d i n other years. R e l a t i v e abundance of the major i n v e r t e b r a t e forms was assessed from Scudder's s e m i - q u a n t i t a t i v e standard sweep-net samples f o r June, August and October, 1970. Volume of benthic f a u n a l elements, as a measure of biomass, shows that t h i s was g r e a t e s t i n East Lake and i n Lake Greer (Table V I I I ) . According to Cannings (1973) chironomid l a r v a l biomass ranged up to 40 cm 3 l a r v a e per m2 of mud s u r f a c e a t a depth of one m (Table V I I ) , and numbers i n excess of 40,000 l a r v a e per square metre were recorded f o r one s p e c i e s alone i n the more freshwater l a k e s . The 1970 plankton i n f o r m a t i o n p a r a l l e l e d data f o r 1972 and show an e a r l y dominance of p l a n k t e r s i n the l i t t e r a l fauna i n the more a l k a l i n e ^ l a k e s , i n c o n t r a s t to t h e i r r e l a t i v e p a u c i t y a t f i r s t i n the more freshwater l a k e s . N e v e r t h e l e s s , there was a c o n s i d e r a b l e biomass of organisms present i n a l l l a k e s at a l l times, although the s p e c i e s composition v a r i e d , being more d i v e r s e i n the freshwater l a k e s . The biomass of organisms was c o n s i s t e n t l y h i g h e s t i n the more freshwater l a k e s , with amphipods the main c o n s i t u e n t s . In the more a l k a l i n e lakes the b e n t h i c fauna was r a t h e r poorer than i n the freshwater l a k e s , but t h e r e was abundant p l a n k t o n i c l i f e , p a r t i c u l a r l y diaptomids, a l l year. 51 Faunsl group: ZYGOPTERANS EFKEMEROPTERANS CHIRONOMIDS CHAOBORIDS / COLEOPTERANS Lake: Jun Aug. Oct. Jun. Aug. Oct. Jun. Aug. Oct. Jun. Aug. Oct. Jun. Aug. Oct. BARNES % volune 1.7 - - - - 1.2 9-1 . -16.7 - . . . Number 17 - - - - 63 2 It8 - - - -ROUND-UP % volune - + - * 14.3 10.0 - 28.6 - - . . . Number - 1 - 2 3 S - 41 - - -L Y E X volume 1.0 - 1.0 0.9 1.0 - - 1.1 Number 10 - 26 1 62 10 - - 27 GREER X volume 0.9 3.9 0.4 * 7.0 0.9 + + 8.1 + + 0.9 • Number 9 113 30 16 761 17 19 8 103 6 2 i» 6 SOCK % volume 1.7 8.3 - - 8.3 + - 0.9 + 2.8 • .• - 1.7 Number 5 112 - 28 7 - 18 10 17 4 2 6 EAST % volume B.9 2.1 3.2 - 0.7 2.0 + • - 47.9 • 0.6 • O.S Number 48 248 98 72 544 2 46 - 944 2 68 1 2 -SUCHOPTERANS AMPHIPODS CORIXIDS PLANKTON OTHERS TOTAL VOLUME Jun. Aug. Oct. Jun. Aug. Oct. Jun. Aug. Oct. Jun. Aug. Oct. Jun. Aug. Oct. • - - - -- - 10.5 163 90.9 33.3 19 83.7 - 50.0 - A l l Excluding plankton 1.3 0.7 1.1 1.1 0.3 0.2 10.0 7 1.6 2 28.5 5 + 1 - -10.0 u 16.7 209 - 70.0 81.7 28.6 - A l l Excluding plankton 0.5 0.2 3.0 0.6 0.1 0.3 1.1 12 0.1 1 -- 3.4 60 56.6 281 91.2 11.5 - M l Excluding plankton 29.6 2.6 5.3 • 3.1 * -62. it •125 81.1 2800 42.0 1737 5.1 68 7.9 206 1-2 17 18.8 5.9 148.0 Notonectids Leeches Hydracarina A l l Lxclud ing plankton 11.7 9.5 25.4 23.8 25.0 13.0 -• - fl3.lt 156 37.5 •172 1.0 17 1.0 9 12.5 110 0.2 . 1 t7.2 30.9 98.2 Hydracarina Al l Excluding plankton S.3 2.8 12.0 8.3 19.9 0.1 - 31.2 182 23.7 23 7S 17.6 1B06 10 2.1 no O.G 3t 1.1 65.3 75.9 Notonectids Leeches Snai Is Hydracarina Al l r.xflmlin^ plankton 11.6 l'l.O 97.0 U.7 99.1 21.0 Table V I I I . V o lumetric abundance data f o r major f a u n a l components of sweep-net samples i n s i x l a k e s . 52 DISCUSSION AND SUMMARY OF FINDINGS The Becher's P r a i r i e l a k e s l i e at a l a t i t u d e cf 52° 0' N whereas Lake LB 2 l i e s a t l a t i t u d e 50 45' » and at a s l i g h t l y lower a l t i t u d e . Lake LB 2 thus has, as a r e s u l t , a s h o r t e r photoperiod but s l i g h t l y warmer water i n the A p r i l to November i c e - f r e e period (Jansson and Scudder, 1974), both cf which might a f f e c t generation success cf a q u a t i c i n v e r t e b r a t e s . The temperature and c o n d u c t i v i t y data i n d i c a t e that the major ph y s i c o - c h e m i c a l .conditions i n the Eecher's P r a i r i e l a k e s i n 1972 f e l l g e n e r a l l y w i t h i n the ranges encountered i n the ten years of previous study. I t was t h e r e f o r e concluded that 1972 was not an a t y p i c a l year and data from 1972 could be u s e f u l l y compared with data obtained by pre v i o u s workers. Among the l a k e s s t u d i e d , the c h i e f d i f f e r e n c e s were seen i n c o n d u c t i v i t y . T h i s determines, d i r e c t l y or i n d i r e c t l y , many of the b i o l o g i c a l parameters of l a k e s , i n f l u e n c i n g f a u n a l composition and the presence or absence of a q u a t i c macrophytes. The r e s u l t s of i n v e s t i g a t i o n s on macrophytes, p r o d u c t i v i t y , zooplankton and macroscopic organisms show c l e a r l y that the la k e s s t u d i e d are q u i t e d i v e r s e and undoubtedly the d i f f e r i n g c o n d i t i o n s impose c e r t a i n l i m i t a t i o n s on the C o r i x i d a e able to e x i s t i n these water bodies. Table IX 'summarises the c o n t r a s t i n g c o n d i t i o n s i n the Becher's P r a i r i e lakes s t u d i e d . In g e n e r a l , the d i s t r i b u t i o n of macrophytes would seem to provide e x t e n s i v e i n s h o r e areas f o r macrofaunal congregation i n 53 Table IX. Summary of the d i s t r i b u t i o n p a t t e r n s of c o n t r a s t i n g c o n d i t i o n s i n the study l a k e s . Macrophytes Primary Pro d u c t i o n Chaobcrus sp. Amphipods Branchinecta Diap_tomus sp. Plankton abundance sp, x x BARKES POUND. LYE | GR SIR | X | ROCK | x 1 EAST | x 1 lew | low | low | high | x 1 x | high | x 1 x 1 high | x 1 x | X | X 1 x 1 ! 1 ! x 1 X 1 x | ] j ] a l l | year | a l l | year | a l l | year | sunnier f a l l | summer f a l l | summer f a l l | 54 the more freshwater l a k e s . F u r t h e r , the s t r u c t u r a l h e t e r o g e n e i t y i n t r o d u c e d by p l a n t stands should r e s u l t i n many d i v e r s e and d i s c r e t e h a b i t a t s there. The absence of macrophytes from the more a l k a l i n e lakes on Becher's P r a i r i e might impose severe l i m i t a t i o n s on the macrofauna i n them; c e r t a i n l y without p l a n t s the h a b i t a t i s l e s s d i v e r s e . The M i l l e r oxygen determination method has r e c e n t l y teen shown to be h i g h l y dependent upon pH, and t h e r e f o r e tc need frequent s t a n d a r d i s a t i o n of the sodium potassium t a r t r a t e reagent ( E l l i s and Kanamora, 1973). Thus, r e s u l t s presented here may be l e s s accurate than was hoped, but give a g e n e r a l p i c t u r e of r e l a t i v e l e v e l s of p r o d u c t i v i t y at d i f f e r e n t times; t h i s i s a l l t h a t the p r o d u c t i v i t y data aimed to do. With these l i m i t a t i o n s i n mind, the r e s u l t s i n d i c a t e a moderate l e v e l of p r o d u c t i o n i n the more a l k a l i n e l a k e s and a higher one i n the freshwater l a k e s . There was a strong decrease i n p r o duction l e v e l s i n September. The l a t t e r may p a r t l y te a f u n c t i o n of o v e r c a s t s k i e s (Bodhe, 1958), lowered temperatures and reduced a l g a l blooms, but may a l s o be owing to the r e l a t i v e l y low angle of the sun; i n Alaska Dugdale and Wallace (1960) a t t r i b u t e d zero or negative r e s u l t s a f t e r mid-July to t h i s f a c t o r . In the three most a l k a l i n e lakes algae were seen i n v e r t i c a l hauls towards the end of summer, but not i n l a r g e q u a n t i t i e s , and p e r i p h y t i c or e p i p e l i c a l g a l development was s i m i l a r l y r e s t r a i n e d . T a i l i n g et a l . (1973) obtained a p h o t o s y n t h e t i c l e v e l c l o s e to the t h e o r e t i c a l upper l i m i t by blue-green algae i n soda 55 l a k e s , and T a i l i n g (1970) shows t h a t , as a broad g e n e r a l i s a t i o n , h e a v i e r phytoplankton s t a n d i n g c r o p s may be maintained i n more s t r o n g l y b u f f e r e d a l k a l i n e waters, and i t i s l i k e l y t h a t a l l Becher's P r a i r i e lakes f a l l i n t o the h i g h - a l k a l i n e category i n t h i s r e s p e c t . However, s t a n d i n g crop data should net be regarded as synonymous with production data. Parsons et a l . (1972) i n d i c a t e t h at standing crop may be a misleading guide to a l g a l p r o d u c t i o n , f o r f e r t i l i s a t i o n of Great C e n t r a l Lake cn Vancouver I s l a n d , B.C. led to l i t t l e change i n a l g a l standing crop, but doubled e u t r o p h i c zone p r o d u c t i v i t y : the p o t e n t i a l a l g a l i n c r e a s e s were l a r g e l y removed by i n c r e a s e d grazing of z o o p l a n k t e r s (LeBrasseur and Kennedy, 1972). Thus, d e s p i t e the r e s t r a i n e d a l g a l development and lack of macrophytes i n the three most a l k a l i n e l a k e s , there i s probably heavy and sustained phytoplankton p r o d u c t i o n s i n c e there i s a c o n s t a n t l y l a r g e herbivorous plankton p o p u l a t i o n present i n these water bodies. No work was done cn the r e l a t i v e c o n t r i b u t i o n of p l a n t s other than p l a n k t o n i c algae t o p r o d u c t i o n . However, much c f the p r o d u c t i o n i n the more freshwater lakes may come from submerged macrophytes and t h e i r a s s o c i a t e d f i l a m e n t o u s green algae, and i n the more a l k a l i n e l a k e s , a p r o p o r t i o n of p r o d u c t i o n may be d e r i v e d from e p i p e l i c blue-green algae. Most taxa of l i t t o r a l i n v e r t e b r a t e s of p o s s i b l e importance as c o r i x i d food were represented through much of the s a l i n i t y range s t u d i e d . However, Chaoborus and amphipods were apparently excluded from h i g h e s t s a l i n i t i e s , while Branchinecta was c o n f i n e d to these. The number of conspicuous grcups decreased 56 with i n c r e a s i n g a l k a l i n i t y , a f e a t u r e seen a l s o f e r p l a n k t o n i c c r u s t a c e a n s and a q u a t i c macrcphytes. In p a r t i c u l a r , Chironomidae, Zygoptera and Ephemeroptera were r a t h e r more abundant i n the low s a l i n i t y l a k e s , a f e a t u r e perhaps r e l a t e d to the g r e a t e r degree of p r o t e c t i o n and camouflage a f f o r d e d by the dense i n s h o r e v e g e t a t i o n . The zooplankton was s h a r p l y d i f f e r e n t i n composition between l a k e s . In the higher s a l i n i t y lakes the planktcn was c h a r a c t e r i s e d by one to two s p e c i e s of Diaptomus; these were fr e q u e n t or abundant throughout the growing season, and peaked i n June to J u l y (Figure 6). In the more freshwater lakes 5i^£i21ii§ w a s absent. In such l a k e s zooplankton p o p u l a t i o n s were low i n s p r i n g , c o n s i s t i n g c h i e f l y of s m a l l c y c l c p c i d copepods; daphniids were almost absent u n t i l mid-June, but formed swarms i n midsummer and again i n l a t e f a l l , f o r very r e s t r i c t e d p e r i o d s of time. In the freshwater l a k e s i n p a r t i c u l a r , plankton was most abundant i n the inshore areas. Apart from p l a n k t e r s , the biomass of the freshwater lake i n v e r t e b r a t e s was c o n s i s t e n t l y higher than that of the more a l k a l i n e water bodies. The bulk of t h i s bicmass was composed of amphipods, but d i v e r s i t y of i n v e r t e b r a t e taxa was a l s o highest i n these lakes. F o l l o w i n g s t u d i e s of Sawson and Moore (1914) on s a l i n e l a k e s i n Saskatchewan, Jansson (1971) assumed t h a t c e n d u c t i v i t y was c o r r e l a t e d p o s i t i v e l y with p r o d u c t i v i t y r a t e s i n the s a l i n e l a k e s of c e n t r a l B r i t i s h Columbia. The present s t u d i e s show that p r o d u c t i v i t y by phytoplankton i s apparently g r e a t e r i n the l e s s 57 a l k a l i n e l a k e s . Secondary p r o d u c t i o n appears tc f e l l o w a s i m i l a r p a t t e r n . Nevertheless there seems to be ample abundance of food i n a l l l a k e s , with a p o t e n t i a l l y wider c h o i c e of f o o d s t u f f s i n the more freshwater l a k e s . I f the c o r i x i d s p e c i e s are g e n e r a l - f e e d e r s , then food l i m i t a t i o n s would appear not to be a f a c t o r of major importance i n t h e i r d i s t r i b u t i o n and c o e x i s t e n c e . However, should the d i f f e r e n t s p e c i e s of C o r i x i d a e prove t c have l i m i t e d food p r e f e r e n c e s e i t h e r a l l year, or s e a s o n a l l y , then the f i e l d data suggest that s e v e r e l y l i m i t i n g s i t u a t i o n s might e x i s t i n c e r t a i n l a k e s . 58 CHAPTER I I I . THE CORIXIDAE INTRODUCTION In 1972 s t u d i e s on the d i s t r i b u t i o n , abundance and phenology of c o r i x i d s p e c i e s i n the lak e s of Eecher's P r a i r i e were c a r r i e d out to confirm b a s e l i n e data obtained by Scudder (1969a) and Jansson and Scudder (1974). These data were not only necessary f o r a measure of any competitive - i n t e r a c t i o n , but were important i n a d e t a i l e d c o n s i d e r a t i o n of the fe e d i n g p a t t e r n s of the s p e c i e s . Further s t u d i e s were made i n the three most a l k a l i n e l a k e s i n 1972 and 1973 to i n v e s t i g a t e p o s s i b l e d i f f e r e n c e s i n abundance of the two s p e c i e s of C e n c c c r i x a i n d i f f e r e n t h a b i t a t s , f o r such h a b i t a t p r e f e r e n c e s could l e s s e n d i r e c t competition. MATERIALS AND METHODS Every four days, c o r i x i d samples were c o l l e c t e d i n each of the s i x Becher's P r a i r i e l a k e s , using a 'standard sweep' c o v e r i n g approximately 1 m of bottom, forewards and backwards (Jansson, 1971) and t a k i n g about one second per movement. A 20 cm diameter round-mouthed net (Freshwater E i o l o g i c a l A s s o c i a t i o n , Windermere, England) was used, with mesh s i z e s u f f i c i e n t l y s m a l l to r e t a i n a l l c o r i x i d i n s t a r s . Depending on c o r i x i d d e n s i t y t h r e e , f i v e or ten sweeps were made at a p r e - s e l e c t e d s t a t i o n i n 10 to 40 cm of water. 59 C o l l e c t i o n areas i n each lake were as s i m i l a r as p o s s i b l e , with g e n t l y s l o p i n g s i l t or mud s h o r e s , u s u a l l y with o c c a s i o n a l rocks or l o g s . Aquatic p l a n t s were unavoidably present i n the three lakes of lowest a l k a l i n i t y (Greer, East and Rock Lakes). On f i v e o c c a s i o n s more i n t e n s i v e sampling was used to i n v e s t i g a t e d i f f e r e n t h a b i t a t s w i t h i n a l a k e . S e v e r a l r e p l i c a t e sweeps were c o l l e c t e d both inshore and o f f s h o r e a t g r a v e l l y , muddy and reedy a r e a s , to o b t a i n around 100 a d u l t s and l a r g e r j u v e n i l e s at each. Sweep-net e f f i c i e n c y was lowered i n deeper waters, as drag reduced sweeping speed, and as the depth provided a c l e a r escape area above the net. C o r i x i d s were c o l l e c t e d and preserved i n 15% e t h a n o l , then counted and i d e n t i f i e d i n the l a b o r a t o r y , using the keys of Hungerford (1948), Cobben and P i l l o t (1960), Scudder (1966) and Jansson (1972a) . RESULTS £2£ixid D i s t r i b u t i o n and Abundance The occurrence of a l l c o r i x i d s p e c i e s i n the lakes s t u d i e d i s summarised i n t a b l e X. The f i n d i n g s c o r r o b o r a t e the data i n Scudder (1969a) and show seme ex t e n s i o n of the c a s u a l ranges of f o u r s p e c i e s i n the higher a l k a l i n i t y l a k e s . I t i s seen that £§£2£2£ixa b i f i d a h u n g e r f o r d i Lansbury occurred i n a l l lakes 60 T a b l e X. Summary of the d i s t r i b u t i o n p a t t e r n s c f c c r i x i d s i n t h e s t u d i e d l a k e s . b r e p r e s e n t s b r e e d i n g p o p u l a t i o n ; parenthes€s i n d i c a t e r e c o r d s from Scudder (1969) but net a u t h e n t i c a t e d i n the p r e s e n t s t u d y . 1 LE 2 | BARNES fiCUKD. I YE | G BEE B | FCCK IAST | C e n o c o r i x a | b i f i d a | b (x) | b x | b x | t X 1 1 x | b x b x | C e n o c o r i x a | ex£leta | b x | b x | t x | b x i i Cymatia | americana | x 1 x 1 | b x | b x b x | il§s£erocorixa | l a e v i g a t a | x | x 1 X | b x | 1 X t x | C a l l i c o r i x a | au d e n i | j x 1 • X 1 x | b x b x | S i g a r a s p . | i 1 X 1 fe x | t X fc x | r a w s o n i | (x) | (x) I <x) ! ! 61 i n v e s t i g a t e d , and, i n the high s a l i n i t y l a k e s Barnes, Lye and Round-up c o e x i s t e d with Cenocorixa e x p l e t a (Uhler) ; the l a t t e r was never captured i n the three freshwater l a k e s . Other s p e c i e s of C o r i x i d a e occurred s p o r a d i c a l l y i n the l a k e s , but only i n the three freshwater lakes can these forms be considered as permanent r e s i d e n t s with breeding p o p u l a t i o n s . The temporal sequence of the i n s t a r s and generations i n C. b i f i d a and C. e x p l e t a i n the Becher's P r a i r i e lakes s t u d i e d i s shown i n f i g u r e 8. The r e s u l t s show a c l o s e correspondence with those of Jansson and Scudder (1974). Breeding ended e a r l i e r i n the more freshwater l a k e s . C. e x p l e t a shewed a t h i r d g e n e r a t i o n i n Barnes Lake, the most a l k a l i n e water body, but i n d i c a t i o n s of t h i s were s l i g h t i n Lake Lye, while i n Rcund-up Lake only two c l e a r generations were produced. C. b i f i d a appeared to produce two summer gen e r a t i o n s i n a l l three a l k a l i n e l a k e s , but i n the more freshwater l a k e s there was evidence of only one generation per year. When temporal d i s t r i b u t i o n s of i n s t a r s are compared ( f i g u r e 8) the l a s t ( t h i r d ) summer ge n e r a t i o n i n Earnes Lake f o r C. e x p l e t a was seen l a t e r than t h a t f o r C. b i f i d a ( i t s second), although second generations of both were approximately contemporaneous, as they were i n Lake Lye. In Round-up Lake j u v e n i l e s of C. e x p l e t a p e r s i s t e d longer i n the f a l l than those of C. b i f i d a . Jansson and Scudder (1974) found a t i m e - l a g of about one week i n the g e n e r a t i o n s and demonstrated that the s t a r t of breeding was a l s o s l i g h t l y staggered, but they c o n s i d e r e d that the breeding p a t t e r n s cf the s p e c i e s were 62 F i g u r e 8. Temporal d i s t r i b u t i o n of stages of C. b i f i d a and C. e x p l e t a i n s i x study lakes on Becher's P r a i r i e , sampled at 4 day i n t e r v a l s i n 1972. (i-v= f i r s t to f i f t h i n s t a r s ; a=adults). 1= Barnes, 2= Lye, 3= Round-up, 4= Greer, 5= Rock, 6= East Lakes. 64 e s s e n t i a l l y contemporaneous. In the present study ycung i n s t a r s of c o r i x i d s were not detected i n sweep-net samples before the end o f May i n the three h i g h - a l k a l i n e l a k e s , and there i s no marked i n d i c a t i o n of genera t i o n l a g i n sympatry, lend i n g substance to Jansson and Scudder's (1974) c o n c l u s i o n . Breeding p o p u l a t i o n s of Cy_matia americana Hussey, Mesperocorixa l a e v i g a t a (Uhler), Sicjara sp. and C a l l i c c r i x a audeni Hungerford a l s o occurred i n the freshwater lakes and t h e i r temporal d i s t r i b u t i o n of i n s t a r s i s c o n t r a s t e d with data f o r C. b i f i d a (Figure 9). In each lake and s p e c i e s , there i s one c l e a r , temporally r e s t r i c t e d g e n e r a t i o n i n each year. S» i l ^ v i g a t a was found i n a l l s i x Becher's P r a i r i e l a k e s , but bred only i n the three most freshwater, A s i m i l a r p a t t e r n was found i n C. americana , but t h i s s p e c i e s was never observed i n Round-up Lake, and f o r Sicjara s p e c i e s - probably S. d e c o r a t e l l a (Hungerford) - which was not seen i n e i t h e r Barnes or Round-up Lakes. C. audeni was l o c a l l y abundant i n the a d u l t , and bred i n East and Rock Lakes but not appa r e n t l y elsewhere; i t has not been recorded from Barnes Lake. C. americana , H. l a e v i g a t a and C. audeni s p e c i e s seemed approximately contemporaneous with C. b i f i d a i n the appearance of t h e i r summer g e n e r a t i o n . By c o n t r a s t , S i g a r a sp., which i s very s i m i l a r i n s i z e and appearance t c Cenocorixa, unexpectedly showed an e a r l y summer genera t i o n i n East Lake, the t h i r d i n s t a r s being present by l a t e May ( f i g u r e 9) although the main groups of l a r v a l i n s t a r s were contemporaneous with the r e s t . Monthly means of c o r i x i d counts f o r a l l s p e c i e s and stages 65 F i g u r e 9. Temporal d i s t r i b u t i o n of stages of c o r i x i d s p e c i e s sympatric with C. b i f i d a i n the three freshwater l a k e s on Becher's P r a i r i e , sampled at H day i n t e r v a l s i n 1972. b= C. b i f i d a , c= C. audeni , cy= C. americana , h= H. l a e v i g a t a and s= Sigara sp. 66 GREER  b ~ 1 _ H s -C M a y J u n e J u l y A u g . S e p t . O c t . A c i .ii Ml Iv 67 ( f i g u r e 10) i n d i c a t e t h a t the mean number of c o r i x i d s per ten sweeps i s h i g h e s t i n Barnes Lake , the most a l k a l i n e , and lowest i n East and Round-up Lakes. The h i g h e s t numbers recorded were 5648 per ten sweeps i n Rock Lake, c h i e f l y r e c e n t l y e c l c s e d f i r s t i n s t a r s . Greatest numbers of c o r i x i d s were present from June to August i n each l a k e , r e f l e c t i n g the presence of many j u v e n i l e s ; September and October numbers were g e n e r a l l y low (means under 200 per ten sweeps) i n a l l l a k e s . The p r o p o r t i o n of a d u l t s of each sex at d i f f e r e n t seasons was examined f o r each lake and s p e c i e s , s i n c e the sexes c o u l d not be assumed to behave and feed i d e n t i c a l l y . In C o r i x i d a e males emerge t y p i c a l l y somewhat before females (Pajunen and Jansson, 1969a) which tended to skew the r a t i o s towards males f o r the e a r l i e r p a r t of each g e n e r a t i o n . Rearing and h o l d i n g evidence a l s o i n d i c a t e d a r a t h e r higher male m o r t a l i t y but no q u a n t i t a t i v e r e s u l t s were a v a i l a b l e . Table XI summarises d e t a i l s of sex r a t i o s f o r C. b i f i d a . O v e r a l l , males were fewest i n s p r i n g but s l i g h t l y more abundant than females i n midsummer and probably more sc i n f a l l . T h i s suggests a r e l a t i v e l y high male m o r t a l i t y i n winter, a f e a t u r e r e p o r t e d to be f a i r l y frequent i n C o r i x i d a e (Jaczewski, 1936; Larsen, 1938). A s i m i l a r p a t t e r n was seen f o r C. e x p l e t a . The o v e r w i n t e r i n g a d u l t s of C. americana , C. audeni and 2* l l ^ v i j j a t a were predominantly female, but summer generation a d u l t s were almost equal i n the f i r s t two s p e c i e s . In H« l a e v i g a t a the early-emerging summer a d u l t s were c h i e f l y males; as more females emerged the sex r a t i o s returned c l o s e r to 68 F i g u r e 10. Numbers of c o r i x i d s per ten standard sweeps i n each Becher's P r a i r i e l a k e s t u d i e d i n 1972, expressed as monthly means. 70 T a b l e X I . Summary o f s e x r a t i o s o f C . b i f i d a i n s a m p l e s , e x p r e s s e d a s p e r c e n t a g e o f m a l e s , and t o t a l c o r i x i d s e x a m i n e d (n ) . i T T T 1 — r r T 1 | BARNES i i ROUND. L Y E | G R E E R | ROCK | EAST A L L : SPRING 1 1 1 1 ] (18 M a y - 1 8 June ) | I I 5 2 . 8 | 4 3 . 4 | 4 5 . 5 4 6 . 5 | 4 2 . 1 I 5 3 . 2 4 6 . 4 n = 1 1112 | i i 632 j 1489 2143 | 1785 498 | 7659 SUMMER 1 1 1 1 ] (18 J u n e - 2 8 Sep) | | 1 5 7 . 6 | 5 9 . 0 | 5 3 . 7 5 6 . 2 | 5 8 . 9 4 1 . 0 | 52 . 4 n = J 1134 | i i 95 | 1374 610 | 846 | 1473 5521 FALL 1 1 1 1 | ( O c t o b e r ) | 4 0 . 7 | 6 9 . 6 | 52 .1 5 9 . 7 | 6 2 . 3 5 8 . 8 j 5 8 . 2 n = 1 59 | 23 | 71 414 | 138 | 97 802 ALL SEASONS | I 1 4 6 . 1 | 4 9 . 5 | 54 . 8 5 0 . 3 | 4 8 . 3 I 4 4 . 8 4 7 . 4 n = I 750 | I I 2933 | 2301 3157 | 2769 2068 ] 13982 71 equal i n f a l l . H a b i t a t Surveys P o s s i b l e h a b i t a t p r e f e r e n c e s were i n v e s t i g a t e d f o r the two s p e c i e s of Cenocorixa i n the Becher's P r a i r i e l a k e s , i n a check f o r p o t e n t i a l f a c t o r s such as s p a t i a l s e g r e g a t i o n , that could l e s s e n i n t e r s p e c i f i c c o m p e t i t i o n . Scudder (1966) r e p o r t e d t h a t i n the f i e l d C. e x ^ l e t a p r e f e r r e d to o v i p o s i t on rocks, but C. b i f i d a on decaying l e a f -sheaths of f l o o d e d g r a s s , while i n c a p t i v i t y C. b i f i d a o v i p o s i t e d on the p l a s t i c s c r e e n i n g placed i n j a r s f o r c l i n g i n g t o , and C. exrileta p r i m a r i l y on the s i d e s and f l e e r cf the j a r i t s e l f . Jansson (1971) suggests that there may be analogous d i f f e r e n c e s i n h a b i t a t preference between the two s p e c i e s i n the f i e l d , C. b i f i d a p r e f e r r i n g i n s h o r e , reedy areas and C. ex£leta the deeper waters with rocks and sandy bottoms, but he d i d not adduce any evidence to support t h i s . My s t u d i e s on c o r i x i d o v i p o s i t i o n did not e n t i r e l y support Scudder's o b s e r v a t i o n s . Most C. b i f i d a i n c a p t i v i t y o v i p o s i t e d on both screen and c o n t a i n e r bottom, e s p e c i a l l y the l a t t e r , whereas C. exj)leta o v i p o s i t e d on the bottom alone i n most ca s e s , o c c a s i o n a l l y a l s o p l a c i n g a few eggs on the screen as w e l l . S e l e c t i o n of o v i p o s i t i o n s i t e was thus not c l e a r - c u t between the s p e c i e s i n c a p t i v i t y . 72 Ha b i t a t p r e f e r e n c e s i n the present study were i n v e s t i g a t e d from two s t a n d p o i n t s ; at midlake s t a t i o n s a d u l t and j u v e n i l e c o r i x i d s were c o l l e c t e d whenever seen swimming near the s u r f a c e , and i n s h o r e , surveys of d i f f e r e n t h a b i t a t s were made cn s e v e r a l d i f f e r e n t occasions i n the three a l k a l i n e l a k e s . Midlake s i g h t i n g s of c o r i x i d s were commonest i n s p r i n g and f a l l , when f l i g h t and d i s p e r s a l may be most pronounced. Although a d u l t s were seen i n most la k e s s t u d i e d , o b s e r v a t i o n s were c o n s i d e r a b l y fewer i n the three more freshwater l a k e s and here, midlake records were a l l f o r C. b i f i d a . Inshore and midlake c o l l e c t i o n s i n the three more a l k a l i n e l akes are c o n t r a s t e d i n t a b l e X I I . Midlake records cf j u v e n i l e i n s t a r s i n d i c a t e t h a t d i s p e r s i n g a d u l t s were not the only forms c o n t r i b u t i n g to midwater l a k e p o p u l a t i o n s , a point f u r t h e r c o r r o b o r a t e d by the absence of any records f o r C. e x p l e t a from the more freshwater l a k e s , even i n midwaters. In a l k a l i n e l a k e s , the p r o p o r t i o n of C. e x p l e t a was higher i n midlake samples than i n s h o r e , suggesting that t h i s s p e c i e s might p r e f e r o f f s h o r e waters or midwater s i t u a t i o n s to inshore benthic s h a l l c w s . T h i s was f u r t h e r i n v e s t i g a t e d i n in s h o r e surveys. D e t a i l e d i n v e s t i g a t i o n s of c o n t r a s t i n g h a b i t a t s were made around the shores of Barnes Lake i n June and September, 1972, and i n J u l y and August, 1973; i n Round-up Lake i n August, 1973 and i n Lake Lye i n mid-October, 1973, j u s t before freeze-up. In each lake s i t e s were s e l e c t e d with c o n t r a s t i n g f a c i e s , such as g r a v e l l y , muddy and reedy. 73 Table XII. Midlake and inshore records of c o r i x i d occurrence i n three l a k e s on Becher's P r a i r i e (Data f o r matched dates o n l y ) . BARNES LYE ROUND-UP t o t a l s MIDLAKE C. e x j i l e t a a d u l t s 2 2 16 24 j u v e n i l e s 2 2 C. b i f i d a a d u l t s 3 15 16 38 j u v e n i l e s 4 percent C. e x g l e t a 36.4 11.8 52.9 38.7 INSHORE C. e x E l e t a a d u l t s 12 3 j u v e n i l e s 21 0 C. b i f i d a n - i n a d u l t s 54 479 246 1710 j u v e n i l e s ^77 2 percent C. e x g l e t a 3.2 0.0 4.4 2.5 74 In June, 1972, h a b i t a t s s t u d i e d were a l l i n s h o r e , and i n c l u d e d flooded Juncus and grass, muddy areas and shcres near f l o o d e d t r e e s . J u l y (1973) samples were c o l l e c t e d i n s i m i l a r r e g i o n s , but i n s h o r e and o f f s h o r e samples were c o l l e c t e d at each s i t e . August (1973) samples again came from i n s h o r e and o f f s h o r e areas but only three of the o r i g i n a l four s t a t i o n s were sampled. In September (1972) the four major s t a t i o n s were again sampled i n s h o r e and o f f s h o r e . The r e s u l t s f o r Barnes Lake are summarised i n f i g u r e 11, where t e n t a t i v e i s o p e r c e n t i l e l i n e s were f i t t e d to sampling data, t a k i n g i n t o c o n s i d e r a t i o n depth and h a b i t a t d i s t r i b u t i o n p a t t e r n s . Although r e l a t i v e abundance of C. e x p l e t a was many times g r e a t e r i n 1973 than i n 1972, d i s t r i b u t i o n p a t t e r n s were not d i s s i m i l a r . There were c o n s i s t e n t l y higher p r o p o r t i o n s of C. e x p l e t a o f f s h o r e than i n s h o r e , but as t o t a l numbers were a l s o lower o f f s h o r e than i n s h o r e (due perhaps p a r t l y tc lowered sampling e f f i c i e n c y ) t h i s s p e c i e s may i n f a c t be a b s o l u t e l y more numerous i n s h o r e than o f f s h o r e , d e s p i t e being r e l a t i v e l y l e s s abundant . In Round-up Lake i n August 1973 samples taken inshore and o f f s h o r e at one pl a c e again i n d i c a t e d t h a t c. ex p l e t a made up a higher p r o p o r t i o n of o f f s h o r e p o p u l a t i o n s than i n s h o r e (61% versus 91$) but i n Lake Lye j u s t before freeze-up nc C. e x p l e t a were among the 13 c o r i x i d s taken t h e r e o f f s h c r e . Since the o v e r a l l p r o p o r t i o n of C. e x p l e t a i n combined samples was only 1.6%, these r e s u l t s are i n c o n c l u s i v e . On the p l a i n bottom h a b i t a t i n Lake Lye C. e x p l e t a made up 9.5% of a l l c o r i x i d s 75 F i g u r e 11. Maps of i s o p e r c e n t i l e l i n e s drawn f o r the percentage of C. e x p l e t a i n samples taken at d i f f e r e n t h a b i t a t s i n Barnes Lake at f o u r dates. A= 24 June 1972. B= 12 September 1972. C= 17 J u l y 1973. D= 25 August 1973. 60 * * Scirpus TV dead trees 77 present, but was absent from shallow muddy areas or g r a v e l l y r e g i o n s with Juncus or l o g s . In the Barnes Lake sequence C. exjpleta was i n i t i a l l y r e l a t i v e l y more abundant over s h e l t e r e d s o f t mud, but a t the st e e p e r s l o p i n g s t a t i o n among f l o o d e d t r e e s the percentage dropped o f f r a p i d l y during the summer, both i n s h o r e and o f f s h o r e . Offshore highest numbers were obtained at the r e g u l a r sampling s i t e where the mud was r e l a t i v e l y f i r m and l o g s and rocks were absent. Percentages were most v a r i a b l e ever very s o f t mud or among flo o d e d t r e e s . In c o n t r a s t to the general p r e f e r e n c e of C. exj>leta f o r a p l a i n s u b s t r a t e and r a t h e r deeper water i n the Eecher's P r a i r i e l a k e s i s the s i t u a t i o n at Lake LB 2, where t h i s s p e c i e s was o f t e n found on the f r i n g e s of the shallow a q u a t i c p l a n t masses as w e l l as on the s i l t y bottom areas. Thus h a b i t a t preferences may be q u i t e complex and c o n c l u s i o n s drawn f o r one place may be untenable elsewhere. Due to the s l i g h t g e n e r a t i o n l a g between the s p e c i e s , h a b i t a t preferences of i n s t a r s were not considered i n d i v i d u a l l y , but f o r Barnes Lake were grouped i n t o i n s t a r s one to three, i n s t a r s f o u r and f i v e , and a d u l t s , f o r each season ( f i g u r e 12). Comparisons i n d i c a t e d that the f i r s t preference of h a b i t a t f o r C. b i f i d a a d u l t s was p l a i n mud only i n June, and was mud with d e b r i s of v a r i o u s kinds a t other times. In C. exp_leta a d u l t s were commonest over o f f s h o r e mud i n J u l y . Fourth and f i f t h i n s t a r s of both s p e c i e s were u s u a l l y found i n s h c r e , with those of C. exjjleta being concentrated i n s h o r e on mud i n J u l y . The 78 F i g u r e 12. D i s t r i b u t i o n o f C e n o c o r i x a s p e c i e s and i n s t a r s i n d i f f e r e n t h a b i t a t s o f B a r n e s L a k e , a t t h r e e t i m e s o f y e a r . B i = C . b i f i d a , ex= C . e x p l e t a . A= J u n e , b= J u l y , c = S e p t e m b e r . U p p e r b l a n k s e c t i o n o f e a c h d i a g r a m r e p r e s e n t s p e r c e n t a g e s o f f i r s t t o t h i r d i n s t a r s ; s h a d e d a r e a , f o u r t h a n d f i f t h i n s t a r s ; l o w e r b l a n k a r e a , a d u l t s . 1= mud and J u n c u s , 2- mud a n d t w i g s , 3=mud, 4=mud and l o g s , 5= S c i r p u s , 6= mud and r o c k s , 7= o f f s h o r e mud, 8= o f f s h o r e mud and r o c k s . 80 f i r s t t h r e e i n s t a r s were p r e d o m i n a n t l y f o u n d on i n s h o r e mud i n J u n e and S e p t e m b e r , b e i n g f o u n d o f f s h o r e g e n e r a l l y i n J u l y . T h u s t h e i n s h o r e - o f f s h o r e s e g r e g a t i o n o f d i f f e r e n t i n s t a r s , as shown by t h e i r p e r c e n t a g e o c c u r r e n c e i n e a c h h a b i t a t , was mcst marked i n m i d - s e a s o n , and a t t h i s t i m e t h e d i s t r i b u t i o n o f i n s t a r s o f e a c h s p e c i e s was a l s o most d i s s i m i l a r . By c o n t r a s t , d i s t r i b u t i o n o f i n s t a r s o f b o t h s p e c i e s f o l l o w e d a r a t h e r s i m i l a r p a t t e r n i n t h e S e p t e m b e r s a m p l e s . A l t h o u g h t h e r e e m e r g e s no s e a s o n a l l y c o n s i s t e n t p a t t e r n o f d o m i n a n c e o f e i t h e r s p e c i e s i n any one f a c i e s , t h e s m a l l e r b o d i e s o f d a t a f r o m R o u n d - u p and l y e l a k e s f o l l o w t h e p a t t e r n d e s c r i b e d f o r B a r n e s L a k e . L i t t l e d i f f e r e n c e was s e e n b e t w e e n t h e s p e c i e s i n f a c i e s o r h a b i t a t p r e f e r e n c e i n f a l l , e x c e p t f o r e f f e c t s r e l a t e d t o t h e s l i g h t g e n e r a t i o n l a g . F o r a l l d a t e s and l a k e s f o r e a c h h a b i t a t ( f i g u r e 13) t h e p e r c e n t a g e o f a d u l t C . b i f i d a among S c i r j > u s , i n s h o r e mud, r o c k s and l o g s i s h i g h e r t h a n t h a t f o r C . e x p l e t a , a l t h o u g h e a c h s p e c i e s had a s i m i l a r o r d e r o f p r e f e r e n c e f o r h a b i t a t s . I n the s m a l l e s t i n s t a r s C . b i f i d a was more p r e d o m i n a n t o v e r o f f s h o r e mud t h a n was C . exp_ le ta , bu t t h e l a r g e r j u v e n i l e s c f the l a t t e r p r e f e r r e d S c i r £ u s , i n s h o r e mud, r o c k s , l o g s and o f f s h o r e mud more t h a n t h o s e o f t h e o t h e r s p e c i e s . T h u s , s i n c e f i f t h i n s t a r C . e x £ l e t a a r e l a r g e r t h a n t h e same i n s t a r o f C . b i f i d a , a s a g e n e r a l i s a t i o n i n b o t h s p e c i e s , t h e l a r g e s t i n d i v i d u a l s p r e f e r r e d t h e most d i v e r s e h a b i t a t s . O b s e r v a t i o n s s u g g e s t e d t h a t i n C . b i f i d a m a l e s t e n d e d to be g r o u p e d a r o u n d o b s t a c l e s s u c h a s l o g s , r o c k s i n s h o r e and r e e d 81 F i g u r e 13. Percentages of C. b i f i d a and C. e x p l e t a o c c u r r i n g i n d i f f e r e n t h a b i t a t s i n the higher s a l i n i t y Becher's P r a i r i e l a k e s i n 1972 and 1973. Ad= a d u l t s , bi= C. b i f i d a , ex= C. e x p l e t a . 1 = §£i££!l§' 2= mud and l o g s i n s h o r e , 3= mud and twigs o f f s h o r e , 4= mud and Juncus, 5= i n s h o r e mud, 6= o f f s h o r e mud, 7= i n s h o r e r o c k s , 8= o f f s h o r e rocks and twigs. 83 beds, whereas females were found most of t e n i n more open areas, not c l u s t e r e d i n groups. Table XIII shows h a b i t a t d i s t r i b u t i o n s of c o r i x i d s i n mid-September, 1972, at Barnes Lake, arranged i n order of i n c r e a s i n g h e t e r o g e n e i t y of h a b i t a t . I t i n d i c a t e s t h a t an i n c r e a s i n g p r o p o r t i o n of males was found i n i n c r e a s i n g l y d i v e r s e h a b i t a t s . In summary, although h a b i t a t preference v a r i e s between sexes, i n s t a r s , lakes and seasons, a l l data summed agree with midlake o b s e r v a t i o n s i n i n d i c a t i n g that, i n sympatric s i t u a t i o n s , C. e x p l e t a shows some preference f o r deeper waters (over 40 cm) and f o r p l a i n mud or s i l t bottoms ( f i g u r e 14). DISCUSSION C o r i x i d s are known to have wide s a l i n i t y t o l e r a n c e s (Eawson and Moore, 1944), although t h e r e i s o f t e n marked v a r i a t i o n between s p e c i e s groups ( i . e . S a i l e r , 1948; Brooks and K e l t o n , 1967; Savage, 1971; Knowles and W i l l i a m s , 1973). Scudder et a l . (1972) reported d i f f e r e n c e s i n p h y s i o l o g i c a l t o l e r a n c e s of the two Cenocorixa s p e c i e s and l i n k e d them to t h e i r d i s t r i b u t i o n i n high s a l i n e waters, but suggested t h a t s i n c e both have t y p i c a l l y freshwater i o n i c and osmotic balances, C. e x p l e t a must be excluded from breeding i n l a k e s with a c o n d u c t i v i t y of l e s s than 5000 umhos cm - 1 through e c o l o g i c a l i n t e r a c t i o n s r a t h e r than p h y s i o l o g i c a l l i m i t a t i o n s . Jansson (1971) found that C. e x p l e t a 84 T a b l e X I I I . S e x r a t i o s o f a d u l t C e n o c o r i x a i n d i f f e r e n t h a b i t a t s o f B a r n e s L a k e i n S e p t e m b e r , 1 9 7 2 . HABITAT n p e r c e n t m a l e s ( t o t a l s ) P l a i n mud 64 5 0 . 0 Mud w i t h t w i g s 220 5 3 . 5 Mud and l o g 296 5 7 . 4 Mud and S c i r p _ u s 486 6 5 . 0 8 5 Figure 14. Overall percentages of C. b i f i d a and C. expleta i n samples from d i f f e r e n t habitats. Bi= C. b i f i d a , ex= C. expleta , G= gravel, Ju= Juncus, L= logs, M= mud, Sc= Scirpus, T= twigs. 86 Bl 0 87 d i d not breed i n f i e l d or l a b o r a t o r y a t lower c o n d u c t i v i t i e s . However, I have reared C. ex£leta eggs l a i d i n Rock Lake water of c o n d u c t i v i t y about 1100 ^ mhos cm-* to the second i n s t a r i n the l a b o r a t o r y . In the Canadian p r a i r i e s c o r i x i d s are abundant ever a wide s a l i n i t y range, but Brocks and Kelton (1967) found highest c o r i x i d d e n s i t i e s i n a lake of about 28,000 ^jmhes cm - 1 c o n d u c t i v i t y , higher than anything i n v e s t i g a t e d i n the present study. Although mean abundances were highest i n Barnes Lake, other parameters of abundance d i d not appear to fo l l o w any d e f i n i t e p a t t e r n with a l k a l i n i t y , and i n p a r t i c u l a r there were marked d i f f e r e n c e s i n c o r i x i d abundance between the r a t h e r s i m i l a r lake p a i r s . Round-up and Lye, and Greer and Reck Lakes. The few c o r i x i d s i n Round-up lake compared with t h e i r abundance i n Lake Lye was a l s o r e f l e c t e d i n numbers of p l a n k t e r s and be n t h i c organisms (Chapter I I ) , and t a b l e VIII i n d i c a t e s that i n the a l k a l i n e lakes c o r i x i d s formed a grea t e r p r o p o r t i o n of the ben t h i c f a u n a l biomass than i n the more freshwater l a k e s . DeBach (1966) suggests t h a t reduced numbers cf twe s p e c i e s i n sympatry compared t o t h e i r abundance i n a l l o p a t r y can be an i n d i c a t i o n of co m p e t i t i o n . However, numbers cf C. b i f i d a and £• §2£l§fS were g r e a t l y v a r i a b l e i n a l l o p a t r y and sympatry. Thus, one can deduce no evidence here f o r com p e t i t i v e i n t e r a c t i o n s . Of the c o r i x i d s l i s t e d f o r the Becher's P r a i r i e l a k e s (Scudder, 1969a) only Dasycorixa rawsoni was not rec o g n i s e d from the 1972-1973 samples; other s p e c i e s were confirmed i n t h e i r 88 occurrence and some c a s u a l d i s t r i b u t i o n a l ranges extended. Scudder (1969a) emphasised that the d i s t r i b u t i o n p a t t e r n i s not s t a t i c , but changes with seasonal and longer-term d i f f e r e n c e s i n weather and c l i m a t e . Almost a l l s p e c i e s were recorded as c a s u a l v i s i t o r s to each l a k e , except f o r C. exp_leta which was not seen i n the three most freshwater l a k e s . In Lake LB 2 C. b i f i d a appeared i n the f a l l and has i n i t i a t e d breeding there the f o l l o w i n g s p r i n g i n some years (Scudder, 1969a). C o r i x i d s are thus o p p o r t u n i s t i c i n t h e i r d i s t r i b u t i o n , the s p r i n g and f a l l d i s p e r s a l p a t t e r n h e l p i n g to ensure t h e i r widest p o s s i b l e s u r v i v a l (Johnson, 1966, Pajunen and Jansson, 1969 b; Pajunen, 1970a, b). T h e r e a f t e r t h e i r breeding success must be r e g u l a t e d by other f a c t o r s . Since they do not feed at low temperatures, t h e i r winter d i s t r i b u t i o n r e p r e s e n t s n e i t h e r s p e c i f i c conductance s e n s i t i v i t y nor food p r e f e r e n c e to any degree. ., The present s t u d i e s confirm p r e v i o u s l y e s t a b l i s h e d breeding p a t t e r n s f o r Cenocorixa s p e c i e s (Scudder, 1969; Jansson and Scudder, 1974), i n d i c a t i n g t h a t i n t h i s r e s p e c t as well as i n p h y s i c o - c h e m i c a l data 1972 was a t y p i c a l year. C. ex£leta seems to breed approximately contemporaneously with C. .bifida , but may produce a t h i r d g eneration i n the most a l k a l i n e l a k e s . T h i s would give the former an advantage i n years f o l l o w i n g a mild or prolonged f a l l , e s p e c i a l l y i f there i s some f e e d i n g or h a b i t a t s e g r e g a t i o n between the i n s t a r s . C. exjjleta would then enter the f o l l o w i n g season with a g r e a t e r number of a d u l t s than a f t e r a sudden f a l l c o l d s p e l l or e a r l y winter, which would be l i k e l y to 89 k i l l o f f many j u v e n i l e s before they could moult t c the a d u l t o v e r w i n t e r i n g stage. S i n c e the second generation c f both s p e c i e s i s approximately contemporaneous, a d u l t s of both t h i s and the t h i r d g e n e r a t i o n may overwinter. However, s i n c e both s p e c i e s of SiiHS-SSlixa, were e s s e n t i a l l y contemporary i n sympatry, there i s no evidence f o r c o m p e t i t i v e i n t e r a c t i o n from phenology data. Martin (1970) suggests that l a r g e r s p e c i e s u s u a l l y develop slower, but s t a r t e a r l i e r , i n sympatry. In terms of competitive s t r a t e g y t h i s does not seem to a f f o r d any s e l e c t i v e advantage and i t was not seen f o r e i t h e r of the l a r g e r s p e c i e s H. l a e v i g a t a or C. americana i n the three more freshwater lakes i n the present study. However, the e a r l y appearance of the summer gen e r a t i o n i n Sicjara sp., a s p e c i e s very s i m i l a r i n s i z e and appearance to C. b i f i d a , may be a s u r v i v a l mechanism a l l o w i n g c o e x i s t e n c e with the l a t t e r , which i s n u m e r i c a l l y dominant, by p e r m i t t i n g the nymphs to develop e a r l i e r tc a s i z e where they do not compete with those of the dominant s p e c i e s . Development of c o r i x i d s seems most favoured i n the more s a l i n e l a k e s on Becher's P r a i r i e ; C. e x p l e t a having two or t h r e e , and C. b i f i d a two g e n e r a t i o n s per y e a r . i n sympatry, whereas i n the three more freshwater l a k e s C. b i f i d a and the other c o r i x i d s p e c i e s produced only one g e n e r a t i o n per year. C. e x p l e t a a l s o produced two to t h r e e generations i n the high s a l i n i t y Lake LB 2 (Scudder and Jansson, 1974). Numbers of g e n e r a t i o n s per year are r e l a t e d by Jansson (1971) p r i m a r i l y to food a v a i l a b i l i t y , r a t h e r than to photcperiod cr temperature i n i t i a t i n g o v a r i a n a r r e s t , and the previous chapter has 90 demonstrated t h a t although biomass of benthic organisms was g r e a t e s t i n the freshwater l a k e s , there appeared to be ample food i n a l l l a k e s , and a s t e a d i e s t supply cf plankton, p a r t i c u l a r l y of diaptomids, i n the more s a l i n e l a k e s s t u d i e d . C. b i f i d a only breeds s u c c e s s f u l l y i n Lake LB 2 i n s p r i n g , and i t s l a t e r e x c l u s i o n there seems c o r r e l a t e d with i n c r e a s i n g s a l i n i t y as the d i l u t e s u r f a c e waters g r a d u a l l y become mixed i n . However, e x c l u s i o n of C. e x p l e t a from the more freshwater l a k e s may not be a r e s u l t of osmotic t o l e r a n c e l i m i t s (Scudder et a l . , 1972) but r a t h e r because of e c o l o g i c a l requirements. Thus, Jansson (1971) suggested the p o s s i b i l i t y of p a r t i a l geographic or e c o l o g i c a l i s o l a t i o n as a p o t e n t i a l i s o l a t i n g mechanism f o r CenocoEi* 3. s i b l i n g s p e c i e s , noting that where the s p e c i e s c o e x i s t e d i n the C h i l c o t i n area of B.C., C. b i f i d a was c h i e f l y found at pond edges i n reed beds but C. e x p l e t a c h i e f l y i n deeper waters, r a t h e r l i k e the d i s t r i b u t i o n p a t t e r n s cf the c o e x i s t i n g s p e c i e s of Sigara d e s c r i b e d by Macan (1938). Martin (1970) a l s o noted t h a t c o e x i s t i n g c o r i x i d s p e c i e s tended to feed " i n d i f f e r e n t a r e a s " of a pond. Brooks and Kelton (1967) g i v e some e c o l o g i c a l i n f o r m a t i o n f o r c o r i x i d genera i n Canada, and suggest that C e n c c c r i x a spp., p a r t i c u l a r l y C. e x p l e t a , are u s u a l l y found i n l a r g e , sandy l a k e s , o f t e n s a l i n e , and l a c k i n g conspicuous v e g e t a t i o n . In c o n t r a s t , C. americana , H. l a e v i g a t a and some C. audeni and S i g a r a s p e c i e s p r e f e r areas with some v e g e t a t i o n . However, i n the freshwater Becher's P r a i r i e lakes C. b i f i d a was the most abundant d e s p i t e e x t e n s i v e weed development, and i n Lake LB 2 91 C. e x ^ l e t a was o f t e n found on shallow weed-masses, perhaps a convenient r e s t i n g p l a c e from which t c dart out on t h e i r prey. According to Brooks and K e l t o n (1967) the m a j o r i t y of c o r i x i d s are most abundant i n c o n d i t i o n s apparently not s u i t e d to heteropteran predators such as Notonecta s p e c i e s . Fernando (1959) suggested t h a t food and s u i t a b l e h a b i t a t s seem to be the density-dependent f a c t o r s u l t i m a t e l y l i m i t i n g c c r i x i d p o p u l a t i o n s , but added that p r e d a t o r s may be r e s p o n s i b l e f o r much of the m o r t a l i t y i n crowded or l e s s than optimal c o n d i t i o n s . P r e d a t o r s on c o r i x i d s i n the Becher's E r a i r i e l a k e s may i n c l u d e waterfowl, which B i r d (1961) noted fe e d i n g on c o r i x i d s i n s i m i l a r water-bodies, e s p e c i a l l y i n the f a l l when duck numbers are g r e a t e s t and a q u a t i c p l a n t s have beccme l e s s dense. Since predators of c o r i x i d s w i l l e l i m i n a t e f i r s t the most obvious c o l o u r mis-matches , according t c Popham (1941), these might help to keep the p a l e r C. exDleta out of the more freshwater l a k e s . P r e d a t i o n as a p o s s i b l e e x c l u s i o n f a c t o r , e s p e c i a l l y f o r the younger i n s t a r s , was not f u r t h e r i n v e s t i g a t e d here. However, swimming patterns may d i f f e r s l i g h t l y between the two Cenocorixa s p e c i e s , or f e e d i n g h a b i t s may make some i n s t a r s and s p e c i e s more conspicuous to i n t e n d i n g predators than e t h e r s . D i s t r i b u t i o n r e s u l t s w i l l o b v i o u s l y be i n f l u e n c e d by the development success of each g e n e r a t i o n , which i s l i n k e d to s i t e of o v i p o s i t i o n . Drowned t r e e s might i n i t i a l l y have o f f e r e d s u i t a b l e o v i p o s i t i o n s u b s t r a t e s f o r both s p e c i e s i n Earnes Lake ( p o s s i b l y v e r t i c a l l y segregated to correspond to t h e i r observed p r e f e r e n c e s ) , but the areas might subseguently have proved l e s s 92 f a v o u r a b l e f o r C. e x p l e t a and numbers of t h i s s p e c i e s then dwindled e i t h e r through predation (ambush pre d a t o r s being w e l l provided with concealment) or through some other f a c t o r such as the high o r g a n i c content of the s o f t mud. Diaptcmus s i c i l i s swarms were p a r t i c u l a r l y abundant among the trunks of f l o o d e d t r e e s , suggesting t h a t food abundance i t s e l f was net n e c e s s a r i l y a d i r e c t f a c t o r (see chapter IV) . The presence or absence of a q u a t i c macrcphytes and f i l a m e n t o u s algae i s r e l a t e d t o c o n d u c t i v i t y l e v e l i n the s i x Becher's P r a i r i e lakes s t u d i e d (Chapter I I ) . T h i s suggested that c o r i x i d d i s t r i b u t i o n might be a f f e c t e d by p l a n t development as much as by s a l i n i t y per se, and h a b i t a t checks helped to c o r r o b o r a t e t h i s . C. e x p l e t a was l e a s t frequent i n sympatric lakes amongst S c i r p u s , Juncus and branches, showing preference f o r a p l a i n s i l t y bottom with l i t t l e h e t e r o g e n e i t y (as summarised i n f i g u r e 14). C. e x p l e t a a l s o seems to be more freq u e n t i n midlake than C. b i f i d a , perhaps r e p r e s e n t i n g g r e a t e r m o b i l i t y i n open waters. S i m i l a r l y , C o r i x a germari i s a l s o recorded as p r e f e r r i n g o f f s h o r e waters ( C r i s p , 1962 b) ; t h i s s p e c i e s , l i k e C. e x p l e t a , markedly p r e f e r s to o v i p o s i t on stones, although i t w i l l a l s o d e p o s i t i t s eggs on v e g e t a t i o n . C. b i f i d a on the other hand was most abundant amcng rocks, l o g s and emergent rushes and reeds. However, the s e g r e g a t i o n between the s p e c i e s i s by no means a b s o l u t e , and s i n c e c o r i x i d s are most numerous i n s h o r e , perhaps the g r e a t e r mass of the p o p u l a t i o n i s sympatric here. Nonetheless, some s p a t i a l s e g r e g a t i o n of the s p e c i e s or i n s t a r s would help to l e s s e n 93 c o m p e t i t i o n f o r space and food; the l a t t e r f a c t c r w i l l te considered i n more d e t a i l i n chapters IV and V. 94 CHAPTER IV. FEEDING EXPERIMENTS INTRODUCTION There i s l i t t l e agreement about the d i e t of C o r i x i d a e . Some authors ( i . e . Martin, 1970; Jansson and Scudder, 1972) see them as predators or c a r n i v o r e s , others ( i . e . Parsons, 1966) c o n s i d e r them to be e s s e n t i a l l y herbivorous and microphagic. F c r a few s p e c i e s there i s some o b s e r v a t i o n a l evidence to support both c o n t e n t i o n s , although Istock (1973) s t a t e s that "...good i n f o r m a t i o n on food h a b i t s does not e x i s t f o r even a s i n g l e s p e c i e s , and would be d i f f i c u l t to o b t a i n . . . " . Gut s t u d i e s i n p a r t i c u l a r are c i t e d as u n s a t i s f a c t o r y by s e v e r a l authors, i . e . C r i s p (1962 b), s i n c e food i n g e s t e d i s l a r g e l y l i q u i d and amorphous. I t seems l i k e l y t h a t c o r i x i d s w i l l accept a wide v a r i e t y of foods, both animal and p l a n t , dead and a l i v e . Thus, while Puchkova (1969) f i n d s t h a t C o r i x i d a e have a mixed type of f e e d i n g , with a predominance of phytophagy, Zwart (1965) re p o r t e d the s u r v i v a l of C o r i x i d a e was longest i n c a p t i v i t y when fed on an animal d i e t . Some i n f o r m a t i o n on food h a b i t s of a s p e c i e s may be obt a i n e d by fe e d i n g i n d i v i d u a l s under c o n t r o l l e d c o n d i t i o n s and obse r v i n g t h e i r r e a c t i o n s to the foods o f f e r e d . Such f i n d i n g s are not d i r e c t l y a p p l i c a b l e to the s p e c i e s i n nature because of the a r t i f i c i a l e xperimental s i t u a t i o n and the f a c t t h at through 95 ignorance t h e i r p r e f e r r e d items of d i e t may net be o f f e r e d . Numerous s t u d i e s , e.g. Bsiao and Frenkel (1968) and Jamieson (1973) have shown t h a t i f a p r e f e r r e d food item i s not chosen ty the experimenters, one can expect the behaviour of the experimental animal to be a t y p i c a l . F u r t h e r , some authors, e.g. Latheef and Harcourt (1972) have i n d i c a t e d that a herbivorous animal may respond best i n t e s t s , i n terms of the amount eaten and a c c e p t a b i l i t y , to foods which may be poorly s u i t a b l e f o r growth and r e p r o d u c t i o n . Jamieson (1973) questions whether such a d i f f e r e n c e i n response i s important i n c a r n i v o r e s , suggesting t h a t q u a l i t a t i v e l y one animal prey may be s i m i l a r to another i n terms of u s e f u l n e s s . Whether t h i s i s i n f a c t true cr not, o b s e r v a t i o n s of the behaviour of animals i n f e e d i n g experiments g i v e an i d e a of t h e i r c a p a b i l i t i e s of handling p a r t i c u l a r items of prey, give data a g a i n s t which gut contents can te compared, and provide one view of the d i f f e r e n c e s between the s p e c i e s with r e s p e c t to t h e i r a b i l i t y to use a v a i l a b l e food items, alone or i n combination. They thus provide one i n d i c a t i o n c f the d i e t a r y parameters of the fundamental niche. PRELIMINARY EXPERIMENTS I n i t i a l experiments were run to t e s t f e e d i n g p o t e n t i a l of C. b i f i d a and C. e x p l e t a i n v a r i o u s experimental s i t u a t i o n s , and to determine t h e i r s u r v i v a l time without food. Observations by Scudder (unpublished) i n d i c a t e a maximum s u r v i v a l time a t 25 C without food of around ten days. However, below 5 C, as when o v e r w i n t e r i n g , f e e d i n g ceases and c o r i x i d s can be stored f o r 96 c o n s i d e r a b l e p e r i o d s w i t h l i t t l e m o r t a l i t y . P r e l i m i n a r y o b s e r v a t i o n s were made i n J a n u a r y , 1972 u s i n g £®£222£ixa b l a i s d e 1 l i and d i a p t o m i d s f r o m a s m a l l pond on t h e U . B . C . C a m p u s . C o r i x i d s were i s o l a t e d i n f i l t e r e d , d e c h l o r i n a t e d w a t e r and o f f e r e d v a r i o u s e n t o m o s t r a c a n s . A t 5 C nc f e e d i n g was o b s e r v e d b u t s u r v i v a l was g o o d . H o w e v e r , a t l a b o r a t o r y t e m p e r a t u r e s (24 C) m o r t a l i t y o f c o r i x i d s and p r e y o r g a n i s m s was h i g h . F u n g a l g r o w t h was p r e v a l e n t on d e a d o r g a n i s m s , and i n e x t e n d e d e x p e r i m e n t s i t was d i f f i c u l t t o d e t e r m i n e i f p r e y d e a t h was due t o c o r i x i d p r e d a t i o n o r some o t h e r f a c t c r . D i a p t o m i d s u s u a l l y a p p e a r e d t o have b e e n b i t t e n t h r o u g h t h e c a r a p a c e at t h e j u n c t i o n o f c e p h a l o t h o r a x and a b d o m i n a l s e g m e n t s , and t h e i r c o n t e n t s p a r t l y s u c k e d c u t ; t h i s was e a s i l y d e t e r m i n e d w i t h b r i g h t r e d c o p e p o d s b u t was n o t s o e a s y w i t h o t h e r e n t o m o s t r a c a n s . C y c l o p o i d c o p e p o d s , w h i c h l e a v e s i m i l a r e v i d e n c e o f f e e d i n g , ( W a l t e r s , p e r s o n a l c o m m u n i c a t i o n ) , were no t mixed w i t h d i a p t o m i d s i n t h i s e x p e r i m e n t . I t was a p p a r e n t t h a t d i r e c t e v i d e n c e o f f e e d i n g w o u l d be r e q u i r e d and t h i s w c u l d have t o be o b s e r v e d t a k i n g p l a c e . F u r t h e r p r e l i m i n a r y e x p e r i m e n t s , a t B e c h e r ' s P r a i r i e i n M a y , 1972 , i n v e s t i g a t e d t h e f e a s i b i l i t y o f d i f f e r e n t e x p e r i m e n t a l m e t h o d s and f a c t o r s s u c h a s s i z e o f t e s t e n c l o s u r e s and v i a b i l i t y o f o r g a n i s m s . In g e n e r a l , c o r i x i d s a d e q u a t e l y s u r v i v e d c r o w d i n g a t d e n s i t i e s o f t e n p e r 100 ml w a t e r . S u r v i v a l was i m p r o v e d w i t h r e g u l a r c l e a n i n g o f d i s h e s and c h a n g i n g o f t h e t e s t w a t e r . H o w e v e r , o w i n g t o t h e d i f f i c u l t y c f o b s e r v i n g f e e d i n g and t h e r e l a t i v e l y h i g h m o r t a l i t y r a t e s , l o n g - t e r m 97 experiments were not judged to be s u i t a b l e . Adult C. b i f i d a were shown to eat dead and l i v e p l a n k t e r s , chironomid l a r v a e , amphipods and zygopterans. A f t e r periphyton was added to one d i s h , c o r i x i d faeces seen subsequently were g r e e n i s h , i n d i c a t i n g t h a t some pl a n t matter might a l s o have been used as food. No h y d r a c a r i n i d s , c o l e o p t e r a n l a r v a e or ephemeropteran nymphs were eaten. Using the i n f o r m a t i o n gained i n the p r e l i m i n a r y t e s t s , a s e r i e s of c o n t r o l l e d , short-term f e e d i n g experiments was i n i t i a t e d i n the summer of 1972 and continued i n t c 1974. The methods devised and used and the r e s u l t s obtained are d e s c r i b e d be low. MATERIALS AND METHODS C o r i x i d s were c o l l e c t e d by the use of a d i p - n e t , throughout the summer season i n 1972, 1973 and 1974 at a l l experimental l a k e s , thus encompassing the range of h a b i t a t s encountered. They were placed i n i n s u l a t e d 2 l i t r e f l a s k s p a r t l y f i l l e d with water from the same l a k e , f o r t r a n s p o r t to the l a b o r a t o r y ; they s u r v i v e d f o r up to two days with l i t t l e m o r t a l i t y under these c o n d i t i o n s . C o r i x i d s not immediately used f o r experiments were held i n c i r c u l a r , p l a s t i c 5 l i t r e troughs at 5 C, a t which temperature they do not feed (Jansson and Scudder, 1972). Prey organisms were c o l l e c t e d s i m i l a r l y c r by plankton tows. In some cases these were supplemented with organisms from 98 the U.B.C. Campus ponds and from l a k e s Marion and Eunice i n the U.B.C. Research F o r e s t , Haney. Prey used (Table XIV), i n c l u d e d groups of organisms t h a t occurred f r e g u e n t l y i n the s t u d i e d l a k e s and so might be suspected to form part of the d i e t . Before t e s t i n g , c o r i x i d s were s t a r v e d so that they might be un i f o r m l y r e c e p t i v e t o food: p r e l i m i n a r y experiments i n d i c a t e d t h a t 24 h at l a b o r a t o r y temperature was most a p p r o p r i a t e . Too prolonged s t a r v a t i o n of an animal may make i t l e s s s e l e c t i v e of food, but such an e f f e c t was not noted i n t h i s t e s t . The f e e d i n g response of C. b i f i d a s t a r v e d f o r two days was compared with t h a t of one-day s t a r v e d animals from the same l c c a t i c n and c o l l e c t i o n date. Feeding was found t c be more prolonged and i n t e n s e i n the two-day s t a r v e d c o r i x i d s (a mean of 1.5 feed i n g bouts observed over ten 20-second periods cf o b s e r v a t i o n , n=46) than i n the one-day starved forms (mean 0.8 bouts, n=109). However, at t a c k and ' i n t e n t i o n movements' were somewhat more freguent i n those s t a r v e d f o r the one day p e r i o d than f o r two days (means 2.7 and 2.3 a t t a c k s per i n d i v i d u a l r e s p e c t i v e l y ) . Jamieson (1973) found t h a t a s i m i l a r food d e p r i v a t i o n i n t e r v a l of 24 h f o r l a r v a e and 48 h f o r a d u l t Gerridae (Hetercptera) was s u f f i c i e n t to a t t a i n maximum i n g e s t i o n i n fee d i n g t e s t s . In the present t e s t s , the 24 h p e r i o d was used throughout. C o r i x i d s were i n d i v i d u a l l y i s o l a t e d i n dishes c o n t a i n i n g 125 ml of f i l t e r e d lake water from t h e i r home l a k e , and a 2 cm sguare of p l a s t i c to c l i n g t o . Three s l i g h t l y d i f f e r e n t c o n t a i n e r types were used i n t e r c h a n g e a b l y f o r experiments. P r e l i m i n a r y t e s t s using C. b i f i d a , f i f t h i n s t a r nymphs c o l l e c t e d 99 Table XIV. D e t a i l s of prey organism groups used i n feed i n g experiments, with numbers of t e s t s u s i n g each. PREY SIZE,SEECIES CETAIIS TESTS Daphniids Daphnia pulex, D. s c h c d l e r i , D. magna, D. s i r o i l i s (carapace length 2.0-2.7, mean 2.4 mm) 2C5 Dead daphniids same forms 161 Diaptomids Diaptgmus s i c i l i s , I. nevadensis (length 0.8~- 3~7o mm) 177 Dead diaptomids Same forms 132 Mixed small plankton: D. s i c i l i s , Ceriodaphnia sp. 175 Dead mixed p l a n k t e r s : Same forms 68 F a i r y shrimp j£a£chinecta sp. from IE 2 55 Dead f a i r y shrimp Same forms 21 A tophi pods Gam mar us l a c us t r i s , H j a l e l l a a z t e c a . (smaller than c o r i x i d tested) 161 Dead amphipods Same forms 132 Hydracarina Viater-mites, red or black 16 Chironomid l a r v a e Mixed forms, 4.0-6.5 mm, mean 5.5 mm long 229 Dead chironomids Same forms 117 Chaobcrid l a r v a e Chaobcrus americanus 44 Dead chaoborids Same forms 4 Zygopteran nymphs Enallagma s p e c i e s (smaller than c o r i x i d tested) 85 Dead zygopterans Same forms 113 Ephemercpterans C a l l i p a e t i s s p e c i e s nymphs, (smaller than c o r i x i d tested) 102 Dead ephemeropt. Same forms 56 Bee t l e l a r v a e Smaller than c c r i x i d t e s t e d 24 J u v e n i l e c o r i x i d s C. b i f i d a f i r s t and second i n s t a r s 4 Macrophytes Pofamogeton, Ruppia, Ceratcphj11um, MXEi°JEj>l2IJ3J s p e c i e s 20 Algae Cladophcra-type and blue-greens 20 100 at one sampling place and date, plus two 'active' prey types (daphniids and diaptomids) and one 'passive' E r®y t j P e (chironomid larvae) showed no marked differences in c o r i x i d response between the di f f e r e n t container types (Table XV). At the st a r t of each single prey type feeding experiment, and i n those using a mixture of planktonic organisms, a pot e n t i a l food was added to dishes containing starved corixids. Prey offered were almost always smaller than the cor i x i d being tested. Each animal was then examined ten times at approximately ten minute i n t e r v a l s , for a period of 20 seconds at each examination. During this examination period, c o r i x i d reactions were scored as 0 : no reaction directed to food. 1 : f l u t t e r movement made by stationary c o r i x i d , involving rapid palar motion, usually in reaction to abrupt movement of prey close by. Corixids t y p i c a l l y showed fast brushing of the abdomen with the hind limbs. 2 : swimming co r i x i d s lunged at prey, singly or repeatedly, but did not succeed in capturing i t . This movement was d i f f e r e n t from exploratory or f l i g h t - o r i e n t e d movements. 3 : swimming or stationary c o r i x i d made a sudden swimming lunge at a prey, capturing i t momentarily. 4 : c o r i x i d captured and fed on prey organism for more than f i v e seconds of the observation period. Feeding corixids made the same fast 101 Table XV. Feeding response data f o r C. b i f i d a i n d i f f e r e n t types of t e s t c o n t a i n e r s , when fed s i m i l a r prey types. (Fcr t e s t c o n d i t i o n s see t e x t d e s c r i p t i o n ) . ATTACKS CONTAINER TYPE (MEAN PER DISH, 10 STUDY BEBIOCS) HIEING (MEAN PER REPLICATES U S E , 10 STDDY EE BICES) 350 ML PLASTIC DISH 1.9 250 ML PLASHC EEAFER 2.2 250 ML GLASS EEAKER 2.0 1.6 1.6 0.8 38 25 5 102 movements of the hind limbs d e s c r i b e d i n r e a c t i o n l e v e l 1. When more than one r e a c t i o n type occurred i n one o b s e r v a t i o n p e r i o d , only the r e a c t i o n of g r e a t e r i n t e n s i t y was noted. At the c o n c l u s i o n of each experiment the water temperature was measured and f o r each c o r i x i d s p e c i e s and i n s t a r and, i f a d u l t , sex and whether t e n e r a l or o l d , was recorded, along with the c o l o u r of the food i n the gut. T h i s was d i s t i n g u i s h e d on a zero to f i v e p o i n t s c a l e ( 0=empty, 1=yellow, 2=tan, 3=red, 4=brown, 5 = o l i v e - g r e e n i s h ) . In l a r v a l i n s t a r s and sometimes i n the young a d u l t s food m a t e r i a l could be seen d i r e c t l y through the body w a l l , but i n most a d u l t s , cr where l i t t l e food was i n g e s t e d , d i s s e c t i o n was necessary to observe the gut contents. Reactions to the prey were not q u a n t i f i e d on an i n t e n s i t y g r a d i e n t , as there seemed to be no c l e a r r e l a t i o n s h i p s between the number of i n t e n t i o n movements ( r e a c t i o n s 1 or 2) i n the experimental s i t u a t i o n , and f e e d i n g success. F u r t h e r , under the system of n o t a t i o n used, m u l t i p l e r e a c t i o n s to a prey i n one time p e r i o d were reco r d e d as s i n g l e o n l y . In the c o l l a t i o n cf r e s u l t s , specimens which showed no r e a c t i o n s to prey were i n c l u d e d , although there may have been reasons f o r t h e i r lack of response, such as damage during t r a n s f e r to the experimental d i s h , or imminent moulting. Moults occurred on 33 o c c a s i o n s (1.6%) i n the 2127 f e e d i n g t r i a l s with s i n g l e prey types. Feeding experiments i n d i c a t e d t h at f e e d i n g can a p p a r e n t l y take place up to f i v e minutes before 103 e c d y s i s and as soon as 15 minutes a f t e r the moult i s complete. Many f a c t o r s , such as the s o f t n e s s and ease of capture of the prey o f f e r e d , may a f f e c t t h i s t i m e - p e r i o d , and i n most cases f e e d i n g ceased some time before the moult (mean of o b s e r v a t i o n s 70 minutes) and the l a s t i n t e n t i o n movement ceased at a mean cf 40 minutes before e c d y s i s . A f t e r the moult, f e e d i n g s t a r t e d again a f t e r a mean lag of 40 minutes. Eata c o l l e c t e d within 70 minutes before and 40 minutes a f t e r moults were t h e r e f o r e e l i m i n a t e d from the a n a l y s i s . O v i p o s i t i o n caused apparently l i t t l e or no d i s r u p t i o n of f e e d i n g . In s e v e r a l i n s t a n c e s eggs were l a i d i n dishes i n which the experimental c o r i x i d was apparently f e e d i n g f c r much c f the o b s e r v a t i o n p e r i o d . In the a n a l y s i s of r e s u l t s , each experiment was assessed, and i f c o r i x i d behaviour was l i s t e d as l e v e l s 3 or 4 (capture or f e e d i n g r e s p e c t i v e l y ) i n any of the ten o b s e r v a t i o n p e r i o d s , the experiment was considered p o s i t i v e evidence f o r f e e d i n g on t h a t prey, while i n t e n t i o n movements ( l e v e l 1 or 2) were considered as n e g a t i v e , along with no r e a c t i o n s ( l e v e l 0 ) . T h i s broad treatment of the r e s u l t s minimised any p o s s i b l e b i a s which might have been introduced through the use of d i f f e r e n t experimental c o n t a i n e r s . Data were summed f o r each s p e c i e s . For f u r t h e r a n a l y s i s , data were subdivided by sex and i n s t a r , and placed i n c a t e g o r i e s r e l a t i n g t o the environmental d i v e r s i t y , as f o l l o w s : lake group 1 : C. exjaleta from Lake LE 2 lake group 2 : C. exp_leta and C. b i f i d a frcm Barnes and Round-up Lakes and Lake Lye 1 04 l a k e group 3 : C. b i f i d a from Greer, Bock and East l a k e s . Season 1 : May and June season 2 : J u l y and August season 3 : September and October. For the prey c h o i c e feeding experiments, except those using mixed p l a n k t e r s , i n i t i a l procedures were i d e n t i c a l to those d e s c r i b e d above, but each d i s h was observed c o n t i n u o u s l y and a l l c o r i x i d r e a c t i o n s recorded under the same headings as b e f o r e . Once feed i n g had o c c u r r e d , the experiment was terminated i n each case, thus a v o i d i n g problems of removal and replacement of prey to prevent d i s t o r t i o n of prey r a t i o s i n a c o n t i n u i n g experiment. Numbers of mixed s m a l l p l a n k t e r s used were so great that t h i s e f f e c t was not judged important. Computer programmes to scan the data, d e s c r i b e each experiment as p o s i t i v e or negative evidence of f e e d i n g on each prey type, and t a b u l a t e the r e s u l t s by c o r i x i d s p e c i e s and i n s t a r , prey, season and lake group, were k i n d l y w r i t t e n by Ms. Dolores L a u r i e n t e . Other s t a t i s t i c a l c a l c u l a t i o n s were done using the U.B.C. Computing Centre programmes CHI 2 and SAS 1.. 105 RESULTS S t r u c t u r e and Behaviour During the experiments some i n s i g h t was gained i n t c f e e d i n g behaviour and i t s r e l a t i o n to s t r u c t u r e of mouthparts and a l i m e n t a r y c a n a l . Most evidence pointed tc c a r n i v c r y , r a t h e r than to a phytophagous h a b i t . In t e s t s i t u a t i o n s c o r i x i d s a l t e r n a t e d bcuts cf r a p i d , r e s t l e s s swimming with p e r i o d s spent p i c k i n g over the bottom of the t e s t d i s h or the p l a s t i c screen with palae and mcuthparts, or c l i n g i n g motionless to the s c r e e n i n g . P r e l i m i n a r y o b s e r v a t i o n s suggested that c o r i x i d s swam more f r e q u e n t l y when l i v e prey were present than with dead prey. However, an a n a l y s i s of the frequency of b u r s t s of u n d i r e c t e d swimming a c t i v i t y showed no s i g n i f i c a n t d i f f e r e n c e s between the two s i t u a t i o n s , i n d i c a t i n g that the p r o p o r t i o n of d i s t r a c t i o n cr ' s t a r t l e ' swimming i s not l i k e l y t c be great i n each case. The p i c t u r e i s r a t h e r d i f f e r e n t when ' i n t e n t i o n movements', s p e c i f i c a l l y d i r e c t e d towards the food item, are c o n s i d e r e d . Data matched f o r date or lake of o r i g i n were analysed .to see i f major d i f f e r e n c e s e x i s t e d i n f e e d i n g i n t e n s i t y between l i v e and dead prey. In a l l c o r i x i d s i n t e n t i o n movements cf i n t e n s i t y l e v e l s 1 to 3 were more fre q u e n t with l i v e prey i n experiments than i n those with dead ( Table XVI). Thus, although some a c t i v i t y may have been the r e s u l t of d i s t r a c t i o n or s t a r t l i n g by the p o t e n t i a l food organism r a t h e r than cf i n t e r e s t i n i t , a l i v i n g prey aroused more d i r e c t e d c o r i x i d a c t i v i t y than 106 Table XVI. I n t e n s i t y of c o r i x i d a c t i v i t y when o f f e r e d dead and l i v e p r e y s . (Data f o r C. b i f i d a ; a c t i v i t y r e g i s t e r e d as i n t e n t i o n movements recorded i n ten o b s e r v a t i o n s i n each fee d i n g experiment. I n t e n s i t y l e v e l s d e s c r i b e d i n t e x t ; l e v e l s 1 and 2 = d i r e c t e d i n t e r e s t , l e v e l 3 = prey c l u t c h e d , l e v e l 4 = feed i n g frcm p r e y ) . l e v e l LIVE PHEY 4 3 2 + 1 n= DEAD PBEY 4 3 2 + 1 PBEY TYPE : AMPHIPODS 15 101 59 53 1 43 Mean 2.09 a c t i v i t i e s / i n d i v i d u a l , 1.71 f o r l e v e l s 2+1 Mean 1.35 a c t i v i t i e s / i n d i v i d u a l , 0.09 f o r l e v e l s 2+1 CHIRONOMIDS 120 5 75 120 Mean 1.67 a c t i v i t i e s / i n d i v i d u a l , o.63 f o r l e v e l s 2+1 126 2 25 50 mean 3.06 a c t i v i t i e s / i n d i v i d u a l , 0.50 f e r l e v e l s 2+1 PLANK TER S 24 2 198 45 Mean 4.98 a c t i v i t i e s / i n d i v i d u a l , 4.40 f o r l e v e l s 2*1 79 7 204 98 mean 2.96 a c t i v i t i e s / i n d i v i d u a l , 2.08 f e r l e v e l s 2+1 107 a dead one. Prey motion may a t t r a c t i n i t i a l a t t e n t i o n through mechanisms such as v i s i o n or pressure d e t e c t i o n . Dead prey, on the other hand, may be seen or d i s c o v e r e d by chance during a r e s t l e s s a c t i v i t y bout, or through f o l l o w i n g a chemical g r a d i e n t to the food-source. when the f r e s h l y k i l l e d prey were halved, the body j u i c e s e n t e r i n g the water of the t e s t d i s h produced f a s t e r c o r i x i d o r i e n t a t i o n to the cut prey than t o an i n t a c t prey organism. L i v e prey organisms were not always detected by the c o r i x i d s (as determined by f e e d i n g lunges cr i n t e n t i o n movements) unless they were wit h i n s t r i k i n g d i s t a n c e , g e n e r a l l y about 1.5 mm away. S i m i l a r l y , c o r i x i d s o f t e n s e i z e d a p r e v i o u s l y i g n o r e d prey when i t was moved c l o s e to them and he l d i n a f i n e f o r c e p s . However, c o r i x i d s were e x c i t e d at a g r e a t e r d i s t a n c e to sp a s m o d i c a l l y swimming organisms, on occasions l u n g i n g across the width of the t e s t d i s h (6 to 7 cm) to take a prey. Late i n s t a r s and a d u l t s could a l s o sometimes be e n t i c e d to attack the po i n t of a needle i f t h i s was moved through the water tc a p o s i t i o n 1 to 2 mm from t h e i r rostrum i n a j e r k y f a s h i o n . i Thus, motion of the prey seems to be an important t r i g g e r i n i t i a t i n g f e e d i n g behavicur, although s i n c e c o r i x i d s became e x c i t e d by prey motion below and behind them, v i s i o n cannot be the only means of d e t e c t i o n . The behaviour of both j u v e n i l e s and a d u l t s thus s t r o n g l y suggests c a r n i v o r y and l i m i t e d p r e d a t i o n . Feeding on p l a n t s was more d i f f i c u l t to c o r r o b o r a t e , and when i t seemed to occur i t was r a r e l y accompanied by r a p i d hind-limb 108 b e a t i n g a s s e e n i n c o r i x i d s f e e d i n g on a n i m a l p r e y . T o c a p t u r e a p r e y o r g a n i s m , t h e c o r i x i d p o u n c e d r a p i d l y and s c o o p e d i t i n t h e p a l a e , a m o t i o n b r i n g i n g i t t i g h t l y a d p r e s s e d t o t h e mouth r e g i o n . T h e n , i f p r e y o r i e n t a t i o n was c o r r e c t , f e e d i n g c o u l d commence . Many a d u l t c o r i x i d s p r e s e n t e d w i t h a p r e y h e l d i n a f o r c e p s g r i p p e d i t w i t h t h e i r p a l a e so f i r m l y t h a t t h e c o r i x i d c o u l d be l i f t e d o u t o f t h e water f o r some s e c o n d s by r a i s i n g t h e f o r c e p s . A d u l t and j u v e n i l e C . b i f i d a were o b s e r v e d t o c a t c h and h o l d d i a p t o m i d s t o t h e r o s t r u m f o r up t c 10 s e c o n d s b e f o r e d r o p p i n g t h e m , s t i l l a l i v e . T h i s may h a v e been due t o i n c o r r e c t o r i e n t a t i o n ; d i a p t o m i d s were u s u a l l y s u c k e d t h r o u g h t h e d o r s a l c a r a p a c e , l e a v i n g a c h a r a c t e r i s t i c c r u s h e d o r b r o k e n a r e a . T h e y were a l s o s u c k e d o c c a s i o n a l l y t h r o u g h t h e a n t e n n a . S i m i l a r l y , a D a £ h n i a c a u g h t and h e l d by t h e c a r a p a c e s p i n e f c r 35 s e c o n d s e s c a p e d u n h a r m e d , o n l y t o be r e c a p t u r e d and r e o r i e n t e d f o r s u c c e s s f u l f e e d i n g , w h i c h was u s u a l l y o b s e r v e d t o o c c u r t h r o u g h t h e v e n t r a l a r e a b e t w e e n t h e v a l v e s . S u t t o n (1951) n o t e d t h a t c o r i x i d s f r e q u e n t l y p i e r c e d d a p h n i i d e y e s f o r f e e d i n g ; p e r h a p s t h e b l a c k c o l o u r p r o v i d e d v i s u a l o r i e n t a t i o n f c r c l o s e m a n i p u l a t i o n and s e l e c t i o n o f f e e d i n g s i t e . T h u s , o n c e a c o r i x i d has c a p t u r e d a p r e y o r g a n i s m , s u b d u i n g o f t h e p r e y a p p a r e n t l y d e p e n d s on f a c t o r s s u c h as i n t e g u m e n t t o u g h n e s s a s much a s on t h e r e p o r t e d venomous s e c r e t i o n s c f t h e m a x i l l a r y g l a n d s ( S o k c l ' s k a y a and Z h i t e n e v a , 1 9 7 3 ) . Once s u b d u e d , b o d y j u i c e s were r a p i d l y s u c k e d o u t and a s t a r v e d c o r i x i d c o u l d c a p t u r e and f e e d on s e v e r a l l a r g e c h i r o n o m i d 109 l a r v a e or one to two f a i r y shrimps i n one hour, these being q u i c k l y reduced to empty s k i n s or s h r i v e l l e d bodies. However, when prey were abundant, a c o r i x i d would o f t e n abandon a p a r t i a l l y - e a t e n organism f o r another l i v i n g one, a f e e d i n g h a b i t a l s o noted by Walton (1943b). Recording of f e e d i n g was r e l a t i v e l y easy f o r some prey; s o f t - b o d i e d forms shrank or l o s t t h e i r c o l o u r as j u i c e s were sucked out. In other cases the prey had to be examined a f t e r being r e l e a s e d by the c o r i x i d f o r s i g n s of damage cr death and l o s s of t u r g o r . A l s o f e e d i n g c o r i x i d s u s u a l l y beat t h e i r hind limbs r e g u l a r l y , at about one h a l f to two beats per second, c o n s i d e r a b l y f a s t e r than the b a l a n c i n g or grooming movements seen when at r e s t . O c c a s i o n a l l y when the c o r i x i d was f a c i n g the prey or i t s own r e f l e c t i o n the same r a p i d limb beating occurred, f o l l o w e d sometimes by a swimming lunge. Sutton (1951)'described a s i m i l a r brushing of the abdomen with the hind l e g s when a c o r i x i d became aware of a prey; i t then darted to face the prey and hovered with s t a t i o n a r y rowing motions producing water c u r r e n t s which might help immobilise the prey, before pouncing upon i t . Wherever p o s s i b l e c o r i x i d s were presented with t e s t food organisms s m a l l e r than themselves, although with c e r t a i n forms, as p l a n t s , or when t e s t i n g the s m a l l e s t c o r i x i d i n s t a r s , t h i s was not f e a s i b l e . The problem of prey s i z e and attack success d i s c u s s e d by Jamieson (1973) f o r g e r r i d s , was net f u r t h e r i n v e s t i g a t e d here, and i t was m i t i g a t e d i n a n a l y s i s of r e s u l t s by i n c l u d i n g l e v e l s 3 and 4 i n evidence c f f e e d i n g . Thus prey 110 s i z e was not such a c r i t i c a l f a c t o r as i t might have teen i f only s u c c e s s f u l capture l e a d i n g to f e e d i n g ( l e v e l 4 alcne) were c o n s i d e r e d as p o s i t i v e evidence of f e e d i n g on that prey. Adult c o r i x i d s would a t t a c k a f a i r y shrimp more than twice t h e i r l e n g t h ; a t t a c k s here were not u s u a l l y s u c c e s s f u l unless d i r e c t e d to the t a i l . C o r i x i d s r a r e l y showed any response to l a r g e r ephemeropteran or zygopteran nymphs, but when the same prey organisms were pro v i d e d dead the c o r i x i d s approached without h e s i t a t i o n and fed from them. Again, many c o r i x i d s seemed to a c t i v e l y avoid amphipods even when these were c l e a r l y s m a l l e r than themselves; t h i s might be due to the r e g u l a r and r a p i d swimming of the amphipods around the c o n t a i n e r , s i n c e dead amphipods were u s u a l l y s e i z e d immediately. Thus, preference f o r d i f f e r e n t prey may have been as much mechanical, concerned with the l o g i s t i c s of c a t c h i n g and g r a s p i n g , as based on b i o t i c c o n d i t i o n s , a c o n c l u s i o n Jamieson (1973) a l s o came tc with g e r r i d s . In the experiments v i s u a l assessments were made c f gut c o l o u r a f t e r each t e s t on a s i x p o i n t s c a l e , as d e s c r i b e d above. R e s u l t s of these r e c o r d s are summarised in Table XVII. For a l l 645 c o r i x i d s t a b u l a t e d the gut c o l o u r s most f r e q u e n t l y seen were c o l o u r l e s s , red or brown. However, c o n s i d e r i n g c n l y those c o r i x i d s recorded as f e e d i n g on the prey o f f e r e d , the p r o p o r t i o n s of c o l o u r l e s s guts not unexpectedly d e c l i n e d , and red guts became dominant. T h i s i s c h i e f l y a r e s u l t of feeding on diaptomids and mixed p l a n k t e r s ; chironomid l a r v a e , even b r i g h t red i n c o l o u r , c o n t r i b u t e d mostly to the brown c o l o u r category. I l l Table XVII. C o r i x i d gut c o l o u r a f t e r o f f e r i n g kncwn preys i n fe e d i n g experiments. Dominant c o l o u r u n d e rlined f o r each prey type. |G DT COLOUR : | 0 I emp ty| 1 | yellow 2 I tan | 3 I red | * I brown| T T 5 | n = | o l i v e | | |PREYS : | 1- -1" PERCEM SHOWING EACH GUT CCIOUR AFTER TESTING | r 1 |Daphniids | 14. 3 | 2 1 . 4 | 8. 3 | 13. 1 | 4 2 . 9 | 0 . 0 | 84 | IDiaptomids | 5. 8 | 8 . 8 | 9 . 5 | 7 2 . 3 | 3 . 7 | 0 . 0 | 137 | |Mixed plankton 5 . 8 | 4 . 6 | 4. 6 | 7 J . 3 | 1 3 . 8 | 0 . 0 | 87 | lAmphipods | 1 4 . 3 | 1 4 . 3 | 0 . 0 | 0 . 0 | 7 1 . 4 | 0 . 0 | 7 | |Chironomids | 12. 0 | 2 0 . 0 | 1 0 . 0 | 4 . 0 | 5 4 . 0 | 0 . 0 | 50 | |TOTALS | 32 | 4 5 | 29 | 174 | 86 | 0 | 365 | | PERCENT EATING |FOOD OFFERED | 8 . 7 | 12. 3 | 7 . 9 | 4 7 . 5 | 2 3 . 5 | 0 . 0 | | 1 12 Co l o u r s were observed through the body w a l l , which would account f o r some d u l l i n g of the hue. The range from brown to yellow through tan seemed s i m i l a r both f o r chironomids and the r e l a t i v e l y c o l o u r l e s s d a p h n i i d s , probably r e f l e c t i n g the amounts of e i t h e r consumed. S i n g l e P r e j Experiments Twenty-three p o t e n t i a l prey items, of which two were p l a n t and nine were f r e s h l y - k i l l e d animals, were t e s t e d i n 2127 s i n g l e - p r e y experiments with approximately equal numbers of C. b i f i d a and C. e x p l e t a (Table X V I I I ) . The numbers of each s p e c i e s and i n s t a r t e s t e d depended upon t h e i r a v a i l a b i l i t y and p r o p o r t i o n s i n samples c o l l e c t e d , as c o r i x i d j u v e n i l e s could not be i d e n t i f i e d to s p e c i e s without m i c r o s c o p i c examination which might cause s t r e s s or damage. Thus no t e s t s were run f o r dead Bra n c h i n e c t a c r j u v e n i l e c o r i x i d s with C. b i f i d a , nor f o r a q u a t i c macrophytes or algae with £• §*£i§iS . However, f a i r y shrimps were uncommon and ver n a l i n those l a k e s which contained both Cenocorixa s p e c i e s and p l a n t l i f e was inconspicuous i n the higher s a l i n i t y l akes i n h a b i t e d by C. e x p l e t a (see Chapter I I ) . R e s u l t s i n d i c a t e t h a t both C. b i f i d a and C. e x p l e t a f ed very s i m i l a r l y o v e r a l l , t a k i n g most items o f f e r e d (Table XIX). R e f u s a l s of food items were g e n e r a l l y a t t r i b u t a b l e to t h e i r l a r g e s i z e or a c t i v i t y r e l a t i v e t o the c o r i x i d i n s t a r s t e s t e d . N e i t h e r s p e c i e s took h y d r a c a r i n i d s and, among items t e s t e d f o r Cenocorixa b i f i d a Cenocorixa expleta N ~ \ ' C  INSTARS ADULTS INSTARS ADULTS FOOD ITEMS t 1 2 3 4 1 5 1 f 1 m TOTAL 1 1 2 3 4 1 5 1 f 1 m TOTAL Daphnia 13 16 28 36 17 21 131 10 10 15 10 19 10 74 Dead Daphnia 1 11 15 10 13 15 14 87 12 10 12 10 18 12 74 Diaptomus 4 15 32 29 13 14 107 11 14 10 11 13 n 70 Dead Diaptomus 1 11 13 17 11 14 8 75 2 10 13 8 10 12 12 67 Mixed plankton 9 10 16 18 4 16 73 6 24 24 14 19 15 102 Dead mixed plankton 1 1 1 14 8 25 9 14 14 3 3 43 Branchinecta 1 2 3 4 3 13 3 4 10 12 13 42 Dead Branchinecta - 3 8 10 21 Amphipod s 2 7 14 24 16 12 83 2 11 10 10 26 18 77 Dead amphipods 1 1 28 11 9 58 6 10 18 11 20 9 74 Hydracarina 3 1 4 5 7 12 Chironomids 11 20 13 25 21 108 21 17 29 28 15 11 121 Dead chironomids 2 7 6 17 8 40 1 ' 10 10 12 21 22 76 Chaoborids 15 5 20 18 6 24 Dead chaoborids 1 1 2 1 1 2 Zygopterans 3 10 11 11 4 2 47 10 2 3 14 7 36 Dead zygopterans 1 1 13 16 41 11 14 13 24 10 72 Ephemeropterans 1 7 10 12 30 2 16 22 15 10 7 72 Dead ephemeropterans i 1 9 6 17 10 14 47 Coleopterans 4 5 1 2 12 1 5 6 12 Juvenile corixids - 2 2 4 Algae 1 9 10 20 -Macrophytes 7 13 20 1043 1084 Table XVIII. Numbers of replicate feeding experiments with each foodstuff for each species and instar of Cenocorixa (totals columns includes unsexed adults). 114 Table XIX. Summary of r e s u l t s of feeding experiments t e s t i n g 2127 c o r i x i d s with 23 d i f f e r e n t s i n g l e food items (x = food i t e i t o f f e r e d and accepted; - = item o f f e r e d but not accepted) c. BIFIDA C. EX PI FT A INSTARS 1 2 3 4 5 ads 1 2 3 4 5 a FOCD ITEM Daphniids X X X X X X X X X X Dead daphniids — X X X X X X X X X X Diaptomids X X X X X X X X X X Dead d i a p t . X X X X X X X X X X Mixed plankton X X X X X X X X X X Dead mixed p i . X X X X X X X F a i r y sprimp - - X X X X X X Dead f a i r y sh. X X X Amphipods X X X X X X X X X Dead amphi. X - X X X X X X X Hydracarina - - — Chironcmids X X X X X X X X X Dead c h i r o . X X X X X X X X Chaoborids X X Dead chaob. - X Zygopterans - X X X X X X - X Dead zygopt. - X X X X X X Ephemeropt. - - X - X X X Dead ephem. X - X X X X B e e t l e l a r v a e X X X X X Juv. c o r i x i d s Macrophytes X Algae - -115 one s p e c i e s of Cenocorixa only, C. b i f i d a d i d not take dead chaoborid l a r v a e or algae while C. exjaleta s i m i l a r l y d i d not take j u v e n i l e c o r i x i d s . The f e e d i n g p a t t e r n r e v e a l e d f o r both s p e c i e s i s d e f i n i t e l y c a r n i v o r o u s and l a r g e l y predatory. Thus i n overview, t h e i r fundamental niches did not d i f f e r markedly, su g g e s t i n g t h a t i n f e e d i n g p a t t e r n at l e a s t , the s p e c i e s might be c o n s i d e r e d e c o l o g i c a l l y homologous. However, Chapter II has shown t h a t the p r o p o r t i o n s of food items a v a i l a b l e f o r t e s t i n g v a r i e d from lake tc lake and from month to month i n each l a k e . I t i s t h e r e f o r e p o s s i b l e that d i e t , as i n d i c a t e d by the r e s u l t s of f e e d i n g experiments, w i l l , d i f f e r with season of c o l l e c t i o n and l a k e of c o r i x i d o r i g i n . As j u v e n i l e c o r i x i d i n s t a r s a l s o d i f f e r e d markedly i n s i z e , m o t i l i t y and appearance and to some extent a l s o i n h a b i t a t p r e f e r e n c e s from each other and from a d u l t s (Chapter I I I ) , i t i s l i k e l y t h a t t h e i r f e e d i n g h a b i t s w i l l a l s o show d i f f e r e n c e s . To i n v e s t i g a t e such p o s s i b l e d i f f e r e n c e s , which could have marked e f f e c t s i n the f i e l d on p o t e n t i a l competition f o r feed, the data were f i r s t examined i n terms of a d u l t s alone and then s u b d i v i d e d by l a k e and season. 116 (i) Feeding p a t t e r n s of a d u l t Cenocorixa c o l l e c t e d i n d i f f e r e n t l a k e s : C o r i x i d s c o l l e c t e d from d i f f e r e n t l a k e s might be expected to have a c e r t a i n d i e t a r y preference r e l a t e d to c o n d i t i o n s i n t h e i r lake of o r i g i n at the time of c o l l e c t i o n . The longer they are kept before t e s t i n g , the more t h i s p o s s i b l e preference p a t t e r n could be expected to d i m i n i s h i n the a r t i f i c i a l c o n d i t i o n s of low-temperature st o r a g e and crowding. To t e s t whether such a p r e f e r e n c e i s evident i n the f e e d i n g experiment r e s u l t s and t h e r e f o r e might b i a s the analyses i f i g n o r e d , feeding data f o r a d u l t s from d i f f e r e n t lakes were compared (Table XX). Most data were too few f o r s t a t i s t i c a l a n a l y s i s , but there seem to be no c o n s i s t e n t trends towards d i f f e r e n t f e e d i n g i n t e n s i t i e s on the f o o d s t u f f s l i s t e d , among c o r i x i d s from d i f f e r e n t l a k e s . In C. b i f i d a c o l l e c t e d i n the higher s a l i n i t y l a k e s on Becher's P r a i r i e there was a non-s i g n i f i c a n t trend towards more f e e d i n g on dead amphipods than i n those c o l l e c t e d i n the lower s a l i n i t y l a k e s , where the f o o d s t u f f occurs n a t u r a l l y (Chapter I I ) , but t h i s i s l i k e l y tc be a r e s u l t of the few data a v a i l a b l e . Thus no s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e s between c o r i x i d s c o l l e c t e d from d i f f e r e n t l a k e s were observed, and t h e r e f o r e d i f f e r e n t l a k e data were pooled i n f u r t h e r a n a l y s i s cf the f e e d i n g p a t t e r n s . 117 T a b l e XX . C o m p a r i s o n o f r e s u l t s o f f e e d i n g e x p e r i m e n t s u s i n g a d u l t C e n o c o r i x a c o l l e c t e d f r o m d i f f e r e n t l a k e g r o u p s . (Lake g r o u p 1: LB 2 . L a k e g r o u p 2: B a r n e s , R o u n d - u p and L y e . L a k e g r o u p 3: G r e e r , Rock and E a s t L a k e s ) . D a t a e x p r e s s e d a s p e r c e n t a g e o f c o r i x i d s s u c c e s s f u l l y f e e d i n g ; and p r o b a b i l i t y be tween l a k e s , f r o m c h i s q u a r e s . C . e x p _ l e t a p e r c e n t f e e d i n g n= LAKE P= C . b i f i d a p e r c e n t f e e d i n g n= p= L A K E FOOD I T E M : D a p h n i i d s 2 : 5 3 . 8 3: 6 9 . 2 13 13 0 . 8 2 3 5 Dead d a p h n . 1: 38 .1 2: 100 .0 21 0 .7351 Dead a m p h i p o d s 2 : 3 3 . 3 3: 0 . 0 10 0 .3594 E p h e m e r o p t e r a n s 8 . 3 12 1 0 . 0 10 0 . 5 4 4 4 C h i r o n o m i d s 1: 7 9 . 2 24 2: 0 . 7 9 9 7 2 : 5 0 . 0 2 3 : 6 1 . 1 5 0 . 0 36 0 . 9 2 6 6 10 118 ( i i ) Feeding p a t t e r n s of a d u l t Cenocorixa c o l l e c t e d at d i f f e r e n t seasons : Since fewest c o r i x i d s were c o l l e c t e d i n s p r i n g , f e e d i n g p a t t e r n s of a d u l t c o r i x i d s c o l l e c t e d i n J u l y and August were compared with those f o r c o r i x i d s c o l l e c t e d l a t e r i n the year (seasons 2 and 3 r e s p e c t i v e l y ; Table XXI). Although there were no s i g n i f i c a n t d i f f e r e n c e s between the d i f f e r e n t seasons with any f o o d s t u f f , C. b i f i d a c o l l e c t e d i n summer fed more cn daphn i i d s and amphipods than d i d those c o l l e c t e d i n f a l l . A s i m i l a r trend was seen f o r C. ex£leta f e e d i n g on zygopteran nymphs, but when t h i s s p e c i e s of c o r i x i d was o f f e r e d dead amphipods, fee d i n g tended to be g r e a t e r i n those c o l l e c t e d i n f a l l than among those from the summer. The r e s u l t s thus i n d i c a t e t hat c o r i x i d s c o l l e c t e d at d i f f e r e n t times of year showed no s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e s i n feeding i n t e n s i t y on any of the prey o f f e r e d . The few n o n - s i g n i f i c a n t trends seen d i d not f e l l o w a c o n s i s t e n t p a t t e r n s e a s o n a l l y f o r a l l foods. T h i s may again be a r e s u l t of the v a r i a b l e , but o f t e n prolonged storage cf c o r i x i d s before t e s t i n g , and may mask r e a l d i f f e r e n c e s i n feed i n g preference or i n t e n s i t y such as seem t o be present i n f i e l d - c o l l e c t e d animals, from s e r o l o g i c a l a n a l y s i s of t h e i r gut contents (Chapter V). 119 T a b l e X X I . C o m p a r i s o n o f f e e d i n g e x p e r i m e n t r e s u l t s f o r a d u l t C e n o c o r i x a c o l l e c t e d a t d i f f e r e n t s e a s o n s . ( S e a s o n 1: M a y - J u n e . S e a s o n 2 : J u l y - A u g u s t . S e a s o n 3: S e p t e m b e r - O c t o b e r ) . D a t a e x p r e s s e d a s p e r c e n t a g e o f c o r i x i d s s u c c e s s f u l l y f e e d i n g , and p r o b a b i l i t y be tween s e a s o n s , f r o m c h i s g u a r e s . FOOD I T E M : C . b i f i d a p e r c e n t f e e d i n g s e a s o n n= P= C. e x p l e t a p e r c e n t f e e d i n g s e a s o n n= P= D a p h n i i d s 2 : 3 : 7 0 . 0 0 . 0 23 4 0 . 2829 2 : 3: 6 9 . 2 5 0 . 0 13 14 0 . 8 7 2 4 Dead d a p h n i i d s 2 : 3: 2 5 . 0 4 6 . 2 8 13 0 . 8 2 4 0 D i a p t o m i d s 2: 3 : 5 8 . 3 0 . 0 12 1 0 . 7428 2 : 3: 3 0 . 0 4 6 . 2 10 13 0 . 8 6 8 5 Dead d i a p t . 2 : 3: 5 3 . 8 6 6 . 7 t 8 13 3 0. 7495 Dead mx. p i . 2 : 3 : 2 0 . 0 3 5 . 3 5 17 0 . 9101 A m p h i p o d s 2: 3 : 5 0 . 0 0 . 0 4 18 0 . 0841 2 : 3: 1 8 . 5 7 . 7 27 13 0 . 7 5 1 0 Dead a m p h i p . 2 : 3: 0 . 0 5 6 . 5 4 23 0 . 3 7 1 5 E p h e m e r o p t . 2: 3 : 0 . 0 14. 3 8 14 0 . 7833 Z y g o p t e r a n s 2: 3 : 5 0 . 0 0 . 0 2 8 0 . 5993 2 : 3: 2 5 . 0 0 . 0 12 9 0 . 4 3 1 2 Dead z y g o p t . 2 : 3: 100 .0 5 8 . 3 2 24 0 . 9 2 8 5 C h i r o n o m i d s 2: 3 : 5 1 . 6 7 3 . 3 31 15 0 . 6588 2 : 3: 7 5 . 0 8 0 . 0 16 10 0 . 8 2 3 2 Dead c h i r o n . 2 : 3: 4 6 . 2 7 7 . 3 13 22 0 . 5 6 1 5 120 ( i i i ) F e e d i n g p a t t e r n s o f C e n o c o r i x a m a l e s and f e m a l e s : Among t h e a d u l t s s t u d i e d , f e m a l e C . b i f i d a a t e mere i t e m s o f f e r e d t h a n d i d m a l e s , b u t t h e same was n o t o b v i o u s i n £ • e x p l e t a ( T a b l e X X I I ) . F e m a l e s o f b o t h s p e c i e s a t e r a t h e r more d e a d f o o d s t h a n m a l e s ( p=0. 3908 f o r C . b i f i d a , p=0 .2531 f o r £ • ® X £ i ® i § ) ° u t r e s u l t s were l e s s c o n c l u s i v e f o r l i v e f o o d s where f e m a l e C . b i f i d a a t e more t h a n m a l e s (p=0.U770) but t h e r e v e r s e was s e e n f o r C . e x p l e t a ( p = 0 . 6 6 2 2 ) . Ma le C. b i f i d a a t e m o r e 1 d i a p t o m i d s t h a n d i d f e m a l e s (p=0.3480) w h e r e a s t h e f e m a l e s a t e more c h i r o n o m i d l a r v a e t h a n d i d m a l e s (p=0. 1 8 7 9 ) . A l l t h e s e t r e n d s a r e s t a t i s t i c a l l y n o n - s i g n i f i c a n t a t t h e f i v e p e r c e n t l e v e l , and r e s u l t s o f a d u l t s a r e n o t t h e r e f o r e c o n s i d e r e d s e p a r a t e l y by s e x i n t h e g e n e r a l a n a l y s i s o f t h e r e s u l t s . ( i v ) F e e d i n g p a t t e r n s o f t e n e r a l and o l d a d u l t s o f C e n c c o r i x a : In f e e d i n g e x p e r i m e n t s a t r e n d was s e e n t o w a r d s more f e e d i n g i n y o u n g t e n e r a l f o r m s t h a n i n t h e o l d e r a d u l t s ( T a b l e X X I I I ) . T h i s was marked i n C . b i f i d a , b u t t h e few d a t a f o r C . e x p l e t a were a n o m a l o u s i n t h a t b o t h f o r m s f e d a t s i m i l a r i n t e n s i t y l e v e l s . No t r e n d s a p p e a r e d t o be s i g n i f i c a n t , and a s most d a t a were n o t d i s t i n g u i s h e d t o t e n e r a l o r o l d e r a d u l t , t h e r e s u l t s were summed f o r f u r t h e r a n a l y s i s . 121 Table XXII. D i f f e r e n c e s i n f e e d i n g p a t t e r n between the sexes of Cenocorixa , from f e e d i n g experiments. Data expressed as percentage of p o s i t i v e f e e d i n g r e s u l t s per experiment. FOOD ITEM : C. b i f i d a C. e x p l e t a female male female male (%) n= (%) n= {%) n= (%) n= Diaptomids 28 .6 14 53. 8 13 45. ,5 1 1 30. 8 13 Dead d i a p t . 87 .5 8 21 . ,5 14 33. 3 12 41. 7 12 Daphniids 52 .4 21 51. 9 17 50. 0 10 63. 2 19 Dead daphn. 14 . 3 14 20. 0 15 66. 7 12 33. 3 18 Mixed plankton . 46 .2 26 42. 9 14 20. 0 5 22. 2 9 Dead mx. p i . 25 .0 8 35. 7 14 33. 3 3 0. 0 3 Amphipods 0 .0 12 18. 8 16 11. 1 18 19. 2 26 Dead amphip. 1 1 . 1 9 18. 7 1 1 55. 6 9 45. 0 20 F a i r y shrimp 33 .3 3 25. 0 " 4 8. 3 12 41. 7 12 Chironomids 71 .4 21 48. 0 25 81. 8 11 73. 3 15 Dead c h i r o n . 37 .5 8 17. 6 17 68. 2 22 66. 7 21 Zygopterans 0 .0 2 25. 0 4 14. 3 7 14. 3 14 Dead zygopt. 31 .3 16 30. 8 13 70. 0 10 50. 0 24 Coleopterans 50 .0 2 0. 0 1 50. 0 6 20. 0 5 Chaoborids 20 .0 5 13. 3 15 33. 3 6 22. 2 18 Dead chaob. 0 .0 1 0. 0 1 0. 0 1 100. 0 1 Ephemeroptera 16 .7 12 0. 0 10 0. 0 7 10. 0 10 Juv. c o r i x i d s 0. 0 2 0. 0 2 Hydracarina 0. 0 1 0. 0 7 0. 0 5 Macrophytes 15 .4 13 14. 3 7 Algae 0 .0 10 0. 0 9 TOTALS 33 .7 205 28. 1 221 40. 4 171 38. 1 247 ALL PLANKTON 38, .4 99 37. 9 87 45. 3 51 39. 2 74 ALL DEAD FOOD 31 .3 66 23. 5 85 58. 0 69 47. 5 101 ALL LIVE FOOD 39, .8 118 34. 2 120 28. 7 101 32. 2 146 122 Table X X f i r . Summary of data on feeding i n t e n s i t y c f t e n e r a l s versus o l d e r a d u l t Cenocorixa. SPECIES: C. b i f i d a TENERALS percent feeding CLE percent feeding 68.8 77 51.2 84 c h i sguare p r o b a b i l i t y p=0.3106 C. e x p l e t a 60.0 15 61.1 18 c h i square p r o b a b i l i t y p=G.7889 123 (v) General feeding p a t t e r n of Cenocorixa a d u l t s : Twenty-one food items common or frequent i n the l a k e s s t u d i e d were t e s t e d with 454 a d u l t C. b i f i d a and 19 feeds with 439 a d u l t C. e x p l e t a . R e s u l t s expressed as percentage acceptance of each f o o d s t u f f are c o n t r a s t e d i n F i g u r e 15. T h i s i n d i c a t e s , f i r s t , t h a t C. e x p l e t a i n general accepted more prey items i n t e s t s than d i d C. b i f i d a , and a l s o that the percentages of items eaten by the two s p e c i e s o f t e n d i f f e r e d markedly. Both s p e c i e s took chironomid l a r v a e most f r e q u e n t l y of a l l foods, but i n g e n e r a l C. e x p l e t a p r e f e r r e d dead organisms to l i v i n g . Thus, when percentage f e e d i n g i s ranked f o r each item of prey, mean rank of a l l dead organisms eaten by C. e x p l e t a was 6.6, compared with 11.4 f o r C. b i f i d a which does not show the same b i a s . The most marked d i f f e r e n c e s between the s p e c i e s on i n d i v i d u a l f o o d s t u f f s e n t a i l e d a g r e a t e r f e e d i n g i n t e n s i t y of C. e x p l e t a than C. b i f i d a upon dead chirencmid l a r v a e (p=0.0639), dead daphniids (p=0.1346), dead amphipods (p=0.1480) and dead zygopteran nymphs (p= 0.3138). On the other hand, l i v e mixed sma l l p l a n k t e r s were r a t h e r more f r e q u e n t l y taken by C. b i f i d a than C. e x p l e t a (p=0.4450). None of these trends are s t a t i s t i c a l l y s i g n i f i c a n t , but they i n d i c a t e a d e f i n i t e tendency of C. e x p l e t a tc p r e f e r dead organisms more than does C. b i f i d a , which seems to take dead and l i v e items about e q u a l l y , and to have a r a t h e r more p l a n k t o n i c food-base. The d i f f e r e n c e i n f e e d i n g between the 124 F i g u r e 15. Percentage acceptance of food items o f f e r e d to a d u l t C. b i f i d a and C. e x p l e t a i n 893 t e s t s . R e s u l t s arranged i n order of acceptance by a d u l t C. b i f i d a : Data f o r C. e x p l e t a shaded. C . b i f i d a £ • e x p l e t a PREY C h i r o n o m i d s D a p h n i i d s Dead d i a p t o m i d s D i a p t o m i d s C o l e o p t e r a n s Dead mixed, p l a n k t o n Mixed p l a n k t o n F a i r y shr imp Dead c h i r o n o m i d s Dead z y g o p t e r a n s Dead d a p h n i i d s C h a o b o r i d s Macrophytes Dead amphipods Ephemeropterans Amphipods Z y g o p t e r a n s Dead ephemeropterans Dead c h a o b o r i d s A l g a e H y d r a c a r i n a J u v e n i l e c o r i x i d s n 6 0 % 8 0 126 s p e c i e s on dead chironomid l a r v a e i s p a r t i c u l a r l y s t r i k i n g . (vi) General f e e d i n g p a t t e r n of Cenocorixa j u v e n i l e s : In each s p e c i e s immature forms showed g e n e r a l l y higher f e e d i n g l e v e l s , i n terms cf percentage acceptance of prey, than d i d a d u l t s . In C. b i f i d a , more j u v e n i l e s than a d u l t s tcok dead p l a n k t e r s , and i n both s p e c i e s , other dead food items were a l s o more eaten by j u v e n i l e s than by a d u l t s . In C. ex£leta , the f i r s t s i x food items ranked by freguency of f e e d i n g f c r a l l j u v e n i l e s combined were dead prey, as were e i g h t out of the f i r s t ten i n C. b i f i d a j u v e n i l e s . Among j u v e n i l e s of Cenocorixa s p e c i e s d i f f e r e n c e s were not as marked as they were i n a d u l t s . A f a i r l y s i m i l a r p a t t e r n of food acceptance was seen f o r members of the same i n s t a r group, i r r e s p e c t i v e of s p e c i e s . T h i s i s shown i n Figure 16, where data are grouped f o r the f i r s t three i n s t a r s and f o r i n s t a r s four p l u s f i v e . In both s m a l l and l a r g e i n s t a r s , C. b i f i d a took r e l a t i v e l y more d a p h n i i d s , dead or a l i v e , than did C. exjrleta . The s m a l l e s t i n s t a r s of C. b i f i d a a l s o took r e l a t i v e l y mere sma l l mixed p l a n k t e r s than d i d those of C. e x p l e t a , although t h i s p o s i t i o n was reversed i n the l a r g e r i n s t a r s . C. exp.leta took more diaptomids than d i d C. b i f i d a i n s m a l l i n s t a r s , but both s p e c i e s took about equal numbers i n i n s t a r s four and f i v e . Numbers of r e p l i c a t e t e s t s were too few f o r d e f i n i t e trends to 127 F i g u r e 16. Percentage acceptance of food items o f f e r e d to s m a l l and l a r g e i n s t a r s of C. b i f i d a and C. e x p l e t a . R e s u l t s arranged i n order of acceptance by a d u l t C. b i f i d a ; data f o r C. e x p l e t a shaded. 1 2 8 PREY Chironomids Daphniids Dead diaptomids Diaptomids Coleopterans Dead mixed plankt, Mixed plankton Fairy shrimp Dead chironomids Dead zygopterans Dead daphniids Dead amphipods Ephemeropterans Amphipods Zygopterans Dead ephemeropt. Algae Hydracarina Dead fairy shrimp || BI n 11 19 25 19 19 1 1 9 1 27 1 1 9 13 1 EX n BI n EX n 37 25 18 21 1 38 28 14 22 27 22 29 37 20 5 2U 18 100 100 So 129 be i n d i c a t e d f o r other prey. J u v e n i l e s of both s p e c i e s seemed to take l e s s chironomid l a r v a e than d i d a d u l t s , and mere dead pl a n k t o n . S i g n i f i c a n t d i f f e r e n c e s i n f e e d i n g i n t e n s i t y cn d i f f e r e n t prey were seen between d i f f e r e n t i n s t a r groups, tut these f o l l o w e d no c o n s i s t e n t p a t t e r n between s p e c i e s and f o r a l l foods. In the main, the youngest i n s t a r s fed l e a s t on the items o f f e r e d , a d u l t s occupied an i n t e r m e d i a t e p o s i t i o n while f o u r t h and. f i f t h i n s t a r s were the most a c t i v e f e e d e r s . T h i s i s e v i d e n t l y r e l a t e d to the f a c t t h a t the l a r g e s t i n s t a r s are a l s o those most a c t i v e l y growing. Table XIX i n d i c a t e s t h a t f i r s t i n s t a r nymphs of both s p e c i e s d i d not feed on any prey o f f e r e d , but only three f o o d s t u f f s were t e s t e d i n only f i v e r e p l i c a t e experiments o v e r a l l . However, second and l a r g e r i n s t a r s fed v o r a c i o u s l y on anything they could t a c k l e , n o t a b l y the s m a l l e r mixed p l a n k t e r s and on dead food at f i r s t , t a k i n g more i n t e r e s t i n l i v e foods at l a r g e r stages. The s p e c i e s have been s u c c e s s f u l l y reared through s e v e r a l generations on f r o z e n b r i n e shrimp (Jansson and Scudder, 1972) suggesting t h a t f i r s t i n s t a r nymphs, i f they feed a t a l l , may be able to u t i l i s e dead animal food, u n l e s s they e x i s t on protozoans or other m i c r o s c o p i c l i f e forms. T h e i r scavenging h a b i t i n the youngest stage would f o l l o w from the i n c r e a s i n g dependence on dead foods with d e c r e a s i n g s i z e cf i n s t a r seen a l r e a d y . 130 ( v i i ) Summary of Cenocorixa f e e d i n g p a t t e r n : O v e r a l l , C. ex£leta s i g n i f i c a n t l y p r e f e r r e d dead to l i v e foods (p=0.0000) although the same trend, seen i n C. b i f i d a , was not s i g n i f i c a n t (p=0.3060). Thus, C. b i f i d a ate a s i g n i f i c a n t l y higher p r o p o r t i o n of l i v e foods than did £• ex£leta (p=0.0023) but v i c e versa f o r dead foods (p=0.0003). T h i s d i f f e r e n c e has been shown above to be a r e s u l t of f e e d i n g d i f f e r e n c e s i n the a d u l t s and l a r g e r i n s t a r s ; C. b i f i d a as immatures p r e f e r s dead foods to l i v e as s t r c n g l y as d i d £• ®X£i®il t but i n l a t e r i n s t a r s t h i s preference becomes l e s s e v i d e n t . S i g n i f i c a n t d i f f e r e n c e s were seen between the s p e c i e s o v e r a l l i n t h e i r consumption of dead chironomid l a r v a e (p=0.0530), dead ephemeropteran nymphs (p=0.0073), dead zygopterans (p=0.0073) and dead amphipods (p=0.0465), i n a l l of which C. ex f i l e t a consumed s i g n i f i c a n t l y more than C. b i f i d a . However, with dead p l a n k t e r s there was no evident d i f f e r e n c e between the s p e c i e s . The same s i m i l a r i t y i n d i e t of the two s p e c i e s was seen f o r a l l l i v e p l a n k t e r s and f o r ephemeropterans, amphipods, b e e t l e l a r v a e and B r a n c h i n e c t a , but C. ex£leta took more chaoborids than d i d C. b i f i d a , although the predator and prey i n t h i s case d i d not c o e x i s t i n the Becher's P r a i r i e l a k e s . In summary, although both s p e c i e s ate the same foc d s , fundamental niches were demonstrably not i d e n t i c a l on c l o s e r examination, and the s p e c i e s cannot t h e r e f o r e be c c n s i d e r e d true e c o l o g i c a l homologues. 131 i j e n t s with c h o i c e cf £rey In p r e l i m i n a r y experiments C. b i f i d a a d u l t s accepted dead diaptomids, l i v e daphniids, chironomid l a r v a e , l i v e diaptomids and l i v e amphipods i n that order of pref e r e n c e , when presented with a mixture of prey. F u r t h e r experiments were t h e r e f o r e c a r r i e d out to determine r e a c t i o n s to s p e c i f i c prey mixtures, and r e s u l t s are given i n Tables XXIV and XXV. Mixed sma l l plankton, c o n s i s t i n g l a r g e l y cf Dia^tgmus s i c i l i s with some Ceriodajghnia s p e c i e s was c o l l e c t e d from Earnes Lake i n J u l y and August, 1972. F i v e r e p l i c a t e subsamples from a plankton c o n c e n t r a t e f o r mid-July, 1972, had a mean r a t i o of 94.2% Diajgtomus to 5.8% Ceriodaphnia . For the s i x dates when 59 C. b i f i d a were t e s t e d , the r a t i o of prey eaten showed a marked r e v e r s a l cf p r o p o r t i o n s i n the t e s t samples, i n d i c a t i n g a d i s t i n c t preference f o r the slower-moving Ceriodajahnia. However, of 96 C. e x p l e t a t e s t e d on e i g h t dates i n the same months, the r a t i o of prey eaten was much c l o s e r t o p r o p o r t i o n s i n the lake at t h a t time (Table XXIV). When dead p l a n k t e r s from the same source were o f f e r e d as prey, p a t t e r n s f o r each s p e c i e s d i d not change markedly, thus d i s c o u n t i n g i d e a s t h a t d i f f e r e n c e s i n c o r i x i d a g i l i t y , c r i n response to p l a n k t e r motion, might produce the r e s u l t s . When l a r g e r p l a n k t e r s were mixed with the s m a l l e r cnes, there seemed to be d i s t i n c t s e l e c t i o n f o r the l a r g e r forms. Other t e s t s using p l a n k t e r s were too few t c be s i g n i f i c a n t . 132 T a b l e XX IV . F e e d i n g s u c c e s s o f C . b i f i d a and C . e x p l e t a when o f f e r e d n a t u r a l m i x e d p l a n k t o n , l i v e a n d ~ d e a d ( N a t u r a l r a t i o o f p l a n k t e r s 942 : 58 D i a p t o m i d s t o C e r i o d a p h n i a ) . PREY L i v e p l a n k t o n D i a p t o m u s C e r i o d a p h n i a C . b i f i d a number e a t e n 5 17 t i m e s t e s t s n a t u r a l n = r a t i o 0 . 2 1 3 . 3 59 C . e x p l e t a number e a t e n 41 10 t i m e s t e s t s n a t u r a l n = r a t i o 0 . 9 3 .4 96 Dead p l a n k t o n D i a p t o m u s 9 C e r i o d a p h n i a 5 0 . 7 6 . 0 25 54 6 1.0 1.7 43 133 Table XXV. Feeding s u c c e s s , expressed as percentage o f s t r i k e s and o f f e e d i n g i n each t e s t , with mixed preys. LAKES PREY : Daphniids Chironomids BARNES C. b i f i d a feed s t r i k e N= <*) (%) 24.1 7 5 . 9 6 5 . 5 3 4 . 5 29 ROCK C. b i f i d a feed s t r i k e N= (55) {%) 2 5 . 0 2 8 0 . 0 4 5 . 0 100 .0 20 L E 2 C. e x p l e t a feed s t r i k e N= {%) (%) 3 7 . 5 3 8 7 . 5 8 5 0 . 0 3 0 0 . 0 Daphniids Zygopterans 5 5 . 0 8 0 . 0 10 .0 5 0 . 0 20 4 1 . 7 150 .0 8 . 3 7 5 . 0 12 7 1 . 4 1 8 5 . 7 14 .3 7 1 . 4 Chironomids Zygopterans 8 3 . 3 9 4 . 4 5 .6 1 0 0 . 0 18 6 5 . 0 125 .0 0 . 0 120 .0 20 4 2 . 9 8 5 . 7 2 1 . 4 1 8 5 . 7 14 134 i n further tests i n 1974, 67 C. b i f i d a from Earnes Lake and 52 from Rock Lake on Becher's P r a i r i e were presented with mixtures of a plankter (Daphnia ), a benthic mud organism that sometimes enters the water column (chironomid) and a l i t t e r a l organism (zygopteran). Results are given i n Table XXV. The reaction of C. b i f i d a to the choice situation was so si m i l a r between the d i f f e r e n t lakes of o r i g i n that i t lends weight to the conclusion, above, that there i s no s i g n i f i c a n t difference i n c o r i x i d behaviour or prey preference attributable to lake of o r i g i n , under the test situation used. Overall, £• b i f i d a f i r s t preferred chironomids, then daphniids, feeding on zygopteran larvae at a much lower l e v e l in choice s i t u a t i o n s . However, when only unsuccessful feeding attacks were tabulated (same Table) attacks were d e f i n i t e l y greatest cn daphniids, followed by almost equal numbers of attacks on the chironomid larvae and the zygopteran nymphs. Hence, although plankters may a t t r a c t the most responses from a predatory c o r i x i d due to t h e i r almost continuous motion and small size, there seemed tc be no d e f i n i t e preference for either chironomids or zygopterans of a d e f i n i t e size although the l a t t e r was less e a s i l y subdued or captured. A further set of experiments with C. expigta from Lake LB 2 was run using the same prey organism combinations, and although fewer organisms were available for testing (Table XXV) results followed a similar pattern to that seen for C. b i f i d a . 135 Foods accepted bj other s p e c i e s In p r e l i m i n a r y t e s t s , s e v e r a l H. l a e v i g a t a a d u l t s were o f f e r e d v a r i o u s foods, and were seen to accept a range of these s i m i l a r to t h a t demonstrated f o r C. b i f i d a and C. e x p l e t a . In p a r t i c u l a r , H. l a e v i g a t a took dead chironomids, l i v e and dead amphipods and zygopteran nymphs, but avoided h y d r a c a r i n a . In the main s e r i e s of experiments a second i n s t a r nymph, i n a d v e r t e n t l y i n c l u d e d f o r t e s t i n g , fed on a dead chircnomid and a f o u r t h i n s t a r took a l i v e zygopteran naiad. Thus, although data f o r t h i s s p e c i e s are scanty, the r e s u l t s i n d i c a t e a s i m i l a r f e e d i n g p a t t e r n f o r H. l a e v i g a t a and Cenocorixa i n terms of foods accepted i n feeding t e s t s . T h i s t o p i c i s i n v e s t i g a t e d f u r t h e r i n the f o l l o w i n g Chapter, where guts of f i e l d - c o l l e c t e d i n s e c t s were examined by s e r o l o g i c a l technigues. DISCUSSION The l i t e r a t u r e on c o r i x i d f e e d i n g extends back almost 200 years. However, s i n c e i t has been summarised to seme extent by Hungerford (1919) and more r e c e n t l y by G r i f f i t h (1945), only key r e f e r e n c e s before the l a t t e r date w i l l be mentioned here. The e a r l i e s t mention of c o r i x i d f e e d i n g h a b i t s i s found i n 136 De Geer (1788) who s t a t e d that they are c a r n i v o r o u s ; t h i s was g e n e r a l l y accepted and c o r r o b o r a t e d with s u p p o r t i n g evidence u n t i l the second decade of the present century. Thus, Westwcod (1871) quoted a f r i e n d ' s r e p o r t t h a t C o r i x a o y i y a r a i n Indian streams fed on the spawn of Barbus sp. Furneaux (190U) d e s c r i b e d c o r i x i d s as " c a n n i b a l s " and Buenc (1909) mentioned t h e i r " e x c e e d i n g l y hot tongue". S i m i l a r deductions were . a l s o made from morphological s t u d i e s , i . e . Dufour (1833), a f t e r d e s c r i b i n g the limbs, concluded that c o r i x i d s were c a r n i v o r e s and Geise (1883) con s i d e r e d the barbed mandibular s t y l e t s to be u s e f u l f o r s e i z i n g simple animals. In the same v e i n , Kalgatz (1911) and o t h e r s s t a t e d t h a t the f o r e l e g s were w e l l adapted f c r grasping s m a l l organisms and c l u t c h i n g them to the mouth while j u i c e s were being sucked out. P o i s s o n (1923) c i t e d t o x i c m a x i l l a r y glands and the s m a l l pharangeal t e e t h as evidence of c a r n i v o r y . However, some morphologists had doubts about the c a r n i v o r o u s s u i t a b i l i t y of the limbs; f o r i n s t a n c e , Eueno (1916) r e v e r s e d h i s e a r l i e r o p i n i o n and suggested t h a t Rham^hocorixa may not i n f a c t be c a r n i v o r o u s , s i n c e i t was "not f i t t e d with the means to s e i z e l i v i n g t h i ngs ". White i n 1873 had c a r e f u l l y watched C o r i x a s c r a p i n g the s u r f a c e of a rock on which were algae and r o t i f e r s , and then p a s s i n g the palae r a p i d l y and a l t e r n a t e l y to the mouth. However, u n t i l 1917 most authors c o n s i d e r e d c o r i x i d s , l i k e other a q u a t i c hemipterans, to be c a r n i v o r e s . In t h a t year Hungerfcrd examined guts of one genus, f i n d i n g d i s o r g a n i s e d p l a n t matter and e n t i r e 137 a l g a e , and concluded that the f a m i l y as a whole represented "producers"; he thus ranked them with the Entomostraca i n importance. He observed a l l i n s t a r s of c o r i x i d s s t r i p p i n g the c h l o r o p h y l l from fil a m e n t o u s algae and, although he a l s o saw them capture l i v e animal prey, he s t a t e d that t h i s d i d not re p r e s e n t the "u s u a l feeding behaviour" (1917a,b, 1919). His morphological s t u d i e s again s t r e s s e d t h i s ; " c o r i x i d s possess the most obvious a d a p t a t i o n of t h e i r limbs to d e f i n i t e v e g e t a t i v e f e e d i n g of any water bug...", the r e l a t i v e l y l a r g e intake c a n a l a l s o a l l o w i n g i n t a k e of some s o l i d s . However, he a l s o recorded f i r s t i n s t a r C. americana as being c a n n i b a l i s t i c (Hungerford, 1923) . About the same time R i l e y (1918) found diatoms and 0§cillatoria i n guts, and he assumed these organisms were c o l l e c t e d from the s u r f a c e of the ooze. However, he poi n t e d out t h a t even the h i g h l y predatory g e r r i d s a l s o feed o c c a s i o n a l l y on vegetable matter, p a r t i c u l a r l y s m a l l f r u i t s , suggesting that p l a n t feeding may s i m i l a r l y not be dominant i n c o r i x i d s , but r a t h e r an i n c i d e n t a l f e a t u r e . Hungerford (1919), however, b e l i e v e d i n a fundamentally m i c r o p h y t i c d i e t f o r c o r i x i d s . S i nce that date, Hungerford's p r e s t i g e has l e d t c c o r i x i d s being g e n e r a l l y considered a l g a l and d e t r i t u s f e e d e r s , although a body of evidence f o r c a r n i v o r y continued to accumulate. Many s t u d i e s showed c o r i x i d s to be sometimes important p r e d a t o r s on mosquito l a r v a e , i n A u s t r a l i a (Hale, 1922), H a i t i (Woclcott, 1927), Alaska (Jenkins, 1948) and Canada (Jenkins and Knight, 1950; James, 1966), mostly summarised i n S a i l e r and Leink 138 (1954) , However, G r i f f i t h (1945) r e i n f o r c e d the m i c r c p h y t i c d i e t i d e a with her s t u d i e s of ghamphocorixa i n Kansas which she reared to maturity on a Ty£ha (reed) i n f u s i o n , although she pointed out t h a t t h i s contained not only algae but protozoans, r o t i f e r s , C y c l o p s , Daphnia, o s t r a c o d s , s n a i l s , clams and chironomids. Stomach contents, although o f t e n unrecognisable, c h i e f l y i n c l u d e d f i n e l y comminuted a l g a l remains. Eungerford (1948), d e s c r i b i n g t h i s as the d i e t of the "average c o r i x i d " , showed evidence of a c o r i x i d sucking Sjairocryra f i l a m e n t s and c i t e d t h i s as " c o n c l u s i v e proof of the herbivorous t a s t e s of a water boatman", s t a t i n g t h a t that the abundance of t h i s food i s an e x p l a n a t i o n of the numerical abundance cf c o r i x i d s i n many water bodies. He suggests, as does Walton (1943 a ) , t h a t e v o l u t i o n of c o r i x i d s r e p r e s e n t s a gradual a l t e r a t i o n from p r e d a t i o n to a m i c r o p h y t i c d i e t , with the s h o r t e n i n g of the s t y l e t s and rostrum, enlargement of the pharynx and mastigatory apparatus and a d a p t a t i o n of the pala from a r a p t o r i a l to a s i f t i n g f u n c t i o n . Most recent r e p o r t s of c o r i x i d f e e d i n g i n c l u d e an o b s e r v a t i o n by Macan (1962) that G l a e n o c o r i s a £ro£ingua captured and fed upon zoo p l a n k t e r s . C r i s p (1962 b) found that a d u l t s of C o r i x a germari fed i n l a b o r a t o r y and f i e l d upon chironomid and l u m b r i c u l i d s , and nymphs ate small chironomid and dead f i r s t and second i n s t a r s of t h e i r own s p e c i e s . Puchkova (1969) o f f e r e d S i g a r a s t r i a t a a mixture of foods, and concluded that phytcphagy was predominant i n t h e i r d i e t s , although v a r i o u s i n v e r t e b r a t e s 139 were a l s o accepted. P l a n k t o n i c crustaceans did not a t t r a c t the bugs i n the same way as d i d the tube-dwelling organisms such as Tu b i f e x and t e n d i p e d i d chironomids. P e t e r s and U l b r i c h (1973) a l s o i n d i c a t e d a predominantly phytophagous d i e t f o r IlickoSSEiiJSlIa SSZiSaSS * D u t S o k e l 1 s k a y a and Zhiteneva (1973) found S i g a r a and Cj[mat-ia s p e c i e s t o be predatory, a t t a c k i n g f i s h f r y . T h i s dichotomy of views between the h e r b i v c r c u s and c a r n i v o r o u s modes of l i f e f o r c o r i x i d s s t i l l continues i n the more general l i t e r a t u r e . Cobben and P i l l o t (1960), Pcpham (1959, 1964) , Parsons (1966) and Fernando (1959) saw c o r i x i d s as c h i e f l y h e r b i v o r o u s , with r e f e r e n c e s to c a r n i v o r y considered as r a t h e r s p e c i a l c a s e s , although Martin (1970) and Pajunen (1970) both d e s c r i b e d c o r i x i d s as important predators i n smal l ponds. Brooks and Kelt o n (1967) and Istock ( 1973) , almost alone, take a more ••omnivorous" viewpoint, although the l a t t e r s t a t e d t h a t no good i n f o r m a t i o n as yet e x i s t e d f o r any s p e c i e s on d i e t . The whole h i s t o r y of research on c o r i x i d feeding c o n t a i n s a s e r i e s of g e n e r a l i s a t i o n s based on s t u d i e s of a few species i n one area with e x t r a p o l a t i o n to other s p e c i e s through s i m i l a r i t i e s i n h a b i t a t and morphology, as pointed out more g e n e r a l l y by Cummins (1973). I t might be t r u e r to say t h a t c o r i x i d s p e c i e s are g e n e r a l l y o p p o r t u n i s t f e e d e r s , seme h i g h l y c a r n i v o r o u s and o t h e r s tending towards microphagy. P a l a r m o d i f i c a t i o n s may not be easy to a s c r i b e to f u n c t i o n ; thus, d e s p i t e t h e i r very d i f f e r e n t p a l a r shapes, both C. americana and S l ^ s n o c o r i s a are powerful swimmers and predators (Walton, 1943 140 b) . Sutton (1951) d e s c r i b e s the v a r i e d uses of the palae; to hold prey, winnow d e b r i s and c r e a t e water curents. Such d i v e r s i t y would l i k e l y p r e c l u d e s p e c i a l i s a t i o n i n cne d i r e c t i o n . Both Walton and Sutton (o££. c i t . ) observed capture and fe e d i n g upon l i v e prey. According to Macan (1962) " c o r i x i d s r a r e l y c o n t a i n anything i d e n t i f i a b l e , and attempts to study t h e i r food have l e d nowhere". Gut examination (Walton, 1943 b; S u t t c n , 1947a; Fernando, 1959) u s u a l l y r e v e a l s a mass of u n i d e n t i f i e d brownish or g r e e n i s h m a t e r i a l with some r e c o g n i s a b l e diatoms. These may come from the a l i m e n t a r y t r a c t of a prey organism, but i t i s l i k e l y t h a t they are a l s o i n g e s t e d d i r e c t l y . Thus feeding h a b i t s can best be e s t a b l i s h e d through other means of d i e t a r y study, such as using r a d i o - a c t i v e t r a c e r s (James, 1960) or s e r o l o g i c a l means (see the next Chapter), through r e a r i n g experiments or through fe e d i n g s e l e c t e d f o o d s t u f f s . Despite the views of Hungerford and G r i f f i t h (C££. c i t . ) there i s only one example of s u c c e s s f u l r e a r i n g through s e v e r a l c o n s e c u t i v e g e n e r a t i o n s on wholly p l a n t m a t e r i a l ; that of Peters and U l b r i c h (1973) who r e p o r t e d t h a t T r i c h o c o r i x e l l a mexicana could be reared on autoclaved mud plus branched algae. These authors a l s o f r e q u e n t l y added Tubifex t o other r e a r i n g c u l t u r e s . Jansson and Scudder (1972) have r e a r e d Cenocorixa s p e c i e s using both l i v e and f r o z e n b r i n e shrimp, while P e t e r s (1962) used Elodea, algae and t u b i f i c i d s to r e a r ten s u c c e s s i v e generations °f K r i s o u s a c o r i x a femorata. I t i s l i k e l y that there i s a need f o r some animal f o o d , perhaps to ensure s u c c e s s f u l r e p r o d u c t i o n . 141 Zwart (1965) suggested t h a t most forms c f animal fcod were adequate f o r c o r i x i d r e p r o d u c t i o n . T h i s animal p r o t e i n need was a l s o suggested by Pajunen (1970a), who s a i d that i n c a p t i v i t y female c o r i x i d s would suck t h e i r eggs at a l e v e l c o r r e l a t e d with food l a c k . The same h a b i t has been observed here f o r Cenocorixa. In the present study C. b i f i d a and C. e x p l e t a were tes t e d i n f e e d i n g experiments with as many f o o d s t u f f s , plant and animal, as c o u l d commonly be found i n the l i t t o r a l of the l a k e s s t u d i e d , to assess the p o t e n t i a l f e e d i n g range of these s p e c i e s under experimental c o n d i t i o n s . R e s u l t s demonstrated a p r i m a r i l y c a r n i v o r o u s nature f o r both Cenocorixa s p e c i e s . E i f f i c u l t y was experienced i n o b s e r v i n g the response of c o r i x i d s t c algae and higher p l a n t s , as a determined movement towards a p l a n t r e s t i n g place might appear s i m i l a r to a feeding a t t a c k cn an animal prey. However, the r a p i d l e g - b e a t i n g of a f e e d i n g c o r i x i d was unmistakeable. The ooze and a l g a l feeding p a t t e r n so f r e q u e n t l y a s s o c i a t e d with c o r i x i d s ( i . e . Hungerford, 1919) was seen o c c a s i o n a l l y ; one sample of c o r i x i d s c o l l e c t e d i n f a l l of 1973 i n Lake LB 2 c o n t a i n e d s e v e r a l a d u l t s whose guts were f i l l e d with a b r i g h t blue-green substance, probably cyanophyte m a t e r i a l . Also, a d u l t s o f f e r e d periphyton l a t e r l e f t g r eyish-green f a e c a l m a t e r i a l which night have been a l g a l , and s e v e r a l guts examined contained r e c o g n i s a b l e diatoms. The p o s s i b i l i t y a l s o e x i s t s that these may have come from the guts cf prey organisms. I t was observed t h a t e a r l y i n s t a r s u s u a l l y picked ever the bottom of the t e s t d i s h , and a d u l t s often s y s t e m a t i c a l l y probed 1 42 with t h e i r mouthparts the p l a s t i c s creening provided f o r them to c l i n g t o , square by square, as i f s e a r c h i n g f o r food. Many r e p o r t s of c o r i x i d s f e e d i n g on a q u a t i c macrophytes are suspected t o be i n f a c t the probing of the s u r f a c e f o r smal l i n v e r t e b r a t e s i n a s i m i l a r manner. Eggs of Enalla^gma s p e c i e s , the ccirmon genus of d a m s e l - f l y i n the s t u d i e d l a k e s , are i n s e r t e d i n t o s l i t s i n submerged v e g e t a t i o n , and f e e d i n g might be upon m a t e r i a l such as t h i s r a t h e r than on the p l a n t s themselves. Evidence of c a r n i v o r y was more c o n c r e t e ; both s p e c i e s seemed to p r e f e r dead foods to l i v e , perhaps being connected with a l i m i t e d a b i l i t y to capture some prey. E l a n k t o n i c organisms and chironomid l a r v a e comprised the majcr feed items. Only a few organisms were i g n o r e d , among these being h y d r a c a r i n a . These may be d i s t a s t e f u l to c o r i x i d s , as they appear to be to many predators (Frost and Smyly, 1952). E l t o n (1923) suggests t h a t t h i s may be p a r t l y a response t c t h e i r red c o l o u r , but feeding on red diaptomids would seem to r e f u t e t h i s . As well as demonstrating c a r n i v o r y , evidence was obtained of a predatory h a b i t , both i n f e e d i n g r e s u l t s and i n the i n c r e a s e d i n t e r e s t evinced by a c o r i x i d i n a moving prey organism. I t appears t h a t d e t e c t i o n of motion need not be by v i s i o n alone, as even prey behind the c o r i x i d were detected and captured, suggesting t h a t the c o r i x i d s as a group are s e n s i t i v e t o pressure changes i n the water. Dahm (1972) noted t h a t underwater v i b r a t i o n s were important s t i m u l i f c f the r e l a t e d hemipteran Notonecta to l o c a t e t h e i r prey, and these c o n s i s t e n t l y responded to i r r e g u l a r motion of models through the 143 water. L i g h t and v i s i o n c o n d i t i o n s did' not a f f e c t n c t c n e c t i d prey capture success, but t h i s was lowered by the presence of dense a q u a t i c v e g e t a t i o n , or by constant d i s t u r b a n c e of the water s u r f a c e . Murphey (1971) a l s o shewed that G e r r i d a e s i m i l a r l y responded to water d i s t u r b a n c e s , through l e g s e n s o r s . Thus the r e l a t e d c o r i x i d group may a l s o r e l y h e a v i l y cn pressure d e t e c t i o n to l o c a t e prey a t a d i s t a n c e . In the experimental s i t u a t i o n used, one majcr problem seemed to l i e i n the d i f f i c u l t y of c o r i x i d s i n i n i t i a l l y d i s c o v e r i n g t h e i r prey organisms; r e f l e c t i o n s from the c o n t a i n e r w a l l s p o s s i b l y adding to problems of o r i e n t a t i o n . The presence of a piece of s c r e e n i n g i n the bottom of each c o n t a i n e r provided some degree of het e r o g e n e i t y and an area f o r the c o r i x i d to c l i n g t o , but i t a l s o acted as a camouflage f o r seme types of prey, and i t s removal at the end of the experiment o f t e n t r i g g e r e d r a p i d p r e d a t i o n on p r e v i o u s l y ignored prey. Once a prey organism i s captured and subdued, perhaps by i n j e c t i o n of the venomous m a x i l l a r y gland s e c r e t i o n s r e f e r r e d to e a r l i e r , i t s body j u i c e s are r a p i d l y sucked out by the a c t i o n of the pharangeal pump. T h i s s t r u c t u r e has been l i n k e d with c a r n i v o r y and the predatory h a b i t , but Hungerford (1917a) has shown t h a t c e r t a i n c o r i x i d s w i l l p i e r c e i n d i v i d u a l c e l l s of a fi l a m e n t o u s green a l g a , and suck out the contents. Further evidence f o r a mixed f e e d i n g h a b i t i n c l u d i n g phytcphagy and d e t r i t u s - e a t i n g has been adduced from the presence of te e t h i n the pharangeal pump, con s i d e r e d to be present tc conminute or s t r a i n out the l a r g e r i n g e s t e d s o l i d s (Slack, 1947; Parsons, 1 44 1957b). Reports of a p e r i t r o p h i c membrane i n the c o r i x i d gut (Sutton, 1951) are apparently r e l a t e d to the presence cf s o l i d food p a r t i c l e s . Parsons (1957a) i n v e s t i g a t e d North American s p e c i e s of S i g a r a , T r i c h o c o r i x a and Hesperccorixa. H i s t o l o g i c a l s e c t i o n s through the midgut o f t e n showed fragments of crustacean exoskeleton w i t h i n the foodmass; these may be i d e n t i f i e d by the b r i s t l e s , and by r e a c t i n g with a t e s t s p e c i f i c f o r c h i t c s a n . In one feeding experiment, a c o r i x i d fed a g r a v i d Eaphnia was found to have s e v e r a l whole daphniid embryos i n the gut, as the s t y l e t c a n a l i s wide enough to allow s o l i d s to pass through (Parsons, 1957a). Also i n the present study and i n other o b s e r v a t i o n s ( i . e . Hungerford, 1948) diatoms were seen i n the gut. However, i t i s p o s s i b l e that these might have come from the alimentary system of a herbivorous prey such as an ephemeropteran or a chironomid. Feeding on red chironomids u s u a l l y r e s u l t e d i n a deep red c o l o u r to the midgut, v i s i b l e i n j u v e n i l e s through the tody w a l l . T h i s c o l o u r r a r e l y p e r s i s t e d to the hindgut r e g i o n , d i g e s t i o n r a p i d l y c o n v e r t i n g the haemoglobins to a brownish mass. On the other hand, orange-pink f a t t y d r o p l e t s c c l o u r e d with c a r o t e n o i d s from diaptomid foods were sometimes seen at the c o r i x i d anus, so that t h i s food could be detected at a l a t e r stage i n the feed i n g process. Red guts were seen i n f i e l d -c o l l e c t e d samples from f i r s t i n s t a r to a d u l t , suggesting diaptomid food at a l l stages of each s p e c i e s . Jansson (1971) and Jansson and Scudder (1974) a l s o mentioned the appearance cf red 1 45 guts i n f i e l d - c o l l e c t e d c o r i x i d s cf a l l i n s t a r s , and suggest t h a t t h i s r e p r e s e n t s f e e d i n g on Dia^tomus s i c i l i s , a l i v e or dead. In the l i t e r a t u r e , a g r e e n i s h or brownish gut has o f t e n been c o r r e l a t e d with a l g a l f e e d i n g ( G r i f f i t h , 1945; Fernando, 1959); t h i s c o l o u r was seen o c c a s i o n a l l y i n f i e l d - c o l l e c t e d c o r i x i d s i n the present study (see Chapter V) but was not recorded i n feeding experiments. Given abundant foods, c o r i x i d s f r e g u e n t l y abandoned h a l f -eaten prey f o r other l i v e ones. These o b s e r v a t i o n s p a r a l l e l l a b o r a t o r y experiments with G e r r i d a e fed excess prey (Jamieson, 1973). Jamieson suggests t h a t such a s i t u a t i o n would be rare i n the f i e l d . However, on Becher's P r a i r i e excess prey might have been f r e g u e n t l y encountered, i n the form of synchronous emergence of c h i r c n o m i d s , or of plankton swarms i n s h o r e . Prey c h o i c e experiments i n d i c a t e d t h a t p l a n k t e r s e l i c i t e d the s t r o n g e s t predatory r e a c t i o n s , d e s p i t e a r e l a t i v e l y low capture success. The f e e d i n g h a b i t of Cenocorixa t a k i n g both l i v e and dead prey c o u l d have added s i g n i f i c a n c e i n t h i s l i g h t , s i n c e temporary plankton blooms or i n s h o r e d r i f t s could be e x p l o i t e d while l i v e , and the excess k i l l e d and p a r t l y eaten would provide food f o r l a t e r p e r i o d s as w e l l as f e e d i n g s m a l l e r i n s t a r s which show the most pronounced preference f o r dead f o o d s t u f f s . The s i n g l e prey f e e d i n g experiments i n d i c a t e d d i f f e r e n c e s i n s p e c i f i c food preference and i n f e e d i n g response to prey o f f e r e d , by sexes, i n s t a r s and s p e c i e s of Cenocorixa . In the main, l a t e i n s t a r s were most voracious, a d u l t s occupied an i n t e r m e d i a t e p o s i t i o n while i n s t a r s one to three fed l e a s t 146 h e a v i l y . T h i s r e f l e c t s the. f a c t that the f o u r t h and f i f t h i n s t a r s are the p e r i o d s of most r a p i d growth and development of a d u l t body s t r u c t u r e s , and consequently maximal food intake i s needed. The p a t t e r n p o s s i b l y a l s o r e f l e c t s h e t e r o g e n e i t y among a d u l t s i n r e p r o d u c t i v e stage, l i n k e d to feeding i n t e n s i t y . Young ad u l t c o r i x i d s may need to feed h e a v i l y i n the t e n e r a l p e r i o d , when egg maturation i s c o n t i n u i n g ; the l e v e l of 67.4% f e e d i n g with 92 t e n e r a l s t e s t e d i s high i n comparison with the mean fe e d i n g i n t e n s i t y of a l l forms. Once the a d u l t e x c s k e l e t c n i s f u l l y hardened, i t i s impossible to d i s t i n g u i s h e x t e r n a l l y whether the c o r i x i d i s at the height of o v i p o s i t i c n . c r i s spent. Reduced fee d i n g might occur i n the l a t t e r s t a t e , but no method was devised to t e s t the r e s u l t s f o r any b i a s that might be thus i n t r o d u c e d . O v e r a l l , C. b i f i d a showed a s l i g h t p r e f e rence f o r l i v e food over dead. However, C. e x p l e t a f a i r l y c o n s i s t e n t l y p r e f e r r e d most types of prey dead to l i v e ; t h i s c o n s t i t u t e s a d i s t i n c t b i o l o g i c a l d i f f e r e n c e between the s p e c i e s at l e a s t i n the c o n t e x t of the experiments, and i g n o r i n g i n the main the e f f e c t s of d i f f e r e n t l i f e s t a g e s. T e s t s showed c o n s i s t e n t l y , i f non-s i g n i f i c a n t l y , g r e a t e r f e e d i n g by C. exjaleta than C. b i f i d a on dead chironomids, dead d a p h n i i d s , dead amphipods and dead zygopterans On the other hand, C. b i f i d a f ed more than C. e x p l e t a on l i v e s m a l l p l a n k t e r s . The low a c t i v i t y s c o r e recorded f o r both s p e c i e s of £§52c.0..Eixa. with chaoborid l a r v a e as prey may i n part be due to 147 the h a b i t of the l a t t e r of f l o a t i n g motionless near the s u r f a c e i n experiments. To C. e x p l e t a from Lake LB 2, where ch a c b o r i d have not been recorded, the prey may to some extent resemble Branchinecta and evoke s i m i l a r f e e d i n g responses. T e s t s c o r e s f o r f e e d i n g on both foods were almost equal and, although these prey sometimes d i f f e r i n s i z e , both have a s i m i l a r l a c k of c o l o u r , hang h o r i z o n t a l or s l i g h t l y i n c l i n e d i n the water, and show r a t h e r s i m i l a r escape movements when approached. Feeding p a t t e r n , as shown by f e e d i n g experiments, d i f f e r e d l e s s between the j u v e n i l e stages than between a d u l t s of each s p e c i e s . Feeding h a b i t s tended to d i f f e r more between c o n s p e c i f i c s of d i f f e r e n t i n s t a r s than between s p e c i e s cf the same i n s t a r ; t h i s i s probably l a r g e l y a f a c t o r of the food a v a i l a b l e t o , and e a s i l y c a p t u r a b l e by, each s i z e - r a n g e of c o r i x i d j u v e n i l e s . In mixed prey experiments, s p e c i e s d i f f e r e n c e s were again seen. Thus, when o f f e r e d mixed plankton, C. b i f i d a s e l e c t e d d a p h n i i d s whether l i v e or dead, whereas C. exjsleta seemed to take both diaptomids and daphniids i n the p r o p o r t i o n s o f f e r e d . I n i t i a l c o n s i d e r a t i o n s suggested that t h i s r e s u l t might be r e l a t e d to the d i f f e r i n g p r o p o r t i o n s of p l a n k t e r s i n s h o r e , where C. b i f i d a predominates, compared to openwater h a b i t a t s where £• i - s p r o p o r t i o n a t e l y more abundant. However, midlake plankton hauls suggested that diaptomids may be l e s s f requent there than i n s h o r e . Perhaps C. e x p l e t a may show s u p e r i o r m o b i l i t y and predatory success; i t s e q u i v a l e n t i n s t a r s are l a r g e r than those of C. b i f i d a , and may be more e f f i c i e n t at 148 c a p t u r i n g t h e i r prey. In other mixed prey experiments, both s p e c i e s behaved s i m i l a r l y , p r e f e r r i n g chironomid over p l a n k t o n i c organisms and t a k i n g zygopteran r e l a t i v e l y r a r e l y . However, u n s u c c e s s f u l a t t a c k s were made most o f t e n on d a p h n i i d s , and there was no s i g n i f i c a n t d i f f e r e n c e between those made on the chircnomid and zygopteran prey o f f e r e d , both being much fewer. B e s u l t s thus s t r o n g l y uphold the i d e a s t h a t c o r i x i d s are f i r s t a l e r t e d to prey presence by motion, which would be more frequent i n a c o n t i n u o u s l y swimming p l a n k t e r than i n the r e l a t i v e l y immobile b e n t h i c and l i t t o r a l organisms. A l s o , Cenocorixa s p e c i e s appear to be r e l a t i v e l y i n e f f i c i e n t p r e d a t o r s , f o r although showing a t y p i c a l l y predatory response to prey movement, they are f r e g u e n t l y u n s u c c e s s f u l i n i t s c a p t u r e . Despite the demonstrated d i e t a r y p r e f e r e n c e s , i t appears t h a t a wide range of food, l i v e or dead but predominantly animal, may be accepted by Cenocorixa , and p o s s i b l y the sympatric H. l a e v i g a t a , i n the f i e l d . Feeding experiments showed no c a n n i b a l i s m , but i n p r e l i m i n a r y t e s t s there were many a t t a c k s on e a r l y i n s t a r s by s t a r v e d c o r i x i d s of both s p e c i e s , and s e v e r a l j u v e n i l e s were found dead, ap p a r e n t l y p i e r c e d through the head r e g i o n and sucked. Many Cenocorixa i n c a p t i v i t y c a r r i e d out a s y s t e m a t i c search of the p l a s t i c s c r e e n i n g i n the t e s t d i s h , and sucked any of t h e i r eggs thus encountered. Other s p e c i e s have a l s o been recorded to suck t h e i r own eggs i n c a p t i v i t y ( S a i l e r , 1948; C r i s p , 1962 b; Pajunen, 1970a) while C. americana s p e c i e s w i l l eat i t s own nymphs, even i n the f i r s t 149 i n s t a r (Hungerford, 1923; Walton, 1943 b). These behaviour p a t t e r n s i n c o r i x i d s need more study, but they r e p r e s e n t i n most forms probably a f e e d i n g h a b i t r e s o r t e d to i n times of s t r e s s or inadeguate food. In summary, r e s u l t s cf f e e d i n g experiments poin t to a predominance of c a r n i v o r y and some predation as a feed p a t t e r n f o r Cenocorixa . Species d i f f e r e n c e s within the genus were not marked, although C. e x p l e t a tended to p r e f e r dead feeds t c l i v e , and the reverse p a t t e r n was seen i n C. b i f i d a . Thus i t would seem t h a t , with regard to f e e d i n g p a t t e r n , fundamental niches of the two s p e c i e s are s i m i l a r i n range, although not i d e n t i c a l i n d e t a i l s . 150 CHAPTER V. SEROLOGICAL INVESTIGATIONS OF GUT CONTENTS INTRODUCTION The p o t e n t i a l of the immune response as a b i o l o g i c a l t o o l was r e c o g n i s e d e a r l y , and while i n use r o u t i n e l y i n medicine f o r •immunisation' i t s i n v i t r o p r o p e r t i e s were a l s o being studied (Crowle, 1961). I t has o f t e n been c a l l e d the ' p r e c i p i t i n r e a c t i o n * because of the v i s i b l e r e s u l t s of mixing, l a y e r i n g or a l l o w i n g to d i f f u s e together, an antigen and i t s corresponding antibody. Thus the p r e c i p i t a t i o n seen becomes d i a g n o s t i c f o r the presence or absence of the antigen, due to the s p e c i f i c nature of the r e a c t i o n . Orlowski et a l . (1972) used the technique t c separate the v i s u a l l y i d e n t i c a l eggs cf two c l o s e l y r e l a t e d s p e c i e s of r o c k f i s h , and i t s use i n taxonomic problems i s widespread ( i . e . Basford et a l . , 1968; Leone, 1947), even at the i n t r a s p e c i f i c l e v e l (Ridgway et a l . , 1962). The technique has even been used to f o l l o w p h y s i o l o g i c a l - m o r p h o l o g i c a l development i n animals, based on the f a c t t h a t j u v e n i l e g l o b u l i n s d i f f e r from those i n a d u l t s (Koeppe and G i l b e r t , 1973; T e l f e r and W i l l i a m s , 1953). The u s e f u l n e s s of the p r e c i p i t i n r e a c t i o n to i d e n t i f y blood meals of e c o n o m i c a l l y important i n s e c t pests has been recognised f o r a long time, and s i n c e N u t t a l l (1904) i t has been used i n f e e d i n g s t u d i e s of mosquitoes (e.g. B u l l and King, 1923; R i d d e l l et a l . , 1947; E l i g h , 1952; Edman et a l . , 1972), t l a c k f l i e s 151 (Downe and M o r r i s o n , 1957) and G l o s s i n a (Emslie and S t e i n b e r g , 1973). The method i n v o l v e s p r e p a r i n g antibody sera s p e c i f i c to d i f f e r e n t p o s t u l a t e d mammalian and avian hosts, and t e s t i n g the gut c o n t e n t s of the f l i e s f o r p o s i t i v e p r e c i p i t i n r e a c t i o n s with these. Somewhat l a t e r , the method was recognised as being p o t e n t i a l l y u s e f u l f o r many other predator-prey r e l a t i o n s h i p s where d i r e c t a n a l y s i s of gut contents or o b s e r v a t i o n s of f e e d i n g p a t t e r n were not p o s s i b l e . Thus Brooke and Prcske (1946) and S e r v i c e (1973) determined the important i n s e c t p redators of j u v e n i l e mosguitoes using the p r e c i p i t i n t e s t . Sergeeva (1970) s t u d i e d p r e d a t i o n on Jurjrgaster s p e c i e s using t h i s technique, as d i d Sutton (1970) on wcodlice. The technique has a l s o been used by Dempster (1958, 1960, 1963; Dempster e t a l . , 1959) to study the predators of broom b e e t l e s , Phytodecta, and by Fox and M a c l e l l a n (1956) i n t h e i r study of c a r a b i d p r e d a t i o n . F i n a l l y , Frank (1967, 1971) has s t u d i e d the feeding of a v a r i e t y of organisms by t h i s technique, and i t has been u t i l i s e d i n s t u d i e s on the feeding r e l a t i c n s h i p s of B r i t i s h t r i c l a d s (Young et a l . , 1964; Eavies, 1969; Pickavance, 1970; Beynoldson and Davies, 1970; Beyncldscn and Bellamy, 1973). west (1950) and H a l l et a l . (1953) have d i s c u s s e d the p r e c i p i t i n t e s t with regard to general i n s e c t p redator-prey r e l a t i o n s h i p s . Reactant sera can be produced i n response to p l a n t antigens as w e l l as to those of animal o r i g i n . Bernhard e t a l . (1969) used an immunofluorescence technique to separate groups of 152 p h y t o p l a n k t e r s , although s p e c i f i c i t y c f the se r a was net high. G e l l et a l . (I960) s t u d i e d the taxonomy of Solanuj s p e c i e s using the p r e c i p i t i n t e s t , and by the same technique Ueckert and Hanson (1971) i n v e s t i g a t e d d i e t a r y overlap i n grasshoppers. Medical understanding of a l l e r g i e s i n v o l v e s the use cf immune t e s t s , as most a l l e r g i e s depend on the s e n s i t i s i n g of the p a t i e n t to p l a n t a l l e r g e n s ( a n t i g e n s ) , o f t e n c a r r i e d i n p c l l e n s . Thus B e r n s t e i n and Safferman (1973) showed c l i n i c a l s e n s i t i v i t y to green algae i n v i t r o , and Weber (1973) reviewed recent s t u d i e s i n v o l v i n g immune r e a c t i o n s to p l a n t s . The h i s t o r y of the technique's development c o n s i s t s e s s e n t i a l l y of i t s refinement and m i n i a t u r i s a t i o n , and i n developing s u i t a b l e s t a i n i n g or f l u o r e s c e n t methods to make the p r e c i p i t a t e r e a d i l y v i s i b l e . Pickavance (1970) has a b l y summarised most major st e p s i n the development of the technique and presented an e m p i r i c a l f o r m u l a t i o n of the most s a t i s f a c t o r y methods. I t i s h i s technique which, minor m o d i f i c a t i o n s , was fo l l o w e d i n t h i s study. The food taken by the two s p e c i e s of Cenocorixa i n nature was assessed by a s e r o l o g i c a l i n v e s t i g a t i o n of the gut co n t e n t s of each s p e c i e s i n a l l o p a t r y and sympatry. In t h i s way i t was hoped to o b t a i n a measure of the r e a l i s e d niches c f each s p e c i e s . 1 5 3 METHODS ££.§£§£ation 2 l s l i d e s and templates Methods f o r s e r o l o g i c a l examination of f i e l d - c o l l e c t e d c o r i x i d guts were taken mainly from Pickavance (1970). The process adopted, i n c o r p o r a t i n g those m o d i f i c a t i o n s found necessary, was as f o l l o w s . Numbered s l i d e s (2.5 by 7.5 cm) were b o i l e d i n a s l i d e rack i n weak detergent f o r 15 minutes, r i n s e d f o r 15 minutes under running tap water and then i n three changes of d i s t i l l e d water, and d r a i n e d . The cleaned s l i d e s were then s l u i c e d i n hot 0.2% ' D i f c o 1 Bacto-agar d i s s o l v e d i n d i s t i l l e d water, then d r a i n e d and d r i e d i n a d u s t - f r e e place on absorbent paper t o w e l l i n g . The t h i n agar f i l m served as a cement f o r the agar g e l l a y e r that was a p p l i e d l a t e r . At t h i s stage, s l i d e s could be s t o r e d i n d e f i n i t e l y , i f a i r was permitted to c i r c u l a t e between them. When needed f o r use, a number (about ten) of s l i d e s were placed with numbers o r i e n t e d and edges opposing, and twc l a y e r s of 1.9 cm Scotch V i n y l p l a s t i c e l e c t r i c tape, each 0.35 mm t h i c k , a p p l i e d so as to enclose a c e n t r a l area cn each s l i d e about 2.5 cm sguare and 0.7 mm deep, i n which the g e l s were to be placed. The l a y e r s of tape were a p p l i e d c l o s e r to the bottom edge of the s l i d e than to the numbered edge f o r r a p i d o r i e n t a t i o n . Masking tape, i n i t i a l l y used f o r cne border to s i m p l i f y o r i e n t a t i o n , was not f u r t h e r used a f t e r i n i t i a l t e s t s confirmed the o b s e r v a t i o n s by Proom (1943) that t h i s may i n t e r f e r e with the s e r o l o g i c a l r e a c t i o n s . 154 Ouchterlony templates were c u t from 3 mm t h i c k perspex s h e e t i n g . E i g h t 1.2 mm holes were d r i l l e d through each 5.5 cm diameter d i s c around a c e n t r a l hole of the same s i z e , and 0.5 cm d i s t a n t from i t ; the nine hole c l u s t e r thus occupied a c e n t r a l p o s i t i o n one cm i n diameter. A l l holes were then p a r t l y r e d r i l l e d to a depth cf 2 mm using a 3 mm b i t , to produce a f u n n e l ( f i g u r e 17). The f i n a l p l a t e s were then assembled as f o l l o w s . An agar-coated, taped s l i d e was warmed s l i g h t l y on a 37 C h o t p l a t e . Then 0.2 ml of molten 1% ,Difco» Bacto-agar made up i n 0.95S s a l i n e s o l u t i o n was p i p e t t e d onto the centr e of the s l i d e and an Ouchterlony p l a t e lowered g e n t l y onto i t . The tapes allowed the agar to flow out to g i v e a l a y e r of uniform t h i c k n e s s , with the f u n n e l s of the Ouchterlony p l a t e i n d i r e c t c o n t a c t with the s u r f a c e . O c c a s i o n a l l y molten agar welled up i n t o the f u n n e l s when the agar was not hot enough; or a i r bubbles were trapped or a i r was drawn through the f u n n e l s by the shrinkage of the s o l i d i f y i n g agar, l e a v i n g an a i r space beneath the Ouchterlony p l a t e . Such s l i d e assemblages were d i s c a r d e d , o t h e r s were placed i n a s a t u r a t e d humidity environment f o r about one hour to harden - i t was found convenient to use a t h r e e l i t r e p l a s t i c c o n t a i n e r with a t i g h t - f i t t i n g l i d , and l i n e d with f i l t e r paper soaked i n d i s t i l l e d water. Fungal growth oc c u r r e d o c c a s i o n a l l y i n t h i s environment as t e s t s were run f o r 48 h at room temperatures; f u n g i grew more p r o l i f i c a l l y i f paper t o w e l l i n g was used i n place of f i l t e r paper. 155 F i g u r e 17. D e t a i l s of s e r o l o g i c a l technique. &: p r e p a r a t i o n of the Ouchterlony p l a t e , B: arrangement of a n t i s e r a i n d i s c f u n n e l s as used i n t e s t s . C: Diagram of f i n i s h e d s l i d e , showing adjacent s t a i n e d p r e c i p i t i n bands. 157 E£€£j|£ation of antigens Based on o b s e r v a t i o n s and experiments on the f e e d i n g p a t t e r n of Cenocorixa s p e c i e s , r e p o r t e d i n Chapter IV, and cn the organisms present i n some abundance i n the experimental la k e s (chapter I I ) , eleven groups of p o t e n t i a l prey organisms were used i n the p r e p a r a t i o n of antigens f c r a n t i s e r a l p r o d u c t i o n (Table XXVI). A l l organisms were c o l l e c t e d with a net, s o r t e d to component groups and s t a r v e d of feed f c r three to f i v e days i n f i l t e r e d water from t h e i r own environments. T h i s water was changed s e v e r a l times during s t a r v a t i o n . Pickavance (1970) s t a r v e d organisms f o r seven days to r i d the gut of f o r e i g n p r o t e i n s , which might have produced n c n - s p e c i f i c r e a c t i o n s , but Young et a l . (1961) used a fou r to f i v e day s t a r v a t i o n p e r i o d . L i v i n g specimens were then b l o t t e d dry and q u i c k - f r o z e n on dry i c e i n 1 - 4 ml a l i g u o t s in s m a l l 'Nalgene' p l a s t i c tubes, then s t o r e d i n a deep-freeze at -20 C. Most organisms were c o l l e c t e d i n summer, 1972, a t the Becher's P r a i r i e f i e l d - s i t e , but some had to be supplemented from other sources, namely Lakes Eunice and Marion i n the U.B.C. Research F o r e s t near Haney, and d i t c h e s i n the U.E.C. Endowment Lands, along Wesbrook Crescent and 16th Avenue. C e t a i l s c f prey organisms and t h e i r c o l l e c t i o n s i t e s are l i s t e d i n Table XXVI. The c o l l e c t e d animals were l a t e r unfrozen and 1 ml aloquots homogenised with equal volumes of 0.9 5? s a l i n e f o r 5 - 1 0 minutes using a ground-glass p e s t l e and mortar with e f f e c t i v e c a p a c i t y of about 5 ml, kept semi-immersed i n crushed i c e to 158 Table XXVI. P o t e n t i a l prey organisms used to produce a n t i s e r a i n order of t h e i r abundance w i t h i n each group, with areas of o r i g i n . ANTIGEN ORIGIN DETAILS 1: DIAPTOMIDS Diaptomus nevadensis (Round-up, Lye) s i c i l i s (Barnes, Round-up, Lye, LB 2) D. jcenae (Eunice L., U.E.C. Research Forest) D. caducus ( U.E.C. Campus) 2: DAPHNIIDS Daphnia magna (Rock, East) D. pulex, D. s c h g d l e r i (Lye, Rcund-up) D. s i mi l i s , C e r i o daphnia sp. (Barnes) 3: AMPHIPODS H y a l e l l a azteca (Greer, Rock, East, Marion) Gam mar us l a c u s t r i s (Greer, Rock, East) 4: CHAOBORUS Chaoborus americanus (East, Eunice) C. n j a b l e i (Eunice) 5: CHIRONOMIDS E i n f e l d i a pagana, G l y p t o t e n d i p e s barbipes ( a l l Becher's P r a i r i e lakes) ^erotanypus a l a s k e n s i s (Lye) E£2£iailius c l a v u s , Chironomus sp. (Barnes) c h i r o n o m i n i (Maricn) 6: EPHEMEROPTERANS C a l l i b a e t i s sp. (Barnes, Round-up, Lye, G r e e r ) . 1: ZYGOPTERANS j£aIIa32S SP« ( a l l Becher's P r a i r i e l akes and U.B.C. Campus). 8: PULMONATES Planorbidae (Rock, Greer, East) Lymnaeidae (Marion, U.b.c.campus) 9 : BLUE-GREEN ALGAE U n i d e n t i f i e d , forms dense blooms, (East) . 10: GREEN ALGAE Cladophora and Spirog_y_ra spp. And other f i l a m e n t o u s forms (Rock, Greer) 11: MACROPH YTES Potamogeton pu s i l l us, Cer atophjjllum demersum, M^riophjr 1 lum spicaturn Ruppia maritima (Greer, Rock, East) 159 prevent heat d e n a t u r a t i o n of p r o t e i n s . The homogenate was then p i p e t t e d i n t o tubes and deep-frozen. A l l homogenates f c r one group of p o t e n t i a l food organisms were then combined and c e n t r i f u g e d at 9000 rpm f o r 15 minutes i n a r e f r i g e r a t i n g u l t r a c e n t r i f u g e . The c l e a r supernatant c o n t a i n i n g the d e s i r e d a n t i g e n i c p r o t e i n s was then decanted and st o r e d i n 1 ml p o r t i o n s at -20 C u n t i l r e g u i r e d f o r i n j e c t i o n . Fickavance (1970) found t h a t t h i s procedure should y i e l d 1% t o 5% p r o t e i n , depending upon prey source, and adeguate to i n i t i a t e antibody p r o d u c t i o n . Davies (1969) used »a few ml* of 0.9% s a l i n e , and t h i s greater d i l u t i o n gave a s o l u b l e p r o t e i n l e v e l of 0.85% to 1.25% p r c t e i n , while Young et a l . (1964) used a 'small volume 1 of s a l i n e to get a p r o t e i n c o n c e n t r a t i o n of 0.4%. The present p r o t e i n l e v e l s were co n s i d e r e d to l i e w i t h i n Pickavance's l e v e l s , although no B i u r e t p r o t e i n estimates were made. P r e p a r a t i o n of a n t i - s e r a Young a d u l t New Zealand r a b b i t s were kept i n outside cages at the U.B.C. Vivarium, and fed a standard d i e t , from which a n t i b i o t i c s were absent to minimise immunological i n t e r f e r e n c e . A f t e r one week's a c c l i m a t i s a t i o n the r a b b i t s were t e s t - b l e d to check f o r any p r e - e x i s t i n g antigens which might i n t e r f e r e with the r e a c t i o n s . Rabbits were hand-held i n t h e i r normal posture d u r i n g a l l b l e e d i n g and immunisation procedures. Before t e s t - b l e e d i n g the ear was shaved and l i g h t l y swabbed 160 with x y l o l to i n c r e a s e blcod flow. Although ambient temperatures were c l o s e to f r e e z i n g at the time, there was a dramatic i n c r e a s e i n ear temperature and d i l a t i o n of blcod v e s s e l s i n the ear. The marginal vein was then nicked and 1 to 3 ml of blood c o l l e c t e d i n a tube, which was then l i g h t l y capped with • P a r a f i l m 1 . Treatment of blood samples i s d e s c r i b e d below. A patch of s k i n about two inches square was then shaved low down on the r i g h t f l a n k to f a c i l i t a t e i n o c u l a t i o n . I n j e c t i o n s were given subcutaneously f o l l o w i n g Pickavance (1970), although Fink and Quin (1953) reported best r e s u l t s with i n t r a - a b d o m i n a l i n j e c t i o n s . A f t e r four days, during which the t e s t - b l e e d i n g was checked, the shaved f l a n k was swabbed with a l c o h o l and cne ml of a n t i g e n was i n j e c t e d subcutaneously using a 1 ml •Tomac1 d i s p o s a b l e s y r i n g e with a number 21 or 27 needle ; the l a t t e r was p r e f e r r e d as i t s s m a l l s i z e made i n j e c t i o n l e s s d i f f i c u l t . Four i n j e c t i o n s of 1 ml antigen each were given a t four-day i n t e r v a l s . Rabbits were then l e f t f o r 21 days to b u i l d up a n t i -b o d i e s , as Pickavance (1970) i n d i c a t e d t h a t , d e s p i t e c c n t r c v e r s y i n the l i t e r a t u r e , the antibody t i t r e u s u a l l y peaked a f t e r t h a t p e r i o d . Seven days a f t e r the f i n a l i n j e c t i o n r a b b i t s were again t e s t - b l e d to check f o r antibody t i t r e l e v e l s . The f i n a l b l e e d i n g d i f f e r e d from the t e s t procedures as both ears were shaved and swabbed with x y l o l . Marginal v e i n s were then s l i c e d a c r o s s and blcod c o l l e c t e d i n 12 ml c e n t r i f u g e tubes. U s u a l l y about 10 ml could be c o l l e c t e d from one i n c i s i o n before reduced blocd flow 161 or c l o t t i n g n e c e s s i t a t e d a f u r t h e r i n c i s i c n . In a l l , afccut 30 to 40 ml of blood was c o l l e c t e d from each r a b b i t , a process t h a t took about h a l f an hour. Rabbits were then checked to see t h a t b l e e d i n g had ceased, and r e t u r n e d to t h e i r cages. The tops of c e n t r i f u g e tubes c o n t a i n i n g blood were covered with p a r a f i l m . Pickavance (1970) suggested that the bleed should be allowed to stand over n i g h t , then c e n t r i f u g e d and the serum p i p e t t e d o f f and f r o z e n . However, ex t e n s i v e haemolysis was o f t e n observed when t h i s was attempted. T h i s phenomenon i s common with r a b b i t blood (Ms. J . Hards, p e r s o n a l communication) and might have been due to x y l o l fumes, the abrupt changes of temperature unavoidable when working out of doors i n winter, c r a number of other causes. Although not c r i t i c a l i n r e l a t i o n to the p r e c i p i t i n t e s t i t s e l f , i t was d e t r i m e n t a l to the o p e r a t i o n of some antiserum t i t r e t e s t s (see below). The standing p e r i o d was thus reduced to s h o r t e r p e r i o d s (under s i x hours) cf c l c t t i n g at l a b o r a t o r y temperatures; c l o t s were then g e n t l y loosened from the edge of the c e n t r i f u g e tubes to minimise t h e i r rupture, and any supernatant present c a r e f u l l y p i p e t t e d o f f . The blood was then l i g h t l y c e n t r i f u g e d i n a BHG Roto-Oni c e n t r i f u g e a t 2000 to 2500 rpm f o r about 5 minutes to compress the c l o t s and r e l e a s e pockets of serum. The serum was drawn o f f and s t o r e d at -20 C. In some cases antibody p r o d u c t i o n was i n s u f f i c i e n t . Rabbits were t h e r e f o r e r e - i n j e c t e d with two weekly booster shots each of 1 ml c o n t a i n i n g a n t i g e n e m u l s i f i e d volume per volume with Freund's complete Bacto-Adjuvant ( C i f c o Chemicals), and bled seven days a f t e r the second i n j e c t i o n . The booster schedule was 162 arranged d i f f e r e n t l y , s i n c e a more r a p i d r i s e occurred i n antibody t i t r e than with o r i g i n a l immunisation (Pickavance, 1970). According to most authors ( i . e . J u r i n and Tannock, 1972) Freund's adjuvant has p r o p e r t i e s of i n t e n s i f y i n g and spreading the s p e c i f i c i t y of the an t i g e n r e a c t i o n . It a l s o has l e s s w e l l p u b l i c i s e d s i d e e f f e c t s , causing apparent pain cn i n j e c t i o n ( t h i s was not observed when an t i g e n s alone were i n j e c t e d ) and f r e g u e n t l y producing a r e a c t i o n at the i n j e c t i o n s i t e , e i t h e r as as open sore or a hard lump under the s k i n . For these reasons i t was not at f i r s t used. However, r e s u l t s produced only weak sera except with chaoborid a n t i g e n , so adjuvant booster shots were necessary i n the p r e p a r a t i o n of a l l other a n t i s e r a . The s e r o l o g i c a l t e s t s Two methods of s e r o l o g i c a l gut study were i n v e s t i g a t e d before a d e c i s i o n was taken to f o l l o w the method of Pickavance (1970). Many authors have f o l l o w e d the method of E u l l and King, who smeared gut contents i n the f i e l d onto f i l t e r paper and allowed i t to dry, then s t o r e d i t i n a d e s s i c a t o r ( i . e . Sutton, 1970), or deep-frozen ( i . e . Davies, 1969). In a v a r i a n t cf t h i s technique, Downe and Morrison (1957) r e p o r t the use of guts from d r i e d , pinned i n s e c t specimens. The smear method using f i l t e r paper was attempted using guts of starved c o r i x i d s and of those f e d diaptomids and chironomids, but f o l l o w i n g r e c o n s t i t u t i o n of the smear with 0.9% s a l i n e and t e s t i n g i t a g a i n s t a n t i s e r a cn an Ouchterlony p l a t e , as d e s c r i b e d below, no p o s i t i v e r e s u l t s of 163 f e e d i n g were obtained, although p a r a l l e l experiments using guts f r o z e n i n s t e a d of d r i e d produced the s e r o l o g i c a l r e a c t i o n s expected. With the a v a i l a b i l i t y of dry i c e and deep-freeze f a c i l i t i e s near the study area the f i l t e r paper smear technique was not i n v e s t i g a t e d f u r t h e r . Immunological r e a c t i o n s were a l s o looked at f o l l o w i n g the methods i n Davies (1969), who used the M o r r i s (1964) v e r s i o n of P r e e r ' s (1956) modified Oakley-Fulthorpe method; t h i s method was e a r l i e r used by B u l l and King (1923) to t e s t mcsquitc meals. The technique i n v o l v e d c o a t i n g the i n s i d e of 4 cm l e n g t h s of 2 mm g l a s s tubing with 0.1 % agar d i s s o l v e d i n d i s t i l l e d water, and l e a v i n g these to dry. A 6 mm column of hot agar p l u s b u f f e r was then p i p e t t e d i n t o the c e n t r e , and egual volumes of antiserum and t e s t a n t i g e n l a y e r e d on e i t h e r end. The tube ends were then s e a l e d with p a r a f i l m and the tube incubated at room temperature f o r t hree days. D i f f i c u l t i e s were encountered i n c e n t r a l i s i n g the median agar plug and i n l a y e r i n g the reagents onto t h i s without t r a p p i n g a i r bubbles. Fungal contamination a l s c a f f e c t e d a s m a l l p r o p o r t i o n of the t e s t s . Although p r e c i p i t i n r i n g s formed i n most t e s t s s a t i s f a c t o r i l y , the method was not continued as i t was time-consuming, used l a r g e volumes of reagents and c o u l d not t e s t more than one antiserum a t cne time. Th e r e f o r e the Pickavance (1970) method was f o l l o w e d i n a l l t e s t i n g of the c o r i x i d s . C o r i x i d samples were unfrozen and placed on a c l e a n g l a s s p l a t e f o r m i c r o s c o p i c i d e n t i f i c a t i o n . The abdomen cf each was d i s s e c t e d from the thorax and the gut removed and macerated i n a 164 d r o p o f 0.9% s a l i n e . T h e r e s u l t i n g f l u i d was p i p e t t e d i n t o t h e c e n t r a l f u n n e l o f an a s s e m b l e d p l a t e , w h i l e t h e p e r i p h e r a l f u n n e l s were f i l l e d w i t h a n t i - s e r a , a r r a n g e d i n a s t a n d a r d p a t t e r n ( f i g u r e 1 7 ) . F u n n e l s were f i l l e d u s i n g d i s p o s a b l e 5 lamda ( m i c r o l i t e r ) m i c r o p i p e t t e s (Drummond H i c r o c a p s ) : w i t h c o a r s e r p i p e t t e s t r a p p e d a i r - b u b b l e s o f t e n p r e v e n t e d t h e r e a g e n t s f r c m r e a c h i n g t h e a g a r s u r f a c e . P l a t e s when s o s e t up were l e f t i n a s a t u r a t e d e n v i r o n m e n t t o i n c u b a t e f o r 48 h s o a s t o e n s u r e c o m p l e t e p r e c i p i t a t i o n , a l t h o u g h the r e a c t i o n was n o t e d t o be w e l l a d v a n c e d i n a few h o u r s . T e s t s were u s u a l l y r u n w i t h 24 c r 36 s l i d e s s i m u l t a n e o u s l y , t a k i n g one t o two h o u r s t o s e t u p . A f t e r 48 h o u r s t h e O u c h t e r l o n y p l a t e s were s n a p p e d o f f the g e l s and t h e t a p e s p e e l e d o f f . To l e e c h o u t u n p r e c i p i t a t e d p r o t e i n s t h e s l i d e s were t h e n s o a k e d o v e r n i g h t i n a p h y s i o l o g i c s o d i u m p h o s p h a t e b u f f e r a t pH 7 . 4 , p r e p a r e d u s i n g G o m o r i ' s method (Humason , 1 9 6 6 , p . 5 0 2 ) . F o r t h i s , s t o c k s o l u t i o n s o f m o n o b a s i c and d i b a s i c s o d i u m p h o s p h a t e were m i x e d i n the r a t i o o f 19 : 81 t o g i v e t h e c o r r e c t p H , and made up t o a 0 .1 M s o l u t i o n . S l i d e s were t h e n r e m o v e d f r o m t h e b u f f e r and s t e e p e d i n two c h a n g e s o f d i s t i l l e d w a t e r f o r a b o u t t e n m i n u t e s i n e a c h . T h e y were t h e n s o a k e d f o r e x a c t l y t e n m i n u t e s i n a p r o t e i n s t a i n , t h e n d e s t a i n e d i n t h r e e b a t h s o f "\% a c e t i c a c i d u n t i l t h e g e l s were c l e a r o r o n l y f a i n t l y p i n k , and t h e p r e c i p i t a t e b a n d s s t o o d o u t a s b r i g h t l y c o l o u r e d a r c s . A r e d s t a i n a l l o w e d d i r e c t p r o j e c t i o n o f t h e s l i d e o n t o g r a d e 3 b r o m i d e p a p e r , a s shown i n 165 F i g u r e s 18 and 19. Acid f u c h s i n i n 1% a c e t i c a c i d was i n i t i a l l y used, but T h i a z i n e red was found to give a s u p e r i o r c o l o u r . S l i d e s were then d r i e d at room temperature i n a d u s t - f r e e p l a c e . T h i s caused the g e l s to s h r i n k to a f r a c t i o n of t h e i r o r i g i n a l t h i c k n e s s and to become permanent mounts, withstanding i n d e f i n i t e storage. S l i d e s were examined by eye, and a l l p r e c i p i t i n bands noted. R e s u l t s of the t e s t s were analysed using a computer programme wr i t t e n by Ms. Dolores L a u r i e n t e , which subdivided the data by food item, s p e c i e s , i n s t a r , l a k e group and season. In a l l , 1872 t e s t s of gut contents which y i e l d e d v a l i d r e s u l t s were used i n computations. One problem i n v o l v e d bonding of the agar g e l tc the s l i d e base. In c e r t a i n batches of s l i d e s the g e l s f l o a t e d o f f the s l i d e s at some stage i n the s t a i n i n g process. S e v e r a l t h i n g s seemed t o account f o r t h i s ; repeated washing duri n g s t a i n i n g may be a minor c a u s a l f a c t o r . However, i t i s l i k e l y t h a t f l o a t i n g o f f of the agar g e l s i s c h i e f l y caused by b a c t e r i a l a c t i o n , s i n c e i n cases where c o r i x i d samples were l e f t unfrozen f o r some hours before t e s t i n g , g e l s subseguently f l o a t e d o f f at a r a t e of n e a r l y one i n three. Unless the g e l was detected before i t had s h i f t e d a p p r e c i a b l y , the t e s t had to be d i s c a r d e d as i t s o r i e n t a t i o n was l o s t . 166 F i g u r e 18. R e s u l t s of some s e r o l o g i c a l experiments, showing t y p i c a l p r e c i p i t i n band pa t t e r n s . A: 3 contiguous bands (amphipods, chironomids, daphniids) . B: 4 bands (chironomids, ephmeropterans, daphniids, zy gopterans). C: f o u r contiguous bands. D: Strong r e a c t i o n with daphniid, weak with amphipod a n t i s e r a . E,F,G: two double bands at d i f f e r e n t l o c i . G: Strong band f o r chironomids; no band f o r zygopterans. I: Bands c l o s e to r a d i a l f u n n e l p o s i t i o n s . (A-D,G,H: note d e p o s i t i o n of suspended m a t e r i a l around f u n n e l openings) . 167 168 F i g u r e 19. R e s u l t s of some s e r o l o g i c a l experiments, showing t y p i c a l p r e c i p i t i n band p a t t e r n s . A: s i n g l e band f o r amphipods. B: S i n g l e band f o r diaptomids. C: S i n g l e band f o r zygopterans (no r e a c t i o n f o r chironomids). D: s i n g l e r e a c t i o n with ephemeropterans (double band). E: S i n g l e band f o r daphniids. F: s i n g l e r e a c t i o n f o r chironomids (mu l t i p l e band). G,H: d i f f e r e n t c o n f i g u r a t i o n s of contiguous r e a c t i o n s with d a p h n i i d s and chironomids. I: s i n g l e r e a c t i o n with chironomids ( s i n g l e band; compare with • f ) . 169 170 Tes t s of the s e r o l o g i c a l method The accuracy of the s e r o l o g i c a l techniques used i n t h i s study was checked i n s e v e r a l ways. S p e c i f i c i t y cf each a n t i -serum was most important; other t e s t s i n v o l v e d i s o l a t i n g fed and s t a r v e d c o r i x i d s f o r v a r y i n g l e n g t h s of time t c estimate how long food remained d e t e c t a b l e i n the gut, and attempts to judge the r e l a t i v e s e n s i t i v i t y or s t r e n g t h of the a n t i s e r a . (i) S p e c i f i c i t y of the a n t i s e r a Sera were t e s t e d on three o c c a s i o n s . F i r s t , r a b b i t s were checked before i n o c u l a t i o n s commenced, f o r any p r e - e x i s t i n g a n t i b o d i e s t hat would r e a c t with the anti g e n s to be i n j e c t e d . R e s u l t s were u n i f o r m l y negative. Then, seven days a f t e r the f i n a l i n j e c t i o n , t e s t - b l e e d i n g s were checked a g a i n s t the range of a n t i g e n s to look f o r an i n c r e a s e i n antibody t i t r e . At t h i s stage most sera gave weak but p o s i t i v e r e a c t i o n s . A f t e r 21 days the f i n a l b l e e d i n g s were performed and sera t e s t e d a g a i n s t the f u l l range of a n t i g e n s . Of these, a g u a t i c macrophyte antiserum alone produced no p o s i t i v e r e a c t i o n s . Filamentous green a l g a antiserum gave a f a i n t but s p e c i f i c r e a c t i o n , and o t h e r s were c o n s i d e r a b l y s t r o n g e r . The 21 day sera were s p e c i f i c f o r chaoborids, zygopterans, ephemeropterans, daphn i i d s , diaptomids and s n a i l s . One cf two t e s t s of the chirononiid antiserum showed a f a i n t r e a c t i o n with 171 zygopteran a n t i g e n , while one of f o u r t e s t s of the amphipod antiserum showed a s i m i l a r f a i n t r e a c t i o n a l s o with d a p h n i i d a n t i g e n . The l a t t e r i s suspected t o be a r e s u l t c f a c c i d e n t a l contamination of t e s t p l a t e s , and was not recorded subsequently. However, the f a i n t zygopteran band using chironomid antiserum was more of a problem as these, the dominant c o r i x i d f o o d s t u f f s , o f t e n co-occurred i n t e s t r e s u l t s . T e s t s were a l s o run using unstarved whole prey organisms i n place of p u r i f i e d a n t i g e n i c s o l u t i o n s , versus a range of a n t i s e r a . These t e s t s gave e g u i v o c a l r e s u l t s , f o r although both r e p l i c a t e s with diaptomids were s p e c i f i c , one of s i x r e p l i c a t e s with chironomids showed a f a i n t r e a c t i o n with zygopteran antiserum i n a d d i t i o n to showing s t r o n g chironomid s p e c i f i c i t y . Three chaoborid r e p l i c a t e s gave s t r o n g responses with chaoborid antiserum and weak ones with daphniid, diaptomid and chironomid a n t i s e r a , probably t y p i c a l d i e t a r y c o n s t i t u e n t s of these forms. The f a i n t r e a c t i o n s o f some chironomid t e s t s with i -zygopteran antiserum, and v i c e v e r s a , were p u z z l i n g as they appeared i n a few t e s t s only (see F i g u r e s 18G, 19C). The s i n g l e r e a c t i o n of chironomids with zygopteran antiserum which appeared i n t e s t s with unstarved forms might i n d i c a t e t e n d i p e d i d p r e d a t i o n on zygopteran eggs or very young nymphs. To e x p l a i n the o c c a s i o n a l l y seen r e v e r s e r e a c t i o n , i t i s p o s s i b l e t h a t the zygopterans c o l l e c t e d f o r anti g e n production were not adeguately s t a r v e d and t h a t some chironomids as prey p e r s i s t e d i n t h e i r guts. Many guts of wild-caught c o r i x i d s showed a r e a c t i o n with 172 o n l y one of these two a n t i s e r a , zygopteran and chironomid, although others r e a c t e d with both, which might be expected i n the w i l d . Since the s i n g l e r e a c t i o n band was o f t e n i n t e n s e , t h i s i n d i c a t e d t h at c r o s s - r e a c t i o n s probably played nc cr a t mcst a n e g l i g i b l e p a r t i n the a c t u a l t e s t s . Thus, a b s o r b t i o n was not performed to i n c r e a s e s p e c i f i c i t y of each antiserum. T h i s process, i n v o l v i n g adding to the serum a n t i g e n i c m a t e r i a l of the unwanted r e a c t a n t to p r e c i p i t a t e i t s a n t i b o d i e s (Downe and Morrison, 1957) reduces antibody t i t r e s t r e n g t h c o n s i d e r a b l y . C o n t r o l s t e s t i n g i n d i v i d u a l a n t i s e r a and antigens with 0.9% s a l i n e s o l u t i o n showed un i f o r m l y negative r e s u l t s , i n d i c a t i n g no contamination of the s a l i n e s o l u t i o n used i n the t e s t s . C o n t r o l c o r i x i d s with a p p a r e n t l y empty guts g e n e r a l l y a l s o shewed neg a t i v e r e s u l t s , although some showed c r o s s - r e a c t i o n s with a n t i s e r a i n d i c a t i n g the presence of some food m a t e r i a l s i n the gut. No a n t i s e r a r e a c t e d with c o r i x i d s themselves, as was i n d i c a t e d by negative r e s u l t s f o r s t a r v e d animals. C o n t r o l c o r i x i d s s t a r v e d f o r s h o r t periods of time did not always produce c l e a r negative r e a c t i o n s , however. C o r i x i d s were fed s p e c i f i c f o o d s t u f f s , then s t a r v e d f o r p e r i o d s of 0 to 90 h before t e s t i n g with a range of a n t i s e r a . In a d u l t s feeds were r e a d i l y d e t e c t a b l e two hours a f t e r f e e d i n g but a f t e r 20 h p r e c i p i t i n bands were absent or weak, and o f t e n not s p e c i f i c to the r e l e v a n t antiserum. S t a r v a t i o n of the younger i n s t a r s f o r 24 h or more produced u n i f o r m l y negative s e r o l o g i c a l r e s u l t s , perhaps r e l a t e d to t h e i r s m a l l e r s i z e . Pickavance (1970) s i m i l a r l y found t h a t a s i z e d i f f e r e n c e i n p l a n a r i a n s a f f e c t e d 173 the time i n which gut contents remained i d e n t i f i a b l e . The length of time i n which guts are c l e a r e d i s not w e l l known f o r c o r i x i d s . In r e l a t e d hemipterans such as g e r r i d s , which are a l s o known to feed very o c c a s i o n a l l y cn vegetable matter ( R i l e y , 1918) guts are c l e a r e d i n four to e i g h t hours at room temperatures (Jamieson, p e r s o n a l communicaticn) , but with c o r i x i d s with some p a r t i c u l a t e matter i n the guts t h i s may take r a t h e r l o n g e r . Leeches and p l a n a r i a n s may hold food i n the gut f o r weeks, although the food substance i s s e r o l o g i c a l l y i d e n t i f i a b l e f o r up to f i v e days o n l y , before d e n a t u r a t i o n of the p r o t e i n s (Davies, 1969). I t i s safe to assume f c r c o r i x i d s t h a t foods eaten more than 24 hours p r i o r to t e s t i n g would produce weak or no s e r o l o g i c a l r e a c t i o n s , e s p e c i a l l y i n the s m a l l e r i n s t a r s . ( i i ) Strength of the a n t i s e r a A n t i s e r a l s t r e n g t h was t e s t e d i n c c r i x i d s r e c e n t l y f ed s p e c i f i c foods; s i n c e f o r f i e l d c o l l e c t e d forms there i s a l a c k of knowledge of how long p r i o r to capture they had i n g e s t e d foods, and i n what q u a n t i t y . T i t r e checks were run on most of the a n t i s e r a t c estimate t h e i r s t r e n g t h . R e s u l t s were u n s a t i s f a c t o r y using the method of Proora (1943), suggested a l s o by Pickavance (1970), i n which a n t i s e r a d i l u t e d with 0.9% s a l i n e were t e s t e d by adding the r e l e v a n t antigen to each d i l u t i o n and to a c o n t r o l tube 174 c o n t a i n i n g 0.9% s a l i n e a l o n e , and then i n c u b a t i n g and examining f o r s i g n s of p r e c i p i t a t i o n . P r e c i p i t a t e s were obscured by c l o u d i n e s s i n most samples, a p p a r e n t l y as a r e s u l t of the haemolysis of the bleed during c o l l e c t i o n , r e s u l t i n g i n dark p r e c i p i t a t e s or sediments i n the serum tubes. These d i d not a f f e c t the Ouchterlony t e s t s , s i n c e plasma was deposited around the p o i n t where each funnel met the agar g e l ( f i g u r e s 18 and 19), but i t rendered the method of Proom unworkable. T i t r e s t r e n g t h was t h e r e f o r e t e s t e d by using the r e g u l a r Ouchterlony p l a t e s . D i l u t i o n l e v e l s of antiserum ranged from f u l l s t r e n g t h through 1:50, 1:100, 1:200, 1:400, 1:8C0, 1:1600 and 1 : 3200 and a c o n t r o l f u n n e l c o n t a i n i n g only 0.95! s a l i n e . R e s u l t s from t h i s t e s t i n d i c a t e d a minimum t i t r e s t r e n g t h of 1:50 f o r chaoborid, daphniid and diaptomid a n t i s e r a ; of 1:400 f o r ephemeropteran, amphipod, chironomid and zygopteran a n t i s e r a , and no r e a c t i o n s , even a t f u l l s t r e n g t h , f o r higher p l a n t antiserum. Reactions were seen at f u l l s t r e n g t h only with f i l a m e n t o u s green a l g a antiserum, while no blue-green alga serum was a v a i l a b l e f o r t e s t i n g . Pickavance (1970) suggested that a • s a t i s f a c t o r y ' l e v e l of t i t r e s t r e n g t h f o r h i s a n t i s e r a was a response with a d i l u t i o n of 1:8000, while Young et a l . (1964) recorded a d i l u t i o n s e n s i t i v i t y of 1:512 at best. These are higher than the present l e v e l s , even a l l o w i n g f o r lower s e n s i t i v i t y of the Ouchterlony d o u b l e - d i f f u s i o n method than i n Proom's tube method (Davies, 1969). However, Pickavance's l e v e l p e r t a i n s t o P l a n a r i a , which 175 feed i n f r e q u e n t l y , and an a c c e p t a b l e l e v e l f o r c o r i x i d s need not be n e a r l y as high, due to t h e i r d i f f e r e n t f e e d i n g h a b i t s . Indeed, s i n c e whole guts are examined, r e a c t i o n with an antiserum d i l u t i o n of 1:50 would ensure the r e c o r d i n g of foods r e p r e s e n t i n g as l i t t l e as 2% of the gut contents. In t e s t s the weaker a n t i s e r a would be expected t c produce g e n e r a l l y weaker p r e c i p i t i n bands than the o t h e r s f o r an e q u i v a l e n t meal s i z e , and not to r e c o r d these foods i n s m a l l e s t q u a n t i t i e s . Since c o r i x i d s feed f r e q u e n t l y and are capable of f e e d i n g on a d i v e r s e c o l l e c t i o n of organisms (chapter I V ) , some foods might be detected i n s m a l l e r amounts than o t h e r s . Against t h i s must be s e t the problems of d i f f e r e n t s i z e s cf each food organisms, depending somewhat upon t h e i r i n s t a r s . So many unknowns a r i s e here t h a t i t i s s a f e s t to ignore d i f f e r e n c e s i n band s t r e n g t h when q u a n t i f y i n g d i e t s . In an organism f e e d i n g f r e q u e n t l y , as do c o r i x i d s , the d i e t a r y turnover i s g r e a t e r than i n something l i k e a p l a n a r i a n ; t h e r e f o r e i t i s not as c r u c i a l to te able to recognise t r a c e s of p r e v i o u s meals. One f u r t h e r p o i n t remains, the number of feed items t h a t can be detected i n one gut by the t e s t method. Mo e m p i r i c a l t e s t s were run u s i n g double or t r i p l e feedings cf s p e c i f i c foods, but s e v e r a l t e s t s used a mixture of diaptomids and d a p h n i i d s crushed and t e s t e d a g a i n s t the range of a n t i s e r a as i n a gut contents t e s t , and these re a c t e d with t h e i r r e s p e c t i v e a n t i s e r a , producing two bands i n each t e s t . Since nearly a l l t e s t s using animals fed a s i n g l e f o o d s t u f f produced a s i n g l e 176 r e a c t i o n band (see s e c t i o n above on s p e c i f i c i t y of a n t i s e r a ) , i t can be i n f e r r e d t h a t the method i s capable of r e v e a l i n g s e v e r a l foods s i m u l t a n e o u s l y . I t i s thus s u p e r i o r t c the Oakley-F u l t h o r p e method. F i g u r e s 18 and 19 i n d i c a t e the number and range of r e a c t i o n s seen i n a c t u a l t e s t s cf f i e l d - c o l l e c t e d c o r i x i d s ; r e a c t i o n s of adjacent a n t i s e r a are separable from each other by the angle of the band to the r a d i u s , even when band ends are c o n f l u e n t . RESULTS The g e n e r a l c o r i x i d food p a t t e r n f o r c e r t a i n l a k e s i n c e n t r a l B r i t i s h Columbia was determined by s e r o l o g i c a l a n a l y s i s of gut contents of 1879 c o r i x i d s of s i x s p e c i e s , l i s t e d i n Table XXVII. These guts were t e s t e d with 11 a n t i s e r a , one of which (aquatic macrophytes) proved to be n o n - r e a c t i v e . In a l l c o r i x i d s p e c i e s l e s s than one percent of antiserum r e a c t i o n s were p o s i t i v e f o r blue-green algae and s n a i l s , and most a n a l y s e s showed a p o s i t i v e r e a c t i o n with chironomid and zygopteran a n t i s e r a , i n d i c a t i n g a pronounced c a r n i v o r o u s h a b i t f e r these C o r i x i d a e (Table XXVIII). Despite major s i m i l a r i t i e s between the o v e r a l l f e e d i n g p a t t e r n s of each s p e c i e s , d i f f e r e n c e s i n the food spectrum were seen i n these analyses. T h i s i s to be expected s i n c e c o r i x i d s p e c i e s i n the more freshwater l a k e s s t u d i e d would encounter 177 T a b l e XXVII. Summary of s e r o l o g i c a l data f o r s i x c o r i x i d s p e c i e s . Number of ti m e s each a n t i s e r u m was t e s t e d w i t h each s p e c i e s . CORIXID : ANTISERUM Diaptomid D a p h n i i d Amphipod C h a o b o r i d C h i r o n c m i d Ephemeropt Zygopteran Pulmonatan B l u e - g r e e n Cladophora Macrophyte Ceno-c o r i x a b i f i d a 1232 1109 1232 1232 1215 1218 1215 421 80 3 17 102 £§£2" Hespero- C a l l i - C_jmat i a Sicjara £crixa c o r i x a c o r i x a a m e r i - s p e c i e s e x ^ l e t a l a e v i g a t a audeni -cana 565 509 565 565 565 565 565 168 394 0 33 38 33 38 38 37 38 38 15 17 0 11 23 17 23 23 23 23 23 22 1 0 6 10 10 10 10 10 10 10 0 3 0 7 0 0 0 TOTALS TESTED 1232 565 38 23 10 11 178 Table XXVIII. Summary of s e r o l o g i c a l data f o r s i x c o r i x i d s p e c i e s . Percentage of p o s i t i v e r e a c t i o n s to each antiserum i n t e s t s . CORIXID : Ceno-c o r i x a b i f i d a Ceno-c o r i x a e x p l e t a Hespero-c o r i x a l a e v i g a t a C a l l i c o r i x a audeni C y j a t i a aroeri-- cana Sigar, specie ANTISERUM : Diaptomid 14.9 22.8 5.3 0.0 10.0 9. 1 Daphniid 28. 3 28. 5 3.0 0.0 20.0 9.1 Amphipod 4.1 0.0 2.6 8.7 10.0 0.0 Chaoborid 2. 4 1.4 18. 4 0.0 0.0 0.0 Chironomid 75.4 69.6 86.5 73.9 80.0 81.8 Ephemeropt 23. 7 21. 1 21. 1 26. 1 0.0 0.0 Zygopteran 56.6 55.9 63.2 60.9 60.0 63. 6 Pulmonatan 0.7 0.6 0.0 0.0 0.0 0.0 Blue-green 0.6 0.0 0.0 0.0 0.0 0.0 Cladophora 17. 7 0. 0 0.0 0.0 0.0 0.0 Macrophyte 1.0 0.0 0.0 0.0 0.0 0.0 TOTALS TESTED 1232 565 38 23 10 11 179 such p o t e n t i a l food items as amphipods and chaobcrids i n abundance, organisms which are absent from or very s c a r c e i n the higher s a l i n i t y l a k e s . Conversely, c o r i x i d s i n s a l i n e l a k e s would be exposed i n nature to such forms as diaptomid copepods or f a i r y shrimps, which do not occur i n the lower s a l i n i t y water bodies (Chapter I I ) . A l l o p a t r i c and sympatric p o p u l a t i o n s cf C. b i f i d a and £. I2Ei§.£a may be fe e d i n g r a t h e r d i f f e r e n t l y i n nature, due a t l e a s t p a r t l y to t h i s d i f f e r e n c e i n occurrence of many food-organisms, and perhaps a l s o to some i n t e r s p e c i f i c i n t e r a c t i o n process. Thus, s i n c e food taken may d i f f e r between i n s t a r s , seasons and l a k e s ; i n s t a r , season and lake data must be cons i d e r e d s e p a r a t e l y f o r each s p e c i e s . R e s u l t s f e r the two major s p e c i e s of c o r i x i d s , C. b i f i d a and C. e x p l e t a , are con s i d e r e d f i r s t and i n most d e t a i l , s i n c e most data are a v a i l a b l e f o r these. Feeding p a t t e r n of C. b i f i d a Tables XXIX to XXXV l i s t the f e e d i n g data, from s e r o l o g i c a l a n a l y s i s , f o r each i n s t a r of C. b i f i d a , broken down by lake groups and seasons. As would be expected from a knowledge of the l i f e c y c l e of these i n s e c t s , f i r s t i n s t a r s were corcmcnest i n May and June c o l l e c t i o n s (season 1), and t h i r d to f i f t h i n s t a r s were abundant i n midsummer. Almost no young i n s t a r s were a v a i l a b l e f o r c o l l e c t i o n i n f a l l (Chapter I I I ) , l i m i t i n g the s e a s o n a l 180 comparisons t h a t can be made. I n 108 f i r s t i n s t a r nymphs t e s t e d ( t a b l e XXIX) o n l y z y g o p t e r a n s were r e c o r d e d s i g n i f i c a n t l y (at the 5% l e v e l ) more o f t e n i n guts o f c o r i x i d s c o l l e c t e d i n the more f r e s h w a t e r l a k e s t h a n i n tho s e from t h e more a l k a l i n e l a k e s (p=0.03). Out of 205 second i n s t a r nymphs examined ( t a b l e XXX) t h i s z y g o p t e r a n food p a t t e r n was a g a i n seen, but as a n o n - s i g n i f i c a n t t r e n d (p=0.10). S i g n i f i c a n t l y more p o s i t i v e p r e c i p i t i n r e s u l t s were r e c o r d e d w i t h d a p h n i i d a n t i s e r u m (p=0.02) i n f i r s t and second i n s t a r l a r v a e from the more a l k a l i n e l a k e s compared w i t h the f r e s h w a t e r l a k e s . No p o s i t i v e r e a c t i o n s were r e c o r d e d f o r amphipods or c h a o b o r i d s i n f i r s t and second i n s t a r c o r i x i d s from t h e more a l k a l i n e l a k e s , where t h e s e p o t e n t i a l food items do net o c c u r r e g u l a r l y . I n both l a k e g r o u p s , ephemeropterans were more f r e q u e n t l y r e c o r d e d i n t h e d i e t o f f i r s t and second i n s t a r s i n summer than i n s p r i n g (p=0.02 f o r the more a l k a l i n e l a k e s ; p=0.07 f o r t h e more f r e s h w a t e r l a k e s ) . In t h e gu t s of 224 t h i r d i n s t a r C. b i f i d a t e s t e d ( t a b l e X X X I ) , the number o f p o s i t i v e r e a c t i o n s t o d a p h n i i d s d i f f e r e d between l a k e s i n both s p r i n g (p=0.01) and summer (p=0.02); i n both seasons d a p h n i i d s were r e c o r d e d s i g n i f i c a n t l y more f r e q u e n t l y i n gu t s from h i g h e r s a l i n i t y l a k e s . A s i m i l a r though s t a t i s t i c a l l y n o n - s i g n i f i c a n t t r e n d was seen f o r d i a p t o m i d s (p=0.06) i n summer samples. More t h i r d i n s t a r guts c o n t a i n e d ephemeropterans i n summer than i n s p r i n g samples i n both l a k e groups (p=0.00 f o r the more a l k a l i n e l a k e , but r e s u l t s were non-s i g n i f i c a n t f o r the more f r e s h w a t e r b o d i e s ) . 181 Table XXIX. Feeding pat t e r n of f i r s t i n s t a r C. b i f i d a arranged by l a k e s and seasons. Data expressed as percentages fe e d i n g cn each f e e d s t u f f , and t o t a l s examined, f o r each lake and season. |Serum | lake 2 1 lake 2 I lake 2 | lake 3 I lake 3 | lake 3 I t o t a l £ !_ | s p r i n g | sum mer I f a l l H |spring | summer | f a l l -4 |Diapt I 0.0 29 | 16. 7 6 | I 0.0 72 | 0.0 11 I 0.9 108 | Daph I 3.5 29 | 0.0 4 I I 2.8 72 | 0.0 1 | I 2.8 106 | A oophi | 0.0 29 | 0. 0 6 I I 0.0 72 | 0.0 1 | I 0.0 108 |Chaob | 0.0 29 | 0. 0 6 I I 0.0 7 2 | 0.0 1 | I 0.0 108 |Chi r c I 34.5 29 | 16. 7 6 I I 50.0 72 | 100.0 1| I 44.4 108 1 Eph I 0.0 29 | 16. 7 6 I I 1.4 72 | 100.0 1| I 2.8 108 1 zyg I 3.5 29 | 16. 7 6 I | 30.6 72 | 100.0 11 I 23. 1 108 I Pulm 1 0. 0 6 I I 0.0 12| 0.0 1 | I 0.0 19 | Blue-g I 0.0 29 | I I 0.0 60| I 0.0 89 | Macro 1 0.0 2 ', I 0.0 2 J - j j . _ _ -L 182 Table XXX. Feeding habits of second instar C. bifida arranged by lakes and seasons. ~ ~ Data expressed as percentages feeding on each foodstuff, and totals examined, for each lake and season. |Serum | j ^ lake 2 | spring | f lake 2 summer lake 2 f a l l -+ lake 3 spring lake 3 lake summer f a l l -I .j 3 totals |Diapt | 1.4 73 | 9. 1 22 ! I 6. 5 92 |0.0 18 | I 4.4 205 | Daph | 13.7 73 | 11. 1 9 ! |2. 2 92 |0.0 18 | I 6.8 192 |Amphi | 0.0 73 | 0. 0 22 I | 2. 2 92 10.0 18 | I 1.0 205 | Chaob | 0.0 73 | 4. 6 22 ! 12.0 92 |5.6 18 | | 6.3 205 IChiro | 69.9 73 | 90. 9 22 ! 77. 2 92 100.0 18| 78.0 205 1 Eph | 1 .4 73 | 18. 2 22 i |3. 3 92 27.8 18 | | 6.3 205 izyg 1 31.5 73 | 54.5 22 ! 53. 3 92 66.7 18 | 46.8 205 |Pulm | 0.0 20 | 0. 0 21 ! |4. 2 24 10.0 18 | | 1.2 83 | Blue-g| 1.9 53 | 0.0 1 ! | 0. 0 69 I I |0.8 123 |Macro | 1 L 0. 0 13 i I I 1 0.0 13 183 Table XXXI. Feeding pat t e r n of t h i r d i n s t a r of C. b i f i d a arranged by l a k e s and seasons ~ Data expressed as percentages f e e d i n g on each f o o d s t u f f , and t o t a l s examined, f o r each lake and season. r T T — i — |Serum I l a k e 2 | lake 2 I l a k e 2 | l a k e 3 I lake 3 |lake 3 1 t o t a l s [ |s p r i n g |summer | + + f a l l | s pring | -I -j summer | f a l l _{ |Diapt I 1 6 . 4 61 | 2 7 . 7 101 | 0 . 0 1 I 4 . 7 4 3 | 0 . 0 18 | | 1 7 . 9 224 | Daph | 4 9 . 2 61 | 4 0 . 7 91 | 0 . 0 1 I 1 4 . 0 43 J 0 . 0 1 8 | | 3 4 . 1 214 | A mphi I 0 . 0 61 I 0 . 0 101 | 0 . 0 1 I 2 . 3 4 3 | 5 . 6 18 | I 0 . 9 224 |Chaob I 3 . 3 61 I 2 . 0 101 | 0 . 0 1 I 2 . 3 43 J 1 1 . 1 18] I 3 . 1 224 | C h i r o | 8 2 . 0 61 | 8 4 . 2 101 | 1 0 0 . 1 I 8 8 . 4 4 3 | 7 7 . 8 18 1 I 8 3 . 9 224 1 Eph | 1 6 . 4 61 | 5 6 . 4 101 | 0 . 0 1 I 9 . 3 43 | 2 2 . 2 1 8 | | 3 3 . 5 224 I zyg | 5 0 . 8 61 I 7 6 . 2 101 | 1 0 0 . 1 I 7 9 . 1 4 3 | 4 4 . 4 1 8 | I 6 7 . 4 224 | Pulm I 0 . 0 12 I 2 . 3 43 | I o.o 1 0 | 0 . 0 18 | I 1 .2 83 |Blue-g I 0 . 0 49 I 1. 8 57 | 0 . 0 1 I 0 . 0 33 | I 0 . 7 140 |Macro I I 0 . 0 3 | ! | 0 . 0 3 - J 184 Table XXXII i n d i c a t e s t h at f o r 217 f o u r t h i n s t a r s examined, there were again more p o s i t i v e r e a c t i o n s to daphniids (p=0.0 1) and diaptomids (p=0.06) i n summer samples c o l l e c t e d i n higher a l k a l i n i t y l a k e s than i n the lower a l k a l i n e l a k e group (lake group 3). Ephemeropterans were again more f r e g u e n t l y recorded i n f o u r t h i n s t a r guts c o l l e c t e d i n the summer i n a l k a l i n e lakes than i n s p r i n g (p=0.01). A s i m i l a r r e s u l t was fcund with diaptomids (p=0.08), but the opposite s i t u a t i o n was observed f o r d a p h n i i d s (p=0.01). In a l k a l i n e lake samples o c c a s i o n a l r e a c t i o n s were recorded with chaoborid and amphipod antiserum; however, these organisms have only o c c a s i o n a l l y been rec o r d e d i n Round-up Lake (see Chapter I I ) , and thus may not i n f a c t be a v a i l a b l e i n the normal environment as food. These p o s i t i v e r e a c t i o n s may a l t e r n a t i v e l y r e p r e s e n t a spreading of s p e c i f i c i t y from r e l a t e d crustaceans f o l l o w i n g the use of an adjuvant i n antiserum p r e p a r a t i o n . O c c a s i o n a l l y , a s i m i l a r r e a c t i o n occurred with diaptomid antiserum i n guts c o l l e c t e d i n the more freshwater l a k e s , where diaptomids do not occur (chapter I I ) . Table XXXIII l i s t s data f o r 96 f i f t h i n s t a r nymphs of C. b i f i d a , most of which were c o l l e c t e d i n the higher s a l i n i t y l a k e s . Gut content r e a c t i o n p a t t e r n s were g e n e r a l l y s i m i l a r between l a k e s and seasons. Again, o c c a s i o n a l r e c o r d s f o r p o s i t i v e r e a c t i o n s to amphipods and chaoborids occurred with i n s e c t s taken from the higher s a l i n i t y l a k e s . Most of the 175 a d u l t female C. b i f i d a examined were 185 Table XXXII. Feeding p a t t e r n of f o u r t h i n s t a r of C. b i f i d a arranged by l a k e s and seasons. ~ Data expressed as pecentages f e e d i n g on each f o o d s t u f f , and t o t a l s examined, f o r each lake and season. |Serum | l a k e 2 I lake 2 I lake 2 | lake 3 J lake 3 |lake 3 | t o t a l s h s p r i n g h |summer | 4- -r f a l l -+ s p r i n g Isummer | f a l l „_{ |Diapt 10.9 46 | 28. 3 113| 50.0 2| 0.0 1 I 10.9 55 | |20.3 217 | Daph 69.6 46 131.8 107 | 50.0 1 | 100.0 1 I 2.9 34| |36.5 189 |Amphi 0.0 46 I 0.9 113 | 0.0 2| 0.0 1 I 9.1 55 | | 2.8 217 | Chaob 4.3 46 I 1.8 113 | 0.0 2| 0.0 1 I 1.8 55 | | 2.3 217 |Chiro 80.4 46 | 77. 9 113| 50.0 2| 100.0 1 I 92.7 55 | | 82.0 217 1 Eph 8.7 46 | 39. 8 113| 0.0. 2| 0.0 1 I 30.9 55 | |28.6 217 | Zyg 54. 3 46 | 68. 1 113| 50.0 2| 0.0 1 | 70.9 55 I |65.U 217 | Pulm 0.0 19 I 0.0 43 | I 0.0 1 I 0.0 55 | | 0.0 118 | Blue-g 0.0 27 I 0.0 70 | 0.0 2| ! | 0.0 99 |M aero I 0.0 1 | '| I 0.0 21 | | 0.0 22 185 Table XXXII. Feeding p a t t e r n of f o u r t h i n s t a r cf C. b i f i d a arranged by lakes and seasons. ~ Data expressed as pecentages feeding cn each f e e d s t u f f , and t o t a l s examined, f o r each lake and season. r T r r — T T T Serum l l a k e 2 I s p r i n g l l a k e 2 | summer lake 2 I f a l l lake 3 s p r i n g lake 3 |lake 3 tsummer j f a l l | t o t a l s |20.3 217 Diapt 110.9 46 | 28. 3 113 50.0 2 0.0 1 10.9 I 55 | • Daph 69.6 46 3 1.8 107 50.0 1 100.0 1 2.9 I 34| i |36.5 189 A mphi 0.0 46 0.9 113 0.0 2 0.0 1 9.1 1 55 | • | 2.8 217 Chaob 4.3 46 1. 8 113 0.0 2 0.0 1 1.8 1 55| | 2.3 217 Chiro 80.4 46 77. 9 113 50.0 2 100.0 1 92.7 1 55 I I | 82.0 217 Eph 8.7 46 39. 8 113 0.0. 2 0.0 1 30.9 1 55| • |28.6 217 Zyg 54. 3 46 68. 1 113 50.0 2! 0.0 1 70.9 1 55| • I65.4 217 Pulm 0.0 19 0. 0 43 | 0.0 1 0.0 1 55| • | 0.0 118 Blue-g 0.0 27 0. 0 70 | 0.0 2| 1 1 l | 0.0 99 Macro 0.0 1 ! 0.0 1 21| | 0.0 22 186 Table XXXIII. Feeding p a t t e r n of f i f t h i n s t a r C. b i f i d a arranged by lakes and seasons Data expressed as percentages fe e d i n g cn each f o o d s t u f f , and t o t a l s examined, f o r each lake and season. |Serum y llake 2 |spring llake 2 | summer | lake | f a l l 2 |lake 3 |spring I lake 3 |lake Isummer | f a l l 3 I fetal e |Diapt |100.0 1 | 35. 1 77 | 20. 0 10 | |33.3 6| C.O 2 I 33. 3 96 | Daph | 100.0 1 |51. 6 64 | 33- 3 9 I | 16.7 6 I 47.5 80 |Amphi I o.o 1 I 1.3 77 I 0.0 10 | | 66. 7 6| 0.0 2| 5.2 96 |Chaob I 0.0 1 I 1. 3 77 I o.o 10| I 0.0 6| 0.0 2| 1.0 96 | C h i r c | 0.0 1 | 72. 7 77 I70.0 10 | | 66.7 6 | 100.0 2| 71.9 96 | Eph | 100.0 1 |61. 0 77 I20.0 10 | I 66.7 6|50.0 2| 57.3 96 1 Zyg | 100.0 1 | 58. 4 77 I80.0 10 | | 83.3 6|50.0 2| 62.5 96 | Pulm ! I 3.4 29 I 1 I 0.0 4| 0.0 2| 2.9 35 |Blue- g| 0.0 1 I 2. 1 48 I 0.0 10 | | 0.0 2 I 1.6 61 j Macro I 0.0 7 JL j I 0.0 2| _ i 0.0 9 1 87 c o l l e c t e d i n the higher a l k a l i n i t y lakes (Table XXXIV). There was a n o n - s i g n i f i c a n t t r e n d f o r more p o s i t i v e r e c o rds f o r da p h n i i d s i n guts from s p r i n g - c o l l e c t e d i n s e c t s from higher a l k a l i n e l a k e s than i n those from the f r e s h e r water bodies (p=0. 1). Of guts from the higher a l k a l i n i t y lake i n s e c t s t e s t e d with ephemeropteran antiserum, s i g n i f i c a n t l y mere p o s i t i v e r e a c t i o n s were recorded i n summer than i n f a l l (p=0.01) and i n summer than i n s p r i n g (p=0.07). However i n the case of amphipods, more r e a c t i o n s were seen i n s p r i n g (p=0.06) and summer (p=0.01) than i n f a l l samples from freshwaters. In the 205 males of C. b i f i d a t e s t e d ( t a b l e XXXV) s e v e r a l s i m i l a r trends were seen. Ephemeropterans were again s i g n i f i c a n t l y most o f t e n recorded i n guts of i n s e c t s c o l l e c t e d i n the more a l k a l i n e lakes i n summer than i n f a l l (p=0.01) or s p r i n g (p=0.01), and amphipods were detected more f r e q u e n t l y i n s p r i n g (p=0.1) and-summer (p=0.01) than i n f a l l i n the more freshwater l a k e s . In a d u l t males and females, as i n the j u v e n i l e i n s t a r s , o c c a s i o n a l r e a c t i o n s were recorded with diaptomids, amphipods and chaoborids i n guts from lake groups where these organisms app a r e n t l y do not occur. These r e a c t i o n s o c c u r r e d i n 1.7% of guts with chaoborid antiserum and 0.5% with amphiped antiserum i n the higher a l k a l i n i t y l a k e s , and 4.7% with diaptomid antiserum i n the freshwater l a k e s . These discrepancy r a t e s f o r a d u l t s are not a l t e r e d markedly when a l l i n s t a r s are considered together. P o s i t i v e r e a c t i o n s were observed i n 0.63% of t e s t s with a l l 188 Table XXXIV. Feeding p a t t e r n of ad u l t female C. b i f i d a arranged by lakes and seasons. ~ Data expressed as percentages f e e d i n g cn each f e e d s t u f f , and t c t a l s examined, f o r each lake and season. |Serum L l l a k e 2 | s p r i n g i l l a k e 2 | summer | lake I fa l l 2 I J lake 3 s p r i n g lake 3 | summer | lake f a l l 3 t c t a l c r |Diapt | 26. 1 23 T I 15. 0 20 I 13. 1 61 i 6.3 32 10.0 r 10 | C O 29 11.4 175 | Daph |47 .8 23 | 50. 0 6 | 38. 3 60 15.6 32 20.0 10 | 30.8 26| 33. 1 157 |A mph i I 0.0 23 I 0.0 20 I 0.0 61 28. 1 32 60.0 10 | 3.4 29 9.1 175 |Chaob | 4.3 23 I 0.0 20 I 0.0 61 3. 1 32 0.0 10 | C O 29 i 1.1 175 | C h i r c | 80 .5 21 I 65. 0 20 | 68. 9 61 74. 1 27 90.0 10 | 62.1 291 68.8 173 1 Eph I 9 .5 21 | 50. 0 20 I 9.8 61 14.8 27 20.0 10 | 20.7 29| 17.9 168 1 Zyg | 47. 6 21 I50.0 20 I 54. 1 61 48. 1 27 60.0 10 | 51.7 29| 53.0 168 | Pulm I I 0.0 15 I 0.0 2 0.0 2 I C O 10| C O 29 |Blue-g I 0.0 23 I 0.0 5 I 1.7 59 0.0 32 0.0 8 I C O 15| 0.7 142 |Macro I | 0.0 14 C O 6| C O 20 |Green | 0.0 2 20.0 5 14.3 7 I 1 1 1 1 I L L 189 Table XXXV. Feeding p a t t e r n of a d u l t male C. b i f i d a arranged by l a k e s and seasons ~ ~ Data expressed as percentages feeding cn each f e e d s t u f f , and t c t a l s examined! f o r each lake and season. Serum |lake 2 |lake 2 Ispring Isummer i i I lake Ifall 2 I lake 3 | lake 3 | spring | summer | lake fa l l 3 I tctal c Diapt 121.7 23 34. 3 35 | 25. 8 62 | 0.0 T 24| 4 .8 21 I 5 .0 40 | 17.6 205 Daph I 47.8 23 25. 0 8 I37.9 66 I 41.6 24| 23.8 21 | 35.1 37| 36.9 179 A mphi I 8.7 23 0.0 35 I 0.0 62 I 25.0 24| 42.9 21 | 5.0 40 | 9.3 205 Chaob I 4 .3 23 | 2. 9 35 | 0.0 6 2 | 0.0 24 | 0.0 21 | 0.0 401 1.0 205 C h i r c 181.3 16 | 85. 7 35 | 66. 1 62 | 90.5 21| 90.5 21 | 75.0 4C| 77.9 195 Eph I 5.3 19 | 60. 0 35 I 19.4 62 | 4 .2 21| 23.8 21 | 15.0 40| 23.2 198 Zyg | 62.5 16 | 65. 7 35 | 67. 7 62 | 57. 1 21| 66.7 21 | 62.5 4 0 | 64.6 195 Pulm I 0.0 3 | 0. 0 31 I 0.0 3| 0.0 9 I 0.0 ei 0.0 54 Blue-g I 4 .3 23 | 0. 0 4 I o .o 57 | 0.0 24| 0.0 12| 0.0 27 | 0.7 147 Macro ! 4 .0 25 I o .o 2| 0.0 6| 3.0 33 Green I 14.7 7 I I 50.0 2| j. j . L 22.2 9 190 i n s t a r s with blue-green algae antiserum and i n 0.71% with pulmonate antiserum. This low l e v e l of r e a c t i o n s suggests a d e f i n i t e l a c k of f e e d i n g on these items. R e s u l t s with the a q u a t i c macrophyte antiserum were a t a s i m i l a r l e v e l of 0.98%, (one p o s i t i v e r e a c t i o n i n 102 t e s t s ) . T h i s l a s t antiserum gave no p o s i t i v e r e a c t i o n s i n i n i t i a l t e s t s of a n t i s e r a . Feeding pattern of C. e x p l e t a Tables XXXVI to XIII c o n t a i n the s e r o l o g i c a l r e s u l t s f o r a l l C. e x p l e t a t e s t e d , with i n s t a r , season and lake data considered s e p a r a t e l y . There were no s p r i n g samples c o l l e c t e d i n the h i g h e s t s a l i n i t y water (Lake IB 2 ) , and, as expected, f a l l samples d i d not c o n t a i n any of the youngest i n s t a r s . In the 17 f i r s t i n s t a r nymphs t e s t e d ( t a b l e XXXVI), no s i g n i f i c a n t p a t t e r n of v a r i a t i o n i n numbers cf p o s i t i v e r e a c t i o n s to d i f f e r e n t a n t i s e r a was d i s c e r n a b l e . Cf 73 second i n s t a r C. e x p l e t a ( t a b l e XXXVII) the only marked trend seen was i n the numbers of p o s i t i v e r e a c t i o n s to d a p h n i i d s ; these were g r e a t e r i n s p r i n g than i n summer samples (p=0.08) from the Becher's P r a i r i e lakes of group 2, A s i m i l a r trend was seen among the 116 t h i r d i n s t a r nymphs (table XXXVIII) where more p o s i t i v e r e a c t i o n s were recorded i n these l a k e s i n s p r i n g (p=0.01 and i n f a l l (p=0.11) than i n summer. In c o n t r a s t , fewer p o s i t i v e r e a c t i o n s were noted f o r ephemeropterans i n s p r i n g than i n e i t h e r summer (p=0.0U) or f a l l 191 Table XXXVI. Feeding p a t t e r n c f f i r s t i n s t a r C. e x p l e t a arranged by lak e s and seasons Data expressed as percentages feeding cn each f e e d s t u f f , and t o t a l s examined, for each lake and season. r - — r T— __j 1  • I  jSerum (lake 1 | lake 1 |lake 1 |lake 2 lake 2 | lake 2 | t o t a l C I I s pring |summer J f a l l |spring | summer | f a l l f ~ „ { |Diapt | | 0.0 10 ! i I 0.0 7 | 0.0 17 |Daph | |20.0 10 I I I 0.0 7 111.8 17 |Amphi | | 0.0 10 I I I 0.0 7 | 0.0 17 JChaob | | 0.0 10 i i I 0.0 1 I 0.0 17 IChiro | | 90. 0 10 i i I 57. 1 7 I 76.5 17 I Eph | I 10. 0 10 i i I 0.0 7 I 5.9 17 I Zyg | I 50. 0 10 i i | 14.3 7 | 35.3 17 |Pulm j I i i I 0.0 7 | 0.0 7 | Blue-g | | 0.0 10 i i | 0.0 10 1 9 2 Table XXXVII. Feeding pattern of second instar C. expleta arranged by lakes and seasons. Data expressed as percentages feeding cn each feedstuff, and totals examined, for each lake and season. Serum I- + Diapt Daph A it phi Chaob Chirc Eph Zyg Pulm Blue-g Macro lake 1 spring 193 Table XXXVIII. Feeding p a t t e r n of t h i r d i n s t a r C. e x p l e t a arranged by lakes and seasons. Data expressed as percentages feeding cn each f e e d s t u f f , and t o t a l s examined, f o r each lake and season. Serum |lake 1 |sp r i n g l l a k e 1 | Isummer | lake f a l l 1 |lake 2 | |spring | lake 2 |lake summer | f a l l 2 | — 4 t c t a l £ Diapt | I 14. 3 42 | 0.0 2| 4.8 21| 14.3 35|31.3 1€| 14.7 116 Daph | |31. 0 42 | 50.0 2 | 95.2 21| 7.7 13|56.3 16| 46.8 94 Arc phi | I o.o 42 | 0.0 2| 0.0 21| 0.0 35| 0.0 16| 0.0 116 Chaob | I 7. 1 42 | 0.0 2| 4.8 21| 2.9 35| 6.3 16| 5.2 116 Chiro | | 90. 5 42 | 50.0 2|47.6 211 62.9 35|87.5 16| 73.3 116 Eph | I 9. 1 44 | 0.0 2| 4.8 21| 42.9 35|43.8 16| 22.9 118 Zyg | | 76. 2 42 | 50.0 2|66.7 21| 71.4 35|75.0 16| 72.4 116 Pulm | ! I 0.0 9 1 0.0 33( 0.0 2| 0.0 44 Blue-g| I 0.0 42 | 0.0 2| 0.0 12| 0.0 2| 0.0 13| 0.0 71 Macro | I I 0.0 10| C.O 1| 0.0 11 i L i [ L j J 194 (p=0.06) samples. In the t h i r d i n s t a r summer samples, s i g n i f i c a n t l y more guts showed a p o s i t i v e r e a c t i o n f o r ephemeropterans i n the Becher's P r a i r i e lakes than i n Lake LB 2. However, s i n c e the f a u n a l components of lake LE 2 were not i n v e s t i g a t e d i n d e t a i l , the f u l l s i g n i f i c a n c e of t h i s d i f f e r e n c e i s not c l e a r . In the 81 f o u r t h i n s t a r C. e x p l e t a t e s t e d (table XXXIX) d i f f e r e n c e s s i g n i f i c a n t a t the 5% l e v e l were only detected i n the Becher's P r a i r i e l a k e s . Here, more p o s i t i v e r e a c t i o n s to daphniids were recorded i n s p r i n g than i n summer guts (p=0.00) but t h e r e were more p o s i t i v e r e a c t i o n s to ephemeropterans i n summer than i n s p r i n g (p=0.05). A l l other d i f f e r e n c e s ncted were not s t a t i s t i c a l l y s i g n i f i c a n t . Table XL l i s t s the r e s u l t s f o r the 54 f i f t h i n s t a r nymphs of C. e x p l e t a examined. As i s to be expected, no f i f t h i n s t a r i n s e c t s were taken i n the s p r i n g season i n e i t h e r lake group; numbers were g r e a t e s t i n f a l l c o l l e c t i o n s . The f i f t h i n s t a r C. e x p l e t a r e s u l t s are r a t h e r d i f f e r e n t from those i n f i f t h i n s t a r C. b i f i d a , and t h i s i s r e l a t e d to the presence of a l a t e t h i r d g e n e r a t i o n i n c. e x p l e t a i n LE 2 and the highest s a l i n i t y Becher's P r a i r i e l a k e s , thus i n t r o d u c i n g a seasonal e f f e c t i n f e e d i n g of s i m i l a r i n s t a r s i n the two s p e c i e s . Numbers of C. e x p l e t a were only l a r g e enough to compare f a l l c o l l e c t i o n s between the two lake groups. In these, s i g n i f i c a n t l y more p o s i t i v e r e a c t i o n s to daphniids and ephemeropterans were recorded i n guts from the Eecher's P r a i r i e l a k e s than i n those from Lake LB 2, although with ether a n t i s e r a 195 T a b l e X X X I X . F e e d i n g P a t t e r n c f f o u r t h i n s t a r C . e x c l e t a a r r a n g e d by l a k e s and s e a s o n s . ~ ~ ~ D a t a e x p r e s s e d a s p e r c e n t a g e s f e e d i n g on e a c h f o o d s t u f f , and t o t a l s e x a m i n e d , f o r e a c h l a k e and s e a s o n . Serum l a k e 1 s p r i n g l a k e 1 summer * l a k e 1 _ T f a l l l a k e 2 s p r i n g H +  — + — + — — i —H I D i a p t | I 5 0 . 0 4 I 2 3 . 6 21 8 .7 23 11 . 1 27 3 3 . 3 6 18 .5 81 Daph | I 2 5 . 0 n 3 21 91 .3 23 0 .0 18 2 0 . 0 3 6 . 6 71 A cn p h i | I o. 0 4 I o. 0 21 0, .0 23 0, .0 27 0 . 0 6 0 . 0 81 Chaob | | 0 . 0 4 I o . 0 21 0 .0 23 0 .0 27 16 .7 6 1. 2 81 C h i r o | I o. 0 4 I 8 5 . 7 21i 69 . .6 23 | 63 . .0 27 8 3 . 3 6 6 9 . 1 81 Eph | I 2 5 . 0 4 | 1 9 . 0 21 0, .0 23 | 25. .9 27 3 3 . 3 6| 17. 3 81 z*g l | 1C0 .0 4 | 8 1 . 0 21| 4 3 , 5 23j 59 , 3 27 6 6 . 7 6 6 3 . 0 81 Pulm | I 0, 0 2 3. 8 26 0 . 0 3| 3 . 2 31 B l u e - g | I o . 0 4 I o . 0 21 | 0, 0 21 I 0. 0 1 0 . 0 3| 0 . 0 50 M a c r o | I 0. 0 3 | 0 . 0 3 l a k e 2 suamer l a k e 2 f a l l t o t a l s 196 Table XL. Feeding p a t t e r n of f i f t h i n s t a r C. e x p l e t a arrangec by l a k e s and seasons. ~ Data expressed as percentages f e e d i n g on each f o o d s t u i f | and t o t a l s examined, f o r each lake and season. r |Serum 1 IDiapt i — l l a k e 1 j s p r i n g l l a k e 1 |summer | lake | f a l l 1 l l a k e 2 | s p r i n g 4. {lake 2 (lake | SUIT met | f a l l 2 | t o t a l s | 33.3 3 | 32.8 64 | —r | C O 4 | 2 4 . 1 54|28.0 125 I Daph I | 0.0 3 114.1 64| I C O 3 |40 .7 54 j 25.0 124 | A in phi I I 0.0 3 I C O 64| I 0.0 4 | 0.0 5 4 | C O 125 |Chaob I I C O 3 | C O 64 | I 0.0 4 | C O 54 | C O 125 IChiro I I 33.3 3 | 62.5 64 | I 50.0 4 J 68.5 54|64.0 125 1 Eph I I33.3 3 | 14.1 64 | | 50.0 4 | 4 0 . 7 54|27.2 124 |Zyg ! I 33.3 3 151.6 64 | | 50.0 4 | 68. 5 54 |58 .4 125 | Pulm ! | C C 3 I I | C O 4 | 0.0 2| C O 9 |Blue-g I ! | 0.0 64 | I | 0.0 51| C O 115 I Macro t,_.. | C O 1| 0.0 1 197 no d i f f e r e n c e s were observed. R e s u l t s f o r 61 female C. e x p l e t a examined are given i n Table XLI. Seasonal d i f f e r e n c e s were seen i n the high s a l i n i t y lake group 1 (LB 2) f o r numbers of p o s i t i v e r e a c t i o n s f o r diaptomids (p=0.01) ; these were only recorded i n f a l l . There was a l s o a s i m i l a r but n o n - s i g n i f i c a n t trend i n t h i s lake f o r d a p h n i i d s (p=0.14). Table XIII l i s t s r e s u l t s f o r 81 male C. e x p l e t a , c h i e f l y c o l l e c t e d i n the high s a l i n i t y lake LB 2. The d i s t r i b u t i o n of samples precluded s t a t i s t i c a l comparisons c f data between lake groups. However, i n Lake LB 2 there was a trend (non-s i g n i f i c a n t ) f o r the number of guts r e a c t i n g with ephemeropteran antiserum to be g r e a t e r i n summer than i n f a l l (p-0.08). The r e v e r s e was seen f o r diaptomids (p=0.02). In C. e x p l e t a guts, as i n C. b i f i d a , some r e a c t i o n s were seen f o r organisms that had not been recorded from the c o l l e c t i o n s i t e s . Although no r e a c t i o n s were seen i n 844 t e s t s with amphipod antiserum, 1.495 of guts t e s t e d r e a c t e d p o s i t i v e l y with chaoborid antiserum i n lake groups 1 and 2. Guts r e a c t i n g p o s i t i v e l y were on l y found i n second to f o u r t h i n s t a r i n s e c t s . Chaoborids may have been present i n Round-up Lake on Becher's P r a i r i e (chapter I I ) . No p o s i t i v e r e a c t i o n s were recorded with a q u a t i c macrophyte or blue-green a l g a l a n t i s e r a ; s i n c e the former was i n e r t i n t e s t s of s p e c i f i c i t y and s t r e n g t h , i t may be considered a c o n t r o l s o l u t i o n . One out of 157 t e s t s of £• ®2£l§ia guts with pulmonate antiserums shewed p o s i t i v e f o r the higher s a l i n i t y lakes i n Becher's P r a i r i e ; t h i s i s l i k e l y 198 Table XLI. Feeding pat t e r n of female a d u l t C. e x p l e t a arrangec by l a k e s and seasons ~ ~ #data expressed as percentages f e e d i n g on each f e e d s t u f f and t o t a l s examined, f o r each lake and season. "T "T — — | -J——————— — T —1 ISerum | l a k e 1 | l a k e 1 | l a k e 1 | l a k e 2 l l a k e 2 l l a k e 2 I t o t a l s 1 | | s p r i n g |summer I f a l l + s p r i n g |summer I f a l l — + J J D i a p t | I 0.0 20 | 46.9 32 | 0.0 1 | 0.0 1 I o .o 7| 24.6 61 | |Daph | I 0.0 20 | 19.4 31| 0.0 1 I J 57. 1 7| 16.9 59 | lAmphi | | 0.0 20 | 0.0 32 | 0.0 1 | 0.0 1 I 0.0 7| 0.0 61 | IChaob | I o .o 20 | 0.0 32| 0.0 1 | 0.0 1 | 0.0 7| 0.0 61 I J C h i r o | |45.0 20 | 59.4 32| 0.0 1 I 0.0 1 I 57. 1 7| 52. 5 61 | |Eph | | 5.0 20 I 19.4 32| 0.0 1 I 0.0 1 | 0.0 7| 11. 5 61 | IZyg | 115.0 20 | 43.8 32| 0.0 1 I 0.0 1 171.4 7| 36. 1 61 I |Fulm | I 0.0 19 I o .o 1 I I 0.0 1 ] 0.0 '1 I I B l u e - g | | 0.0 1 I 0.0 31 | 0.0 1 I | 0.0 6| 0.0 39 | |Macro j | | 0.0 1 | I 0.0 1 I 0.0 11 0.0 3 I 199 Table XIII. Feeding pattern of adult male C. expleta arranged b y lakes and seasons #data expressed as percentages feeding cn each f c c s t u f f and to t a l s examined, for each lake and season. 1— T ' "T — — y — 1 — 1 r~—————•-•f———— J-|Serum llake 1 llake 1 |lake 1 |lake 2 lake 2 | lake 2 | t c t a l c [ |spring I summer I f a l l | spring t summer | f a l l — 4 |Diapt i I 5.6 18 I 52.6 57 | | 33. 3 6|40.7 81 I Caph ! I 5.6 18 I 14.5 55 | | 1C0.0 4|16.9 77 |A mphi ! | 0.0 18 I 0.0 57 | I 0.0 6| 0.0 81 | Chaob I I 0.0 18 I 0.0 57 J I 0.0 6| 0.0 81 |Chirc ! | 72. 2 18 | 68. 4 57 | I 83.3 6|70.4 81 1 Ech ! | 50. 0 18 I 17. 5 57 l | 50. 0 6|27.2 81 1 zyg I I aa. a 18 I 35. 1 57 | | 3 3. 3 6 | 3 7. 0 81 | Pulm ! I 0.0 18 I 0.0 2| ! | 0.0 20 |Blue-g I I I 0.0 55 | I 0.0 6| 0.0 61 |Macro j j I 0.0 2| I I 0.0 2 I 200 to be an experimental or reading e r r o r , as i t i s at a s i m i l a r l e v e l to t h a t recorded f o r pulmonates i n C. b i f i d a , and as pulmonates were not recorded from the higher s a l i n i t y l a k e s . D i f f e r e n c e s i n f e e d i n g p a t t e r n of Cenccorixa males and females For every serum t e s t e d a g a i n s t gut contents cf f i e l d -c o l l e c t e d a d u l t Cenocorixa , males showed as many as or more p o s i t i v e r e a c t i o n s than females, although i n many cases the trend was not marked ( t a b l e s X L I I I , XLIV). L i t t l e or no d i f f e r e n c e between the sexes was seen i n numbers recorded f e e d i n g on chaoborids, amphipods, filamentous green algae and a q u a t i c macrophytes f o r C. b i f i d a , and on daphniids and zygopterans f o r C. e x p l e t a . S i g n i f i c a n t l y g r e a t e r p r o p o r t i o n s of p o s i t i v e r e a c t i o n s i n males of C. e x p l e t a were recorded f o r ephemercpteran antiserum, and i n C. b i f i d a males with zygopteran antiserum. Other marked tren d s were seen f o r both s p e c i e s with diaptomids and chironomids. None of these t r e n d s were a f f e c t e d by s e a s o n a l d i f f e r e n c e s i n p r o p o r t i o n s of p o s i t i v e r e a c t i o n s . Thus i n the a n a l y s i s of the r e s u l t s there i s some l e v e l of f e e d i n g b i a s i n t r o d u c e d by p o o l i n g f e e d i n g data f o r both sexes cf a d u l t . In the a n a l y s i s of s e r o l o g i c a l r e s u l t s , no i n f o r m a t i o n was recorded on the r e l a t i v e f e e d i n g l e v e l s cf t e n e r a l s and old a d u l t s . Feeding experiments (chapter I V ) , however, i n d i c a t e t h a t t e n e r a l s may feed at a higher l e v e l , and so p o o l i n g r e s u l t s may 201 T a b l e X L I I I . D i f f e r e n c e s i n f e e d i n g p a t t e r n o f ma le and f e m a l e £ • b i f i ^ a and C . e x p l e t a , e x p r e s s e d a s p e r c e n t a g e s r e a c t i n g p o s i t i v e l y w i t h e a c h a n t i s e r u m . C . b i f i d a C . e x p l e t a f e m a l e s m a l e s f e m a l e s m a l e s n= (*) n= {%) n= (*) n= (%) ANTISERUM D i a p t o m i d s 176 11 .9 206 1 7 . 5 6 1 2 4 . 6 81 4 0 . 7 D a p h n i i d s 158 3 2 . 9 170 3 8 . 8 59 18. 6 77 1 6 . 9 A m p h i p o d s 176 9 . 1 206 9 . 2 61 0 . 0 81 0 . 0 C h a o b o r i d s 176 1.1 206 1.0 61 0 . 0 81 0 . 0 C h i r o n o m i d s 169 7 1 . 0 196 7 8 . 1 61 54 . 1 81 7 0 . 4 E p h e m e r o p t . 169 1 7 . 8 199 2 3 . 1 61 1 1 . 5 81 3 7 . 3 Z y g o p t e r a n s 169 5 1 . 5 196 6 4 . 8 61 3 7 . 7 81 3 8 . 8 P u l m o n a t e s 29 0 . 0 54 0 . 0 21 0 . 0 20 0 . 0 B l u e - g r e e n s 143 0 . 7 148 0 . 7 39 0 . 0 61 0 . 0 C l a d o p h o r a 7 14. 3 10 2 0 . 0 M a c r o p h y t e s 20 0 . 0 33 3 . 0 3 0 . 0 2 0 . 0 A L L FOODS 1392 2 3 . 7 1624 2 7 . 9 488 18 .2 646 2 4 . 0 202 Table XLIV. D i f f e r e n c e s i n f e e d i n g p a t t e r n of male and female C. b i f i d a and C. exp l e t a . P r o b a b i l i t i e s from c h i square data. C. b i f i d a C. e x p l e t a FOOD ITEM SUPERIOR P= SUPERIOB P= SEX SEX Diaptomids MALE 0.17 MALE 0.06 Daphniids MALE 0. 32 MALE 0.92 Amphipods MALE 0.86 Chaoborids MALE 0.73 Chironomids MALE 0.15 MALE 0.07 Ephemeropt. MALE 0.25 MALE 0.04 Zygopterans MALE 0.01 MALE 0.89 Cladophora MALE 0.73 Macrophytes MALE 0.79 ALL FOODS MALE 0.01 MALE 0.02 203 mask a b i a s of undetermined p r o p o r t i o n s here. Feeding p a t t e r n s of Cenocorixa from d i f f e r e n t lakes R e s u l t s here are considered f o r d i f f e r e n t lake groups, but pooled f o r seasons of c o l l e c t i o n . In the next s e c t i o n r e s u l t s are pooled by l a k e s but c o n t r a s t e d by season. T h i s has been done to determine how r e s u l t s might be b i a s s e d by p o o l i n g data of d i v e r s e s p a t i a l and temporal o r i g i n , s i n c e the r e a l i s e d niches i n each environment may d i f f e r . F i r s t , c o n s i d e r i n g a d u l t s , d i f f e r e n c e s between lakes are s i g n i f i c a n t only f o r d a p h n i i d s i n the d i e t of a d u l t C. e x p l e t a i n the Becher's P r a i r i e and LB 2 l o c a l i t i e s (p=0.00), r e f l e c t i n g probably the sparseness of d a p h n i i d s i n the l a t t e r l a k e . Marked though not s t a t i s t i c a l l y s i g n i f i c a n t trends with t h i s focd were a l s o seen f o r C. b i f i d a (p=0.09), the p o s i t i v e s e r o l o g i c a l r e a c t i o n s being fewer i n freshwater l a k e s than i n the moderately s a l i n e l a k e s . A s i m i l a r trend i n a d u l t C. b i f i d a was seen with ephemeropterans; more p o s i t i v e r e a c t i o n s being recorded i n the moderately s a l i n e l a k e s (p=0.19). Both these r e s u l t s may r e f l e c t the g e n e r a l l y g r e a t e r v a r i e t y of food items a v a i l a b l e i n the more freshwater lakes (chapter I I ) : thus, i n d i v i d u a l food items might f i g u r e l e s s f r e q u e n t l y i n the d i e t when there i s a g r e a t e r v a r i e t y of food. More a d u l t C. e x p l e t a were recorded as having fed on diaptomids i n Lake LB 2 than i n the Becher's P r a i r i e l a k e s 204 (p=0.29), t h i s food i n LB 2 perhaps t a k i n g the place of d a p h n i i d s i n the d i e t . R e s u l t s with chironomid and zygopteran a n t i s e r a showed no s i g n i f i c a n t trends between the d i f f e r e n t lake groups. When i n s t a r s of each s p e c i e s are examined s e p a r a t e l y , and a l l s i g n i f i c a n t or n e a r - s i g n i f i c a n t (p<0.1) d i f f e r e n c e s i n p o s i t i v e r e a c t i o n s t o foods assessed, r e a c t i o n s to zygopterans were c o n s i s t e n t l y most fr e g u e n t i n the most freshwater l a k e s , and to daphniids and ephemeropterans i n the moderately high s a l i n e l a k e s . The pooled data f o r a l l i n s t a r s of C. e x p l e t a i n d i c a t e s i g n i f i c a n t d i f f e r e n c e between lake groups i n numbers r e a c t i n g with diaptomid (p=0.00), ephemeropteran (p=0.00) and zygopteran (p=0.02) a n t i s e r a , i n c o n t r a s t to the r e s u l t s with a d u l t s alone. However, r e a c t i o n s with daphniid antiserum remain at a s i g n i f i c a n t l y d i f f e r e n t l e v e l (p=0.00) when e i t h e r a d u l t s alone, or a l l i n s t a r s together, are c o n s i d e r e d . T h i s would seem to i n d i c a t e t h a t the j u v e n i l e s may depend more s t r o n g l y on diaptomids, ephemeropterans and zygopterans than dc the a d u l t s , although both j u v e n i l e s and a d u l t s feed a t f a i r l y s i m i l a r l e v e l s on d a p h n i i d s . The pooled data f o r a l l i n s t a r s of C. b i f i d a show s i g n i f i c a n t l y more p o s i t i v e r e a c t i o n s f o r daphniids and ephemeropterans i n the moderately s a l i n e l a k e s than i n freshwater l a k e s (p=0.00 f o r each ), again i n d i c a t i n g d i f f e r e n t d i e t a r y emphasis by d i f f e r e n t l a r v a l i n s t a r s : a d u l t s when con s i d e r e d alone d i d not show these d i f f e r e n c e s . 2 05 Thus i f i n s t a r s are t r e a t e d s e p a r a t e l y , s i g n i f i c a n t b i a s w i l l not be i n t r o d u c e d i n t o the r e s u l t s through p e e l i n g a l l data from d i f f e r e n t l a k e s , except where some food items are r e s t r i c t e d t o a few l a k e s . Feeding p a t t e r n s of Cenocorixa c o l l e c t e d at d i f f e r e n t seasons In most ca s e s , a n a l y s i s of pooled data i n d i c a t e d no s i g n i f i c a n t d i f f e r e n c e s i n p o s i t i v e r e a c t i o n s f o r f e c d s t u f f s i n guts of i n s e c t s c o l l e c t e d a t d i f f e r e n t times of year. However, the numbers of p o s i t i v e r e a c t i o n s f o r diaptomids were s i g n i f i c a n t l y g r e a t e r i n f a l l than i n summer f o r C. e x p l e t a c o l l e c t e d from Lake LB 2 (p=0.00). By c o n t r a s t , numbers of C. b i f i d a r e a c t i n g p o s i t i v e l y with amphipod antiserum i n the more freshwater l a k e s were higher i n summer than i n f a l l (p=0.00) or s p r i n g (p=0.01), and s i m i l a r r e s u l t s were seen i n the more s a l i n e lakes with ephemeropteran antiserum (p=0.00 f o r both seasons). The higher number of p o s i t i v e r e a c t i o n s recorded with summer-collected samples may c o r r e l a t e e i t h e r with higher temperatures and g r e a t e r f e e d i n g , and/or with a period of g r e a t e r abundance of these f o o d s t u f f s at a s i z e s u i t a b l e f o r c a p t u r e . Looking at i n d i v i d u a l i n s t a r s , highest p o s i t i v e r e a c t i o n r a t e s f o r ephemeropteran and amphipod a n t i s e r a were again c o n s i s t e n t l y seen i n summer, while evidence cf feeding cn d a p h n i i d s was c o n s i s t e n t l y most f r e q u e n t i n s p r i n g and f a l l . Approximately equal percentages of p o s i t i v e p r e c i p i t i n 206 r e a c t i o n s i n guts of C. b i f i d a were recorded i n a l l seasons f o r chironomids, zygopterans and diaptomids in a l l lakes s t u d i e d . T h i s i n d i c a t e s t h a t b i a s may only be i n t r o d u c e d when data are pooled f o r c o r i x i d s c o l l e c t e d at d i f f e r e n t seasons, i n the case of c e r t a i n f o o d s t u f f s such as amphipods and ephemeropterans. However, s i n c e over 20% of guts may show p o s i t i v e r e a c t i o n s with ephemeropteran antiserum, f u r t h e r a n a l y s i s should d i s c r i m i n a t e between seasons of c o l l e c t i o n wherever t h i s f e e d s t u f f i s f r e q u e n t . F u r t h e r , s i n c e p o s i t i v e r e a c t i o n s with ephemeropteran antiserum were commonest i n mid-summer, t h i s i n d i c a t e s the d i f f i c u l t y of summarising f e e d i n g pattern by p o o l i n g data f o r d i f f e r e n t l a k e s , seasons, i n s t a r s , and so on, without weighting them somehow to allow f o r d i f f e r e n t numbers present at d i f f e r e n t times i n d i f f e r e n t l a k e s . D i f f e r e n c e s between f e e d i n g p a t t e r n s of Cenocorixa i n sympatry The d e t a i l e d a n a l y s i s of s e r o l o g i c a l r e s u l t s presented above i n d i c a t e s that f o r c e r t a i n f o o d s t u f f s there are marked and o f t e n s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e s i n p r o p o r t i o n s of p o s i t i v e r e a c t i o n s from lake to l a k e , and with season, i n each s p e c i e s of Cenocorixa . T h i s may be due to d i f f e r e n c e s i n the d i s t r i b u t i o n p a t t e r n and/or abundance of the organisms. However, i t may a l s o be due to some i n t e r a c t i v e behaviour p a t t e r n between the s p e c i e s of Cenocorixa i n sympatry. T y p i c a l l y , t h i s might be expected to be r e f l e c t e d as lessened competition i n sympatry f o r those f o o d s t u f f s eaten e q u a l l y by both s p e c i e s . T h e r e f o r e , the 207 s p e c i e s are next compared to look f o r evidence of d i e t a r y d i f f e r e n c e s i n sympatry. R e s u l t s of a n a l y s i s c f f e e d i n g p a t t e r n by i n s t a r f o r both s p e c i e s i n sympatry are given i n Table XLV, expressed as percentages of p o s i t i v e r e a c t i o n s i n guts t e s t e d , and t o t a l numbers t e s t e d . The data i n d i c a t e f i r s t t h a t both s p e c i e s i n gen e r a l feed upon the same items of food i n sympatry. The order of importance of d i e t a r y items i n a l l i n s t a r s of C. b i f i d a was chironomids, then zygopterans, f o l l o w e d u s u a l l y by daphniids, ephemeropterans and diaptomids. T h i s same pa t t e r n was a l s o seen i n most j u v e n i l e i n s t a r s of C. e x p l e t a . However, i n the t h i r d i n s t a r of C. e x p l e t a the g r e a t e s t p r o p o r t i o n of p o s i t i v e r e a c t i o n s was seen i n response to zygopteran antiserum, and i n a d u l t s , most r e a c t i o n s were with daphniid antiserum. Table XLV again i n d i c a t e s an i n c r e a s e i n the l e v e l of p o s i t i v e r e a c t i o n s f o r most a n t i s e r a (representing f e e d s ) , with i n c r e a s e d i n s t a r s i z e , a f e a t u r e seen f i r s t i n the i n d i v i d u a l Tables XXIX to XLII. Whereas t h i s i n general reaches a pla t e a u l e v e l i n C. b i f i d a by about the t h i r d i n s t a r , i n C. ex p l e t a the pl a t e a u l e v e l of mean a c c e p t a b i l i t y i s o f t e n reached by the second i n s t a r . T h i s i s probably r e l a t e d to the s i z e d i f f e r e n c e between the s p e c i e s ; a l l i n s t a r s of C. e x p l e t a being s l i g h t l y l a r g e r than the corresponding stages of C. b i f i d a , and may re p r e s e n t the p h y s i c a l l i m i t s of s i z e , swimming speed and s t r e n g t h needed to capture, each prey type. For i n s t a n c e , p o s i t i v e r e a c t i o n s to diaptomid antiserum jumped t c a l e v e l of over 20% i n C. b i f i d a i n the t h i r d i n s t a r , a f t e r a l e v e l of 2% 208 Table XLV. Feeding p a t t e r n s of separate i n s t a r s cf C. b i f i d a and C. e x p l e t a expressed as percentage p o s i t i v e r e a c t i o n s with each antiserum. INSTARS: a n t i s e r a Diaptomids Daphniids A mphipods Chaoborids Chironomids Ephemeropt. Zygopterans Pulmonates Blue-greens Macrophytes ALL FOODS NUMBERS TESTED ALL FOODS NUMBERS TESTED T I 1 Cenocorixa s p e c i e s 4 I \ _ + _ bifida | 2 . 9 | 3 . 2 | 2 3 . 3 | 2 3 . 6 | 3 4 . 1 | 2 2 . 3 expl. | 0 . 0 | 17. 3 | 1 5 . 3 | 1 2 . 5 | 2 2 . 4 | 1 3 . 3 bifida| 3 . 0 113 .4 | 4 3.8 | 4 3 . 5 | 5 0 . 0 | 4 2 . 6 expJL. | 0 . 0 | 2 9 . 0 | 6 0 . 0 4 7 . 8 | 4 0 . 7 | 7 5 . 0 bifida| 0 . 0 I 0 . 0 | 0 . 0 | 0.6 I 1.1 I 0 . 9 expl. | 0 . 0 I 0 . 0 I 0 . 0 I 0 . 0 I o . o | 0 . 0 bifida| 0 . 0 I 1.1 2 . 5 2.5 I 1. 1 | 1.3 expl. | 0 . 0 | 1.9 4 . 2 1 .8 1 o . o I 0 . 0 bifida| 3 1 . 4 | 7 4 . 7 | 8 3 . 4 | 7 8 . 3 | 7 1 . 6 I 7 2 . 6 expl. | 5 7 . 1 | 7 5 . 0 | 6 3 . 9 ( 6 7 . 9 | 6 7 . 2 I 6 6 . 7 bifida| 2 . 9 | 5 . 3 |41.1 3 0 . 4 I 5 6 . 8 I 23.9 e xpl. | 0 . 0 | 2 5 . 0 [31.9 16. 1 | 4 1 . 4 | 2 0 . 0 bifida | 5 . 7 | 36.8 I 66 .4 | 64.0 161.4 | 59.5 expl. | 14. 3 | 6 1 . 5 [70 .8 | 5 3 . 6 | 6 7 . 2 | 5 3 . 3 bifida| 0 . 0 | 0 . 0 | 1.8 0 . 0 | 3 . 5 | 0 . 0 e xpl. | 0 . 0 | 0 . 0 | 0 . 0 3 . 2 I O.O | 0 . 0 bifida | 0 . 0 I 1.9 I 0 . 9 | 0 . 0 I 1.7 I 1.2 expl. | 0 . 0 | 0 . 0 I 0 . 0 | 0 . 0 I 0 . 0 | 0 . 0 bifida| 0 . 0 | 0 . 0 0 . 0 0 . 0 I 0 . 0 | 2. 4 expl. | 0 . 0 | 0 . 0 0 . 0 0 . 0 I 0 . 0 | 0 . 0 bifida| 5 . 7 | 1 6 . 7 3 2 . 6 30 . 3 I 34 . 2 | 26 . 6 35 | 95 163 161 | 88 | 224 expl. | 8. 9 | 2 5 . 3 | 2 9 . 1 I 24 . 5 | 2 9 . 8 I 27.1 7 I 52 (72 | 56 | 58 I 15 ADS 209 to 3% i n s m a l l e r i n s t a r s , whereas second i n s t a r C. e x p l e t a had al r e a d y reached the pl a t e a u l e v e l f o r t h i s s p e c i e s of over 15%. Si z e d i f f e r e n c e s between e q u i v a l e n t i n s t a r s of C. b i f i d a and £• s ^ E l i i a are n e v e r t h e l e s s not s u f f i c i e n t l y great t c make, f o r example, the t h i r d i n s t a r of C. b i f i d a more comparable to second i n s t a r than t h i r d i n s t a r C. e x p l e t a . Table XLVI shows the r e s u l t s of c h i - s q u a r e t e s t s of s i g n i f i c a n c e i n comparisons between the s p e c i e s at each i n s t a r i n sympatry. There were no s i g n i f i c a n t d i f f e r e n c e s i n the f i r s t i n s t a r , but by the second the p r o p o r t i o n s of p o s i t i v e r e a c t i o n s with diaptomid, ephemeropteran and zygopteran a n t i s e r a were s i g n i f i c a n t l y g r e a t e r i n C. e x p l e t a than i n C. b i f i d a , and almost s i g n i f i c a n t l y so with d a p h n i i d s . In the t h i r d i n s t a r s i g n i f i c a n t l y more guts of C. b i f i d a showed p o s i t i v e r e a c t i o n s f o r chironomids than those of C. e x p l e t a , and i n the f o u r t h , t h e r e were g e n e r a l l y g r e a t e r l e v e l s of p o s i t i v e r e a c t i o n s with a l l foods i n guts of C. b i f i d a than of C. ex p l e t a . In n e i t h e r t h i s nor the f i f t h i n s t a r d i d C. exjaleta show r e l a t i v e l y more p o s i t i v e r e a c t i o n s to any antiserum than C. b i f i d a . In summary, although both Cenocorixa s p e c i e s took the same food items i n sympatry, i n e a r l i e s t i n s t a r s . C. expjleta g e n e r a l l y showed more p o s i t i v e r e a c t i o n s than C. b i f i d a f e r diaptomids, d a p h n i i d s and zygopterans, but i n l a t e r i n s t a r s t h i s was r e v e r s e d . Among a d u l t s more C. e x p l e t a took daphniids than did £• b i f i d a , although r e s u l t s f o r other f o o d s t u f f s were very s i m i l a r between the s p e c i e s . Table XLVII shows the d i f f e r e n c e s between Cenccorixa 210 T a b l e X L V I . D i f f e r e n c e s i n f e e d i n g p a t t e r n b e t w e e n t h e i n s t a r s o f C . b i f i d a and C . e x p l e t a i n s y m p a t r y . D a t a e x p r e s s e d a s c h i s q u a r e p r o b a b i l i t i e s , and s u p e r i o r s p e c i e s i n e a c h c a s e . I N S T A R S : 1 2 3 4 5 ADULTS ANTISERUM D i a p t o m i d B i :0 . 39 Ex : : 0 .01 B i : : 0 . 2 2 B i : : 0 . 11 B i : : 0 . 1 8 B i : : 0 . 62 D a p h n i i d B i : 0 , • 39 Ex; : 0 . 0 7 Ex; : 0 . 0 6 Ex; : 0 . 73 B i : : 0 . 4 0 Ex : : 0 . 06 A m p h i p o d B i : : 0 . 59 B i : : 0 . 8 1 B i : : 0 . 27 C h a o b o r i d B i ; : 0 . 7 5 B i ; : 0 . 7 6 B i : : 0 . 81 B i : : 0 .81 B i : : 0 . 46 C h i r o n o m i d Ex : 0 . ,39 B i ; : 0 . 8 5 B i : : 0 . 0 0 B i : : 0 . 16 B i : : 0 . 0 0 B i : : 0 . 83 E p h e m e r o p t E x : : 0 . 0 0 B i : : 0 . 1 6 B i : : 0 . 05 B i ; : 0 . 0 9 B i : : 0 . 93 Z y g o p t e r a n B i : 0 . ,95 E x ; :0 .01 Ex ; : 0 . 66 B i : : 0 . 22 E x : : 0 . 5 9 B i : : 0 . 82 P u l m o n a t e B i ; : 0 . 8 8 E x : : 0 . 72 B i ; : 0 . 38 B l u e - g r e e n B i : :0 .61 B i : : 0 . 4 6 B i : : 0 . 9 0 B i : : 0 . 32 B a c r o p h y t e B i : : 0 . 03 A L L FOODS Ex :0< .55 E x : : 0 . 0 0 B i : : 0 . 1 4 B i : : 0 . 02 B i : : 0 . 1 3 Ex: : 0 . 94 211 T a b l e X L V I I . F o o d p r e f e r e n c e s o f s y m p a t r i c C . b i f i d a and £ • S i E l e t a a t d i f f e r e n t s e a s o n s . C. b i f i d a C . e x p l e t a s e a s o n s 1 and s e a s o n s 1 and ANTISERUM p r o b a b i l i t y 2 p r o b a b i l i t y 2 D i a p t o m i d summer s p r i n g f a l l f a l l summer s p r i n g 0. 00 0 . 07 0 . 02 0 . 0 2 0 .00 0 . 28 D a p h n i i d s p r i n g f a l l summer s p r i n g f a l l summer 0. 60 0 . 9 2 0 . 66 0 .00 0 . 0 0 0 . 0 0 C h i r o n o m i d summer s p r i n g f a l l f a l l summer s p r i n g 0. 09 0 . 0 2 0 . 44 0 . 0 3 0 . 1 0 0 . 9 0 E p h e m e r o p t summer s p r i n g f a l l f a l l summer s p r i n g 0. 00 0. 00 0. 04 0 .31 0 .00 0 . 0 0 Z y g o p t e r a n f a l l summer s p r i n g f a l l summer s p r i n g 0 .61 0. 00 0 . 0 0 0 . 37 0 . 0 3 0 .21 R e a s o n s l i s t e d i n o r d e r o f g r e a t e s t p e r c e n t a g e f e e d i n g . 2 C o l u m n 1: p r o b a b i l i t y b e t w e e n f i r s t and s e c o n d s e a s o n l i s t e d C o l u m n 2: p r o b a b i l i t y be tween f i r s t and l a s t s e a s o n C o l u m n 3: p r o b a b i l i t y b e t w e e n s e c o n d and l a s t s e a s o n 212 s p e c i e s at d i f f e r e n t seasons i n sympatry. C. b i f i d a showed most p o s i t i v e r e a c t i o n s to diaptomid, chironomid and ephemeropteran a n t i s e r a i n summer and l e a s t i n f a l l , although r e a c t i o n s with zygopteran antiserum were a t a high l e v e l i n both summer and f a l l samples. By c o n t r a s t , r e a c t i o n s to daphniids were seen i n equal p r o p o r t i o n s at a l l seasons. In C. e x p l e t a , most p o s i t i v e r e s u l t s with diaptomids, chironomids, ephemeropterans and zygopterans were seen i n f a l l , but few e a r l i e r i n the year, whereas r e a c t i o n s with daphniid antiserum were most frequent i n s p r i n g and fewest i n f a l l . I t thus seems c l e a r t h a t , when a l l i n s t a r s are con s i d e r e d t o g e t h e r , the d i e t a r y t h r u s t of each s p e c i e s was d i r e c t e d t o d i f f e r e n t stages. However, the p r o p o r t i o n s of stages examined i n s e r o l o g i c a l samples do not n e c e s s a r i l y represent the p r o p o r t i o n s found i n the f i e l d , although the d i s t r i b u t i o n of the stages with season o b v i o u s l y i s a f u n c t i o n of t h e i r presence. To explore t h i s trend f u r t h e r i n the sympatric s i t u a t i o n , the degree of f e e d i n g on each f o o d s t u f f was compared f o r the two Cenocgrixa s p e c i e s . Table XIVIII i n d i c a t e s that i n f a l l , when C. e x p l e t a showed a high p r o p o r t i o n of r e a c t i o n s with a l l f o o d s t u f f s t e s t e d except d a p h n i i d s , markedly mere p o s i t i v e r e a c t i o n s with a l l a n t i s e r a were seen i n guts of t h i s s p e c i e s than i n C. b i f i d a , when a l l a n t i s e r a are co n s i d e r e d together. Conversely, i n summer, when r e a c t i o n s i n C. b i f i d a guts were g e n e r a l l y most frequent (table XLVII) t h i s s p e c i e s shewed r e l a t i v e l y more p o s i t i v e r e a c t i o n s i n guts than did C. e x p l e t a f o r a l l foods t e s t e d , summed. T h i s may be r e l a t e d to the i n s t a r s present at each season, s i n c e the i n i t i a l analyses shewed o f t e n 213 Table XLVIII. Seasonal preferences of Cenocorixa s p e c i e s f o r d i f f e r e n t f o o d s t u f f s , from s e r o l o g i c a l data. ~ ~ ANTISERUM Diaptomid Daphniid Chaoborid Chironomid Ephemeropt Zygopteran SPRING s u p e r i o r sp. P= Bi 0.31 Ex 0.00 Bi 0.80 B i 0.21 B i 0.44 Ex 0.27 SUMMER s u p e r i o r sp. P= B i 0.00 Bi 0.00 Bi 0.75 Bi 0.00 Bi 0.00 Bi 0.52 FALL s u p e r i o r sp. p= Ex 0.36 Ex 0.59 Ex 0.37 Ex 0.19 Ex 0.00 Ex 0.40 214 g r e a t d i s c r e p a n c i e s i n p r o p o r t i o n s o f p o s i t i v e r e a c t i o n s i n guts between th e d i f f e r e n t s t a g e s . A g a i n , the r a t i o of the s t a g e s i n samples may not p a r a l l e l the r a t i o i n the f i e l d at each s e a s o n , so t h a t t h i s t r e n d , w h i l e r e a l as f a r as the s e r o l o g i c a l a n a l y s e s go, may not be a t r u e r e p r e s e n t a t i o n o f the f i e l d c o n d i t i o n . Summary of C e n o c o r i x a f e e d i n g p a t t e r n s O v e r a l l r e s u l t s of f e e d i n g p a t t e r n , as determined l y s e r o l o g i c a l gut e x a m i n a t i o n , were g i v e n i n Table X X V I I I (above), and have been r e - a n a l y s e d i n Table XIIX to i n d i c a t e d i f f e r e n c e s between the two s p e c i e s . When data are pooled f o r i n s t a r s , l a k e s and s e a sons of c o l l e c t i o n , C. e x p l e t a showed r e l a t i v e l y more p o s i t i v e r e a c t i o n s w i t h c h i r o n o m i d and d a p h n i i d a n t i s e r a than d i d C. b i f i d a , a l t h o u g h the r e v e r s e t r e n d was seen w i t h d i a p t o m i d and z y g o p t e r a n a n t i s e r a . These r e s u l t s must now be r e i n t e r p r e t e d i n terms of d i f f e r e n c e s between i n s t a r s and s p e c i e s at d i f f e r e n t seasons and i n each l a k e group, and i n t h e l i g h t of i n f o r m a t i o n gained from f e e d i n g e x p e r i m e n t s . Thus the d i f f e r e n c e i n f e e d i n g on amphipods and c h a o b o r i d s i s a f f e c t e d by t h e r e s t r i c t i o n of these a n i m a l s c h i e f l y t o the lower s a l i n i t i e s . F eeding e x p e r i m e n t s i n d i c a t e d t h a t a l t h o u g h they d i d not encounter them i n n a t u r e , C. e x p l e t a took dead amphipods more r e a d i l y than d i d C. b i f i d a . The f a c t t h a t r e l a t i v e l y more p o s i t i v e r e a c t i o n s t o 215 Table XLIX. D i f f e r e n c e s i n f e e d i n g p a t t e r n between C. b i f i d a and C. e x p l e t a i n terms of percentage of p o s i t i v e r e a c t i o n s to the a n t i s e r a l i s t e d . ANTISERUM SUPERIOR p r o b a b i l i t y (p) SPECIES DIAPTOMID C. BIFIDA 0. 1125 DAPHNIID c. EXPLETA 0. 0814 AMPHIPOD C. BIFIDA 0. 5616 CHAOBORID c. EXPLETA 0. 9235 CHIRONOMID C. EXPLETA 0. 0877 EPHEMEROPTERAN C. BIFIDA 0. 6453 ZYGOPTERAN C. BIFIDA 0. 1786 PULMONATE £• EXPLETA 0. 5558 BLUE-GREEN ALGAE c. BIFIDA 0. 5589 MACROPHYTE C. BIFIDA 0. 6808 216 chironomid antiserum were detected i n C. b i f i d a than i n £• ®2££lsta guts (p=0.09) i s not e x p l a i n a b l e on these terms, as i n both s p e c i e s chironomids were the most f r e q u e n t l y eaten prey i n s e r o l o g i c a l experiments. Although there i s l i t t l e i n f o r m a t i o n on the fauna of Lake LB 2, d i f f e r e n c e s i n percentages of p o s i t i v e r e a c t i o n s t o t h i s antiserum i n a l l o p a t r y and sympatry and between s p e c i e s do not appear to be a t t r i b u t a b l e to d i f f e r e n c e s i n abundance of t h i s f o o d s t u f f . T h i s i s apparent i n Table L, which gi v e s percentages of p o s i t i v e r e a c t i o n s f o r foods present a c r o s s the e n t i r e range of lakes s t u d i e d . These are compared f o r each s p e c i e s and l a k e group, to look f o r p o s s i b l e evidence of lessened competition f o r a food item i n sympatry, and hence f o r some c l u e towards c o m p e t i t i v e displacement. For C. e x p l e t a , although r e a c t i o n s with chironomid a n t i -serum were s l i g h t l y more numerous i n a l l o p a t r y than i n sympatry (p=0.45), the r e v e r s e was s i g n i f i c a n t f o r daphniids, ephemeropterans and zygopterans (p=0.00 i n each c a s e ) . However, a l l these f o o d s t u f f s may have been r a r e i n Lake LE 2, l i m i t e d i n abundance by the high a l k a l i n i t y l e v e l s . Data f o r C. b i f i d a a l s o suggest that the l e v e l of feeding on chironomids, represented ty the percentage of guts showing p o s i t i v e p r e c i p i t i n r e a c t i o n s , f e l l s l i g h t l y though n o n - s i g n i f i c a n t l y i n sympatry (p=0.34), but t h a t p r o p o r t i o n s of guts r e a c t i n g to daphniid and ephemeropteran a n t i s e r a i n c r e a s e d s i g n i f i c a n t l y i n the sympatric s i t u a t i o n (p=0.00 f o r both c a s e s ) , h a r d l y the c l a s s i c c o m p e t itive e x c l u s i o n p i c t u r e . When diaptomids (not presented i n the Table as they do not extend i n t o the more freshwater lakes) are c o n s i d e r e d , however, highest numbers of p o s i t i v e r e a c t i o n s with 217 Table L. Percentages of p o s i t i v e r e a c t i o n s to a n t i s e r a prepared f o r s p e c i e s common a c r o s s the e n t i r e range of lakes s t u d i e d , with t o t a l numbers of guts t e s t e d i n both s p e c i e s , d i v i d e d by lake group. ANTISERUM D a p h n i i d Chironomid Ephemeropt Zygopteran CORIXID LAKES 1 LAKES 2 LAKES SPECIES (sympatric) C. b i f i d a 38. 4 12.9 C. e x p l e t a 17. 5 45. 4 C. b i f i d a 74. a 77.0 C. e x p l e t a 71. 1 67. 7 C. b i f i d a 29. 5 14. 0 C. e x p l e t a 16.0 27. 7 C. b i f i d a 56. 9 56. 1 C. e x p l e t a 5. 2 61.9 TOTALS (N) C. b i f i d a 760 456 C. e x p l e t a 294 260 218 guts (29.6%) were found i n C. e x p l e t a from Lake LE 2, dropping by almost 50% to 16.2% i n sympatry (p=0.00), when s i m i l a r r e s u l t s f o r C. b i f i d a were 20.9% of guts r e a c t i n g p o s i t i v e l y . As the food base may be narrower i n the h i g h e s t s a l i n i t y of LB 2 (chapter I I ) , such a high l e v e l of p o s i t i v e r e a c t i o n s of C. e x p l e t a there might be e x p l a i n a b l e by the r e s t r i c t e d food a v a i l a b l e . Feeding experiments suggested that although with mixed plankton C. e x p l e t a took markedly more diaptomids than d i d £ • i i° s i n g l e prey s t u d i e s both s p e c i e s took diaptomids at an almost egual r a t e , except f o r the s m a l l e s t i n s t a r s . £• fiifi^S t however, took more dap h n i i d s than d i d C. expleta These r e s u l t s are not r e f l e c t e d i n the data f c r f i e l d - c o l l e c t e d c o r i x i d gut c o n t e n t s , but with a l l i n d i r e c t technigues of gut examination there i s no easy way to g u a n t i f y the p o s i t i v e r e a c t i o n s recorded i n terms of amounts of p l a n k t e r s eaten. Thus, a gut c o n t a i n i n g the body j u i c e s of one diaptomid and many dap h n i i d s might s c o r e p o s i t i v e l y f o r each, g i v i n g a f a l s e r e p r e s e n t a t i o n of the d i e t , and t h i s i s e s p e c i a l l y true with small food items of which s e v e r a l may be taken i n a s h c r t period of time. Feeding on l a r g e r preys or l a r g e dead organisms may r e s u l t i n a t r u e r p i c t u r e from s e r o l o g i c a l data, but the technique does not allow f o r d i s c r i m i n a t i o n between l i v e and dead preys. Thus, the f e e d i n g p a t t e r n v a r i e s d y n a m i c a l l y with time of year and i n s t a r s present i n any one lake, and any summary can only be of l i m i t e d u s e f u l n e s s . However, the r e s u l t s o v e r a l l 219 i n d i c a t e c a r n i v o r y i n a l l i n s t a r s , with the bulk of the d i e t being composed o f chironomids and zygopterans i n both Cenocorixa s p e c i e s , with s m a l l e r numbers of other prey. There i s thus no s t r o n g i n d i c a t i o n of d i f f e r e n c e s i n d i e t e i t h e r i n the a l l o p a t r i c or sympatric s i t u a t i o n s , and the s p e c i e s must t h e r e f o r e be co n s i d e r e d as p o t e n t i a l competitors i n the sympatric s i t u a t i o n i n the moderately h i g h l y s a l i n e lakes of Becher's P r a i r i e where they c o e x i s t . Gut c o l o u r and s e r o l o g i c a l l y determined gut contents Gut c o l o u r was noted f o r 1386 of the c o r i x i d s t e s t e d s e r o l o g i c a l l y ; these are l i s t e d with t h e i r gut contents i n Table L I . There i s no c l e a r c o r r e l a t i o n of d i e t with a p a r t i c u l a r c o l o u r . Further, s i n c e i n most cases m u l t i p l e contents were recorded f o r a gut, the c o l o u r observed may not be one c h a r a c t e r i s t i c of any of these f o o d s t u f f s , but a mixture, depending on the amounts of each present. About HH% of a l l guts were recorded as 'brown*. The category ' c o l o u r l e s s ' and o f t e n supposedly empty had the l a r g e s t p r o p o r t i o n of negative r e s u l t s f o r gut c o n t e n t s , although these s t i l l formed under 20% of a l l guts. Low counts of p l a n k t e r s , chironomids and ephemeropterans were recorded i n these c o l o u r l e s s guts. In the c o r i x i d s with yellow guts, chironomids, ephemeropterans and zygopterans were r e l a t i v e l y important, and diaptomids and chironomids dominated tan guts. 220 T a b l e L I . G u t c o l o u r s i n r e l a t i o n t o s e r o l o g i c a l l y d e t e r m i n e d g u t c o n t e n t s o f 1386 f i e l d - c o l l e c t e d C e n o c o r i x a s p e c i e s . L a r g e s t p e r c e n t a g e u n d e r l i n e d f o r e a c h c o l o u r . GUT COLOUR: ANTIS EEA h DIAPTOMIDS DAPHNIID AMPHIPOD CHA0BORID CHIRONOMID EPHEMEROPT ZYGOPTERAN PULMONATE B L U E - G R E E N MACROPHYTES | t o t a l s o ~~T 1 [ 2 none | y e l l o w t a n I I PERCENTAGE REACTING 8 . 8 | 1 5 . 3 I 2 1 . 4 I I 1 3 . 5 | 16 .1 | 2 2 . 7 I I I I 0 . 9 I I 3 . 5 | 3 . 2 | 3 . 6 I I 6 5 . 5 | 8 5 . 5 | 8 0 . 5 I ! 2 0 . 5 | 3 4 . 7 | 2 6 . 4 I I 5 7 . 9 | 6 2 . 9 | 6 1 . 8 I I 0 . 6 | I I | 0 . 5 I I 0 . 5 +-3 I 4 I 5 I n= r e d I brown | o l i v e 1 1 SHOWING EACH | COLOUR J 4 7 . 6 i -I 1 1 2 . 4 | 7 .1 I , , 260 2 7 . 1 1 1 1 2 8 . 6 | 1 2 . 5 ! 330 1 1 1 2 . 5 | 1.8 ! 18 0 . 5 1 1 1 2 . 6 | 5 . 4 38 7 1 . 9 1 1 1 7 3 . 7 | 8 0 . 4 | 1037 2 9 . 5 1 1 1 2 2 . 5 | 2 6 . 8 349 5 2 . 4 1 1 1 5 6 . 0 | 6 9 . 6 | 801 1 1 1 0 . 2 | 3 1 1 1 0 .3 | 3 0 . 5 1 1 — — — — — — i 1 210 -+-1 j_ 60 5 | 56 1386 H I 1 221 Almost 50% of red guts contained diaptomids, the most marked c o r r e l a t i o n of d i e t and gut c o l o u r seen. R e l a t i v e l y high amounts of daphniids and ephemeropterans were a l s c recorded i n re d guts; and among a l l guts showing no s e r o l o g i c a l r e a c t i o n s with the a n t i s e r a t e s t e d , red guts were fewest. Brcwn guts, the dominant group, had no d i s t i n c t i v e food item predominating, while the o l i v e guts, forming about 10% of the t o t a l t e s t e d , had low numbers of p o s i t i v e r e a c t i o n s f o r p l a n k t e r s . The r e l a t i v e l y high p r o p o r t i o n of guts i n t h i s and the l a s t group f o r which r e s u l t s of s e r o l o g i c a l t e s t s were negative i n d i c a t e d t h a t the a n t i s e r a used d i d not cover the e n t i r e range cf feeds of c o r i x i d s i n the f i e l d . D i e t s of other c o r i x i d s p e c i e s Data f o r other c o r i x i d s p e c i e s are much fewer than those f o r the two s p e c i e s of Cenocorixa , and t h e r e f o r e d i e t s are harder to e s t a b l i s h with c o n f i d e n c e . However, they are of i n t e r e s t , s i n c e these other c o r i x i d s p e c i e s c o e x i s t with £• £i£i^l i n t n e three most freshwater lakes cn Becher's P r a i r i e . P o s i t i v e p r e c i p i t i n r e a c t i o n s with pulmonates, b l u e -green and fi l a m e n t o u s green algae and higher p l a n t s were not recorded i n any t e s t s . In terms of the percentage cccurrence as p o s i t i v e p r e c i p i t i n r e a c t i o n s , zygopterans and chironomids were the major items of d i e t i n every c o r i x i d s p e c i e s s t u d i e d . Among other major foods, daphniids were recorded i n over 20% of 222 C . a m e r i c a n a and e p h e m e r o p t e r a n s i n H . l a e v i g a t a and C . a u d e n i . T a b l e X X V I I I (above) i n d i c a t e s t h e f o o d p a t t e r n f o r a l l s i x c o r i x i d s p e c i e s t a k e n i n t h i s s t u d y , a s d e t e r m i n e d by s e r o l o g i c a l a n a l y s i s o f g u t c o n t e n t s . When s p e c i e s were c o m p a r e d u s i n g t h e c h i - s g u a r e t e s t , S i g a r a s p . and C . a m e r i c a n a showed no t r e n d s o f d i f f e r e n c e i n d i e t . H o w e v e r , r e s u l t s were d i f f e r e n t and c o u n t e r - i n t u i t i v e f o r t h e s y m p a t r i c b u t d i f f e r e n t l y s i z e d H. l a e v i g a t a and C . a u d e n i . I n g u t s o f t h e s m a l l C . a u d e n i s i g n i f i c a n t l y more p o s i t i v e p r e c i p i t i n r e a c t i o n s were s e e n w i t h a m p h i p o d a n t i s e r u m t h a n i n t h e l a r g e r H. l a e v i g a t a , and i n C . a u d e n i t h e r e were a l s o l e s s r e a c t i o n s t o c h a o b o r i d s , b u t more t o c h i r o n o m i d s , t h a n i n H. l a e v i g a t a . When r e s u l t s f c r e a c h c o r i x i d s p e c i e s were c o m p a r e d w i t h t h o s e f o r s y m p a t r i c s p e c i m e n s o f C . b i f i d a i n t h e more f r e s h w a t e r l a k e s ( t a b l e L I I ) H . l a e v i g a t a c l e a r l y shewed a g r e a t e r f r e q u e n c y o f p o s i t i v e r e a c t i o n s w i t h c h a o b o r i d s a n d a l s o had somewhat more s e r o l o g i c a l e v i d e n c e o f f e e d i n g on c h i r o n o m i d s t h a n d i d C . b i f i d a . In T a b l e X X V I I I t h e h i g h e s t t o t a l p e r c e n t a g e o f f o o d s t u f f s i n t h e f o u r m i n o r c o r i x i d s p e c i e s was s e e n i n C . a m e r i c a n a , w i t h a l e v e l a b o u t e g u a l t o t h a t o f C . e x p l e t a a l t h o u g h l e s s t h a n t h a t f o r C . b i f i d a . A l e v e l o f 200% t o t a l f o o d s r e p r e s e n t s a mean r e c o r d o f two f e e d i t e m s p e r g u t t e s t e d . C . a u d e n i showed m a r k e d l y more p o s i t i v e r e a c t i o n s w i t h a m p h i p o d s t h a n d i d C , b i f i d a and s l i g h t l y mere w i t h d i a p t o m i d a n t i s e r u m . H o w e v e r , as d i a p t o m i d s d i d n o t o c c u r i n the more 223 T a b l e L I I . C o m p a r i s o n o f f e e d i n g d a t a f o r d i f f e r e n t s p e c i e s o f c o r i x i d s e n c o u n t e r e d i n t h e s t u d y w i t h C . b i f i d a , u s i n g c h i s q u a r e t e s t s . C . b i f i d a C . b i f i d a C . b i f i d a C . b i f i d a v e r s u s v e r s u s v e r s u s v e r s u s H. l a e v i g a t a C . a u d e n i C . a m e r i c a n a S i g a r a sp ANTISERUM: P= P= P= P= D i a p t o m i d B i 0 . 8 5 B i 0 . 6 0 B i 0 . 8 9 B i 0 . 9 2 D a p h n i i d B i 0 . 1 9 B i 0 . 2 5 B i 0 . 7 3 B i 0 . 8 9 Amphi pod B i 0 . 2 8 C a l l 0 . 0 0 Cym 0 . 6 6 B i 0 . 5 7 C h a o b o r i d H e s p 0 . 0 0 B i 0 . 7 4 B i 0 . 75 B i 0 . 84 C h i r o n o m i d Hesp 0 . 0 7 B i 0 . 8 2 B i 0 . 7 3 B i 0 . 9 4 E p h e m e r o p t e r a n Hesp 0 . 2 4 C a l l 0 . 16 B i 0 . 4 8 B i 0 . 38 Z y g o p t e r a n Hesp 0 .21 C a l l 0 . 6 4 B i 0 . 7 6 B i 0 . 8 3 224 freshwater l a k e s , t h i s r e a c t i o n may be s p u r i o u s . There were no c l e a r - c u t d i f f e r e n c e s ,seen i n comparing S i g a r a sp. or C. americana r e s u l t s with data f o r C. b i f i d a , p a r t l y due to the few t e s t s run f o r these r e l a t i v e l y uncommon s p e c i e s (n=11 and 10 r e s p e c t i v e l y ) , but C. b i f i d a tended to show s l i g h t l y higher p r o p o r t i o n s of p o s i t i v e r e s u l t s with almost a l l a n t i s e r a t e s t e d than d i d e i t h e r of these s p e c i e s . DISCUSSION S e r o l o g i c a l technigues rank i n s e n s i t i v i t y with the use of r a d i o - a c t i v e t r a c e r s f o r the study of gut contents (Davies, 1969) and have the advantage of not r e q u i r i n g tampering with the environment. T e s t s of the a n t i s e r a i n d i c a t e that the techniques used were within experimental l i m i t s r e l i a b l e i n d e s c r i b i n g the gut contents of the s t u d i e d c o r i x i d s . Davies (1969) found a n t i s e r a to r e t a i n t h e i r e f f e c t i v e n e s s a f t e r storage f o r two years; no s e r a i n the present study were s t o r e d f o r more than 12 months. Prey homogenates were combined from s e v e r a l r e l a t e d s p e c i e s , sometimes c o l l e c t e d from widely separated l o c a l i t i e s , i n an attempt to produce an antiserum with broad s p e c i f i c i t y f o r a group r a t h e r than f o r a s i n g l e s p e c i e s . On h i n d s i g h t i t might have been p r e f e r a b l e to have used one homogeneous s t r a i n of organisms f o r each antiserum, t r u s t i n g to the use cf an adjuvant 225 ( J u r i n and Tannock, 1972). m u l t i p l e i n j e c t i o n s (Proem, 1913) or in c r e a s e d amounts of antigen (Rabat and Mayer, 1961) tc spread the s p e c i f i c i t y . Thus, even s p e c i f i c a n t i g e n i c m a t e r i a l obtained by homogenising g e n e t i c a l l y i d e n t i c a l human c e l l i s c - s t r a i n s produced 14 d i f f e r e n t s t r o n g bands and as many weaker ones through r e a c t i o n s with the complex mixture c f a n t i g e n s r e p r e s e n t e d even i n t h i s uniform source (Brand, 1965).. Also, the r e a c t i o n s v a r i e d depending on the i n d i v i d u a l serum-producing animal; thus s t r e n g t h of r e a c t i o n and number of bands cannot always be equated with s t r e n g t h of the a n t i g e n i c m a t e r i a l t e s t e d or q u a n t i t y of food present. Q u a n t i t a t i o n o f the p r e c i p i t i n response has been attempted by v a r i o u s authors. J a l a n t i and Henney (1972) s t u d i e d r a d i a l immunodiffusion techniques to q u a n t i f y s t r e n g t h of an t i g e n s and a n t i b o d i e s , and Mulder e t a l . , (1972) a l s o used s i n g l e r a d i a l d i f f u s i o n to q u a n t i f y a n t i g e n i c p r o t e i n s , a process now used e x t e n s i v e l y to measure immunoglobulin c o n c e n t r a t i o n s i n s e r a . Preer (1956) attempted to q u a n t i f y a technique of double d i f f u s i o n i n agar, while Duquesnoy (1973) devised a method of r a p i d q u a n t i t a t i o n by e l e c t r o i m m u n o d i f f u s i o n , using d i s c s of f i l t e r paper of uniform s i z e soaked i n the a n t i s e r a and placed on agar. However, i t seemed i n a d v i s a b l e t o attempt such methods i n s t u d i e s of f i e l d - c o l l e c t e d organisms where there are sc many unknowns. Since the c o r i x i d s seem to i n g e s t a good p r o p o r t i o n of each prey caught (chapter IV), the amount of each f o o d s t u f f present i n the d i e t w i l l depend on the food item s i z e , and perhaps i t s s p e c i f i c i d e n t i t y , as w e l l as cn how long ago i t was eaten; i n f o r m a t i o n that cannot be obtained a £cstericri. 226 T h e r e f o r e , both s t r o n g and weak p r e c i p i t i n bands were taken as p o s i t i v e evidence f o r a food item, and q u a n t i t a t i o n was based simply on p r o p o r t i o n s of i n d i v i d u a l s t e s t e d c o n t a i n i n g that food item, r e l a t i v e to other d i e t a r y components. The main drawback to a n a l y s i s of the r e s u l t s obtained i s common to other i n d i r e c t methods of d i e t a r y e v a l u a t i o n through gut contents, i n that predation cannot be d i s t i n g u i s h e d from c a r r i o n f e e d i n g ( i . e . Sutton, 1970); f o r t h i s reason separate f e e d i n g experiments are necessary to s u b s t a n t i a t e r e s u l t s of a l l such s t u d i e s . The d i f f e r e n c e between p r e d a t i o n and scavenging h a b i t s i n d i c a t e very d i f f e r e n t r o l e s i n a community. A t h i r d p o s s i b i l i t y i s noted by Fox and M a c l e l l a n d (1956) who i n d i c a t e that p o s i t i v e s e r o l o g i c a l r e s u l t s f o r wireworms i n c a r a b i d guts might be the r e s u l t of c a r a b i d s feeding on the wireworm's p r e d a t o r s as much as on wireworms themselves. The g e n e r a l c o r i x i d food p a t t e r n which emerges from t h i s study i s s i m i l a r t o that r e v e a l e d by feed i n g experiments i n the preceding chapter. Thus, the d i f f e r e n c e s between fundamental and r e a l i s e d niches are not great. A l l f i v e genera s t u d i e d were shown to be l a r g e l y c a r n i v o r o u s , on f i r s t o b s e r v a t i o n s C. b i f i d a and C. e x p l e t a showed only s l i g h t d i f f e r e n c e s i n t h e i r food spectrum and c o n c e n t r a t i o n but these take cn greater s i g n i f i c a n c e when r e s u l t s are broken down by i n s t a r s , h a b i t a t s and seasons. Stud i e s to c o r r e l a t e gut c o l o u r with contents i n d i c a t e d a c e r t a i n discrepancy i n r e s u l t s , s i n c e about 10% of guts t e s t e d , although o f t e n s t r o n g l y c o l o u r e d , were according to s e r o l o g i c a l evidence empty. Thus perhaps about a tenth of the 227 d i e t may c o n s i s t of untested foods; among t h i s may fce algae and higher p l a n t s not r e a c t i n g with the s e r a used. Accuracy of the s e r o l o g i c a l t e s t s seems to be f a i r l y high. Thus, under 1% of guts showed a p o s i t i v e r e a c t i o n to the antiserum prepared f o r a q u a t i c macrophytes, which, although immunologically i n e r t , was used i n many t e s t s tc r e p l a c e the c o n t r o l w e l l of s a l i n e used by Pickavance (970). S i m i l a r l y low l e v e l s of r e a c t i o n were seen with pulmonate and blue-green a l g a l s e r a ; t h i s i n d i c a t e d t h a t although these sera were r e a c t i v e i n s p e c i f i c t e s t s , the feed substances r e p r e s e n t e d d i d not g e n e r a l l y appear i n guts examined. It i s thus l i k e l y t h a t s n a i l s and t h e i r eggs do not form any r e g u l a r p a r t of the c o r i x i d d i e t i n c e n t r a l B r i t i s h Columbia. However, blue-green m a t e r i a l , apparently a l g a l , was seen on one occ a s i o n i n the guts of s e v e r a l c o r i x i d s c o l l e c t e d i n l a t e f a l l . a t Lake LB 2, but t h i s was not borne out by gut a n a l y s i s , o n l y 17 t e s t s were made of a d u l t C. b i f i d a using serum f o r fi l a m e n t o u s green algae such as Cladophora, and although three cf these were p o s i t i v e , the r e s u l t s were too s m a l l f o r any wide s i g n i f i c a n c e t o be drawn, s i n c e t h i s antiserum was a l s o weak i n s t r e n g t h t e s t s . A major source of discrepancy between f i e l d o b s e r v a t i o n s and s e r o l o g i c a l gut data i s seen i n the r e l a t i v e l y high number of guts from freshwater l a k e c o l l e c t i o n s r e a c t i n g with the diaptomid antiserum (4.7% f o r C. b i f i d a and up to 10.0% f o r the l e s s common c o r i x i d s p e c i e s s t u d i e d ) . I t seems most l i k e l y that i n t h i s case the s p e c i f i c i t y of the serum was broad, probably as 228 a r e s u l t of i t s p r e p a r a t i o n using an adjuvant, and the r e s u l t s are r e a c t i o n s to r e l a t e d c r u s t a c e a n s , p o s s i b l y the c y c l o p o i d copepods common i n the freshwater l a k e s . By c o n t r a s t , r e s u l t s f o r amphipods and chaoborids i n the higher s a l i n i t y l a k e s were a t a much lower l e v e l (around LOSS) and may be put down e i t h e r to experimental e r r o r or to the o c c a s i o n a l presence of both of these forms i n c e r t a i n of the higher s a l i n i t y lakes (chapter I I ) . The g r e a t e r p r o p o r t i o n of diaptomids i n guts of C. e x p l e t a than i n C. b i f i d a o v e r a l l i s s i m i l a r l y connected with d i s t r i b u t i o n of the two Cenocorixa s p e c i e s and the food item. The examples where r e a c t i o n s to chaoborids, amphipods and diaptomids c o n t r a s t with d i s t r i b u t i o n patterns emphasise the need f o r c a u t i o n i n f o r m u l a t i n g summary statements about d i e t a r y p r e f e r e n c e s . However, geographic d i s t r i b u t i o n cannot te invoked to e x p l a i n why C. b i f i d a ate s i g n i f i c a n t l y more daphniids and ephemeropterans i n sympatry than i n a l l o p a t r y , s i n c e both were ap p a r e n t l y abundant i n a l l l a k e s s t u d i e d . In a l k a l i n e l a k e s , the fauna i s a p p a r e n t l y l e s s d i v e r s e (chapter II) and because of the g e n e r a l absence of s e a s o n a l l y appearing masses of a g u a t i c p l a n t s , most organisms i n these lakes may be g e n e r a l l y l e s s dependent upon t h i s f o r concealment and breeding. Numbers may t h e r e f o r e be more constant from season to season, as was seen i n the p l a n k t e r s a t l e a s t . S i n c e there i s a g r e a t e r v a r i e t y of foods i n the more freshwater lakes than i n the more a l k a l i n e ones, i n d i v i d u a l items of food may a l s o f i g u r e l e s s f r e q u e n t l y i n the d i e t s of c o r i x i d s c o l l e c t e d t h e r e . Most t e s t s focussed on a p a r t i c u l a r l a k e group r a t h e r than 229 on o v e r a l l data. A f u r t h e r area of p o s s i b l e b i a s i n v e s t i g a t e d was the d i f f e r e n c e between d i e t s of Cenocorixa s p e c i e s c o l l e c t e d at d i f f e r e n t times of year. C. b i f i d a fed s i g n i f i c a n t l y mere on diaptomids, chironomids and ephemeropterans i n summer than a t other times of y e a r , whereas C. e x p l e t a fed s i g n i f i c a n t l y h i g h e s t on these items i n f a l l . Examining the s i t u a t i o n with diaptomids i n more d e t a i l , s i g n i f i c a n t l y more were consumed by £• £i£i£a than by C. e x p l e t a i n summer but the r e v e r s e tendency, n o n - s i g n i f i c a n t , was seen i n f a l l . Seasons f o r such seasonal d i f f e r e n c e s may l i e i n the d i f f e r i n g abundance cf organisms i n d i f f e r e n t seasons. Plankton samples i n d i c a t e t hat both daphniids and diaptomids were frequent a l l year i n the Becher's P r a i r i e l a k e s , although showing a summer peak (chapter I I ) . However, o b s e r v a t i o n s at Lake LB 2 i n d i c a t e d t hat i n s p r i n g diaptomids were not e v i d e n t , although abundant l a t e r i n the year. Perhaps the abundant diaptomid prey on Becher's P r a i r i e may have been of d i f f e r i n g a v a i l a b i l i t y t o the two s p e c i e s at d i f f e r e n t seasons i n some manner. If the p a t t e r n i s not e x p l a i n a b l e by the f a c t t h a t many of the C. e x p l e t a examined came from Lake LE 2, an a l t e r n a t i v e e x p l a n a t i o n might be t h a t C. e x p l e t a may have f e d c h i e f l y on, f o r i n s t a n c e , dead and dying p l a n k t e r s , which may have been commonest i n the f a l l . R e s u l t s from f e e d i n g experiments agree i n part with t h i s , but only s u b j e c t i v e o b s e r v a t i o n s on m o r t a l i t y l e v e l s of diaptomids i n the f i e l d a t d i f f e r e n t times of year e x i s t to c o r r o b o r a t e the hypothesis. A l t e r n a t i v e l y , i f , as feeding experiments i n d i c a t e , a d u l t s took r e l a t i v e l y l e s s diaptomids than d i d j u v e n i l e s and i f summer samples c h i e f l y contained C. e x p l e t a a d u l t s and C. b i f i d a 230 j u v e n i l e s and the f a l l samples v i c e v e r s a , the same r e s u l t s could be e x p l a i n e d . Among those examined s e r o l o g i c a l l y , a d u l t s of both s p e c i e s made up 17% of summer samples, but i n the f a l l 92% of C. b i f i d a were a d u l t , but only 36% cf C. e x p l e t a , a f a c t perhaps r e l a t e d to the t h i r d summer generation seen i n the l a t t e r s p e c i e s . Thus the c o n d i t i o n s to support the f i n d i n g s are p a r t l y c o n s i s t e n t with the t h i r d hypothesis. I t would seem l i k e l y t h a t a l l three suggestions have some b a s i s i n forming the observed summary f e e d i n g p a t t e r n f o r each s p e c i e s at d i f f e r e n t seasons. Other foods s e a s o n a l l y evident i n the guts may r e f l e c t d i f f e r e n c e s i n the a c t i v i t y of prey or predator with temperature, thus e x p l a i n i n g the cases of C. b i f i d a shewing most evidence of f e e d i n g on chironomids and ephemeropterans i n summer. The p i c u r e may be f u r t h e r complicated by prey organism emergence p a t t e r n s and s i z e changes with growth. There was some i n d i c a t i o n t h a t i n each s p e c i e s d i f f e r e n t i n s t a r s had t h e i r p r e f e r r e d food item or items. Thus i f one i n s t a r predominated i n a c o l l e c t i o n , t h i s might b i a s o v e r a l l r e s u l t s towards the p r e f e r r e d food of that i n s t a r . from Table XLVI l i s t i n g such i n s t a r food p r e f e r e n c e s , i t could be seen that the d i e t i n c r e a s e d i n complexity with i n c r e a s i n g stage, s t a r t i n g i n young nymphs with ' p l a n k t o n i c food, chironomids and zygopterans and expanding to i n c l u d e other l e s s important items such as chaoborids and amphipods i n l a r g e r i n s t a r s . However, i n the s m a l l e s t i n s t a r s s m a l l volumes of foods may not show up i n the s e r o l o g y t e s t s . The p a t t e r n f o r g r e a t e r f e e d i n g by cne or 231 o ther s p e c i e s on d i f f e r e n t f o o d s t u f f s showed nc c o n s i s t e n t t r e n d between i n s t a r s , but i t n e v e r t h e l e s s r e p r e s e n t e d a r e a l p a r t i t i o n i n g of the food spect rum. O v e r a l l , t h e r e was a g e n e r a l l y g r e a t e r l e v e l of f e e d i n g seen i n C . b i f i d a over C . e x p l e t a f o r the foods t e s t e d . A ' p l a t e a u * l e v e l o f f e e d i n g on most f o o d s t u f f s was reached e a r l i e r i n C. e x p l e t a than i n C . b i f i d a . T h i s i s l i k e l y tc be due t o the s l i g h t l y l a r g e r s i z e (Scudder , 1966) of C . e x p l e t a and i t s more r o b u s t palae a t a l l s t a g e s , p e r m i t t i n g more e f f i c i e n t c a p t u r e and h a n d l i n g of a wider v a r i e t y cf and l a r g e r f o o d s . However, above t h i s t h r e s h o l d s i z e , C. b i f i d a was g e n e r a l l y s u p e r i o r to C. e x p l e t a i n the amounts c f most f o o d s t a k e n . Thus from the t h i r d i n s t a r up C. e x p l e t a l o s e s i t s s u p e r i o r i t y i n terms of percentages f e e d i n g on both p l a n k t e r s and b e n t h i c n a i a d s , to C. b i f i d a . Sex d i f f e r e n c e s i n f e e d i n g were not u s u a l l y marked wi th C e n o c o r i x a , but males tended t o show more p o s i t i v e ev idence of f e e d i n g than females i n every c a s e . For C . b i f i d a those foods p r e s e n t a lmost e q u a l l y i n each sex p layed l i t t l e impor tance i n the d i e t o v e r a l l , but t h o s e foods showing l i t t l e sex d i f f e r e n c e i n C . e x p l e t a males and females i n c l u d e d a p h n i i d s and z y g o p t e r a n s , which compr ised on average 30% or H0% c f the d i e t , o r about s i x t imes as impor tan t as those i tems mentioned i n C . b i f i d a . Sex d i f f e r e n c e s i n f e e d i n g p a t t e r n may thus b i a s r e s u l t s more s i g n i f i c a n t l y i n the l a t t e r than the former s p e c i e s . R e s u l t s f o r o ther s p e c i e s of c o r i x i d s s y m p a t r i c wi th 232 C§aocorixa i n d i c a t e d t h a t H. l a e v i g a t a may eat more chircncmid l a r v a e , and C. audeni r e l a t i v e l y more amphipods and chironcmids, than other s p e c i e s . These two c o r i x i d s p e c i e s are the most d i s p a r a t e i n s i z e . The f e e d i n g p a t t e r n i n d i c a t e d , i f r e a l , may be r e l a t e d to h a b i t a t or swimming behaviour d i f f e r e n c e s , s uggesting t h a t H. l a e v i g a t a may freguent midwaters and C. audeni predominantly the b e n t h i c areas. H. l a e v i g a t a showed r a t h e r more p o s i t i v e r e a c t i o n s with ephemeropterans and zygopterans than i n Cenocorixa s p e c i e s ; t h i s may be r e l a t e d to i t s l a r g e r s i z e , and hence g r e a t e r a b i l i t y to capture these l a r g e r preys. An a l t e r n a t i v e hypothesis i s t h a t , being l a r g e r , £• i^§Ii2§iS guts c o n t a i n r e l a t i v e l y more food than these of Cenocorixa which might mean a g r e a t e r v a r i e t y of food at any one time. Thus a f o o d s t u f f might f i g u r e more f r e g u e n t l y i n ti • - l§L§vigata than i n Cenocorixa although i n volume r e l a t i v e to the other f o o d s t u f f s i t might comprise equal amounts; t h i s i n f o r m a t i o n was not a v a i l a b l e from the s e r o l o g i c a l data on guts examined. C. audeni showed more evidence of feeding than d i d C. b i f i d a on amphipods and perhaps a l s o on d a p h n i i d s . T h i s might be l i n k e d e i t h e r with adeptness of capture, or with the s m a l l sample s i z e , and r e i n f o r c e s the suggestion made e a r l i e r , that C. audeni may be more b e n t h i c - o r i e n t e d than H. l a e v i g a t a or C. b i f i d a . R e s u l t s with chironomid l a r v a e were i n c o n c l u s i v e i n t h i s r e g a r d , but i t should be remembered that i n the higher a l k a l i n i t y l a k e s these were u s u a l l y found i n the mud or water column, whereas i n the more freshwater bodies they were e q u a l l y abundant among the a q u a t i c v e g e t a t i o n and t h e r e f o r e perhaps more 233 v i s i b l e t o a v e g e t a t i o n - p e r c h i n g c o r i x i d . Thus i n sympatric s i t u a t i o n s with p o t e n t i a l f o r i n t e r s p e c i f i c c o m p e t i t i o n some c o r i x i d s p e c i e s are f e e d i n g on the same items. However, the concept of 'refuge' s p e c i e s , i n the sense of Reynoldson and Bellamy (1973) may be v a l i d here, as C. audeni may be using amphipods, and H. l a e v i g a t a chaoborids, as refuge items. These food organisms are not n e c e s s a r i l y the p r e f e r r e d item, but are r a t h e r those f o r which the s p e c i e s has a c l e a r s u p e r i o r i t y i n capture and f e e d i n g when competition becomes i n t e n s e or food l i m i t e d . They thus allow mere e f f e c t i v e p a r t i t i o n i n g of the r e s o u r c e . However, evidence of t h e i r use i s not marked i n the l a k e s s t u d i e d , where i n general fcod seems s u p e r f l u o u s to c o r i x i d needs. Thus although there i s evidence of much o v e r l a p of c o r i x i d d i e t , there i s no corresponding evidence f o r c o m p e t i t i o n . F i n a l l y , with i n c r e a s i n g evidence of d i f f e r e n c e s between i n s t a r s , sexes, h a b i t a t s and seasons, i t becomes p r o g r e s s i v e l y l e s s meaningful i n the present context to t a l k of a ' t y p i c a l ' c o r i x i d d i e t , s i n c e a l l these v a r i a b l e s i n t e r a c t to produce the summary. Thus, r e l a t i v e l y s m a l l samples i n experiments such as those of Young (1973) could produce very d i f f e r e n t r e s u l t s depending on the d i s t r i b u t i o n of i n s t a r s i n them cr even cn the dates of sampling. In the present r e s u l t s , obscuring of the true dynamic p a t t e r n n e c e s s a r i l y r e s u l t s from c o n s i d e r i n g the c o r i x i d s c o l l e c t e d under three main seasonal headings alone, but l o g i s t i c s of time.and f a c i l i t i e s p reclude a f u l l e r i n v e s t i g a t i o n of a narrower spectrum of organisms. With a g r e a t e r knowledge of 234 the background c o n d i t i o n s of the h a b i t a t , such r e s u l t s could be obtained, but i n an environment such as that of Eecher's P r a i r i e the s i t u a t i o n may vary s i g n i f i c a n t l y from year tc year, so that c o n c l u s i o n s drawn from the surveys of one season may not te v a l i d f o r the next. 235 CHAPTER VI. DISCUSSION The concepts of com p e t i t i o n and c o e x i s t e n c e p r o v i d e r i c h grounds f o r misunderstanding, due l a r g e l y to d i f f e r e n c e s i n d e f i n i t i o n s and usage of these and other terms c e n t r a l to the problem, such as ' n i c h e 1 , 'competitive e x c l u s i o n ' and 'sympatry'. The h i s t o r i c a l development of these concepts and d i f f e r e n c e s i n d e f i n i t i o n s have a l r e a d y been reviewed i n Chapter I . T h i s s e c t i o n w i l l d i s c u s s some of the f i e l d s t u d i e s of competition and the problems they attempted t o s o l v e . These are con s i d e r e d here because of the l i g h t they may throw on the Cenocorixa problem. Many l a b o r a t o r y s t u d i e s s u f f e r from founder e f f e c t s or g e n e t i c d r i f t i n the s m a l l , o f t e n i n b r e e d i n g p o p u l a t i o n s i n v o l v e d (e.g. Dobzhansky, 1962)..They a l s o show more r e g u l a r c o n d i t i o n s than are found i n most n a t u r a l environments, and are thus u n s u i t a b l e f o r d i r e c t comparison with f i e l d s i t u a t i o n s . In the present d i s c u s s i o n , o n l y d i r e c t i n t e r a c t i o n i s cons i d e r e d as evidence of com p e t i t i o n , f o l l o w i n g the d e f i n i t i o n of Crombie (19U7) r a t h e r than the D a r l i n g t o n (1972) concept of extended c o m p e t i t i o n . The term 'niche' i s a l s o considered p r i m a r i l y n o n - q u a n t i t a t i v e l y , as a composite of f u n c t i o n and h a b i t a t , with the r e a l i s a t i o n t h a t i t would r e q u i r e c o n s i d e r a b l y more study to e l u c i d a t e a l l the f a c t o r s t h a t may be r e l e v a n t . As B i r c h and E r l i c h (1967) p o i n t out, "...we know nothing whatever of the antecedents of most s p e c i e s f o r thousands of years...". C e r t a i n l y nothing i s known of the past h i s t o r y of 2 3 6 Cenocorixa i n c e n t r a l B r i t i s h Columbia. Past competition can be invoked as a reason f o r present d i s t r i b u t i o n , but never f a l s i f i e d ; thus ongoing c o m p e t i t i o n should be s t u d i e d where p o s s i b l e , r a t h e r than a s s e s s i n g present d i s t r i b u t i o n p a t t e r n s i n the l i g h t of p r e v i o u s c o m p e t i t i o n . P r e v i o u s f i e l d s t u d i e s of a q u a t i c h a b i t a t s , which form w e l l - d e f i n e d ecosystems i n g e n e r a l , have i n c l u d e d s e v e r a l d e a l i n g with z o o p l a n k t o n i c organisms. Amongst these, Goulden (1966), working on c o e x i s t e n c e of moinids, found l i t t l e d i f f e r e n c e between n i c h e s to account f o r the apparently sympatric d i s t r i b u t i o n . Sandercock (1967) looked a t c o e x i s t e n c e of diaptomids i n one l a k e , and suggested c o e x i s t e n c e was due i n that case to the use of d i f f e r e n t e c o l o g i c a l niches i n sympatry. S i z e d i f f e r e n c e s between the s p e c i e s lessened c o m p e t i t i o n ; t h e r e was a l s o v e r t i c a l s e g r e g a t i o n i n the water column i n summer, and some seasonal d i f f e r e n t i a t i o n of abundance peaks. S t a h l (1966) s t u d i e d c o e x i s t i n g s p e c i e s of Chaoborus i n many water bodies. Among more g e n e r a l f a c t o r s which might permit c o e x i s t e n c e , e c o l o g i c a l niche d i f f e r e n c e s were not g e n e r a l l y r e c o g n i s e d as important, and the c h i e f f a c t o r p e r m i t t i n g c o e x i s t e n c e was c o n s i d e r e d to be u n r e s t r i c t e d r e s o u r c e s . C o n s i d e r i n g only the a q u a t i c (non-reproductive) phase, numbers were g e n e r a l l y e f f e c t i v e l y reduced by f i s h p r e d a t i o n , by crowding on the bottom and by weather p a t t e r n s a t emergence f o r the p o p u l a t i o n s he s t u d i e d , so t h a t they never reached a l e v e l where com p e t i t i o n was severe. In c o n t r a s t to t h i s study, S w i f t (1974) found that i n the absence of f i s h p r e d a t i o n development 237 of Chaoborus may be re t a r d e d by a shortage of food, and a two-year l i f e c y c l e imposed on chaoborids i n sympatry, r a t h e r than the e x c l u s i o n of one s p e c i e s . His f i n d i n g s resemble those of Wallace (1974) who found t h a t l a r v a l development of D r o s ^ p j i i l a was prolonged i n overcrowded c u l t u r e s ; s i m i l a r r e s u l t s are common i n i n s e c t l a b o r a t o r y s t u d i e s . In g e n e r a l , many of the previous s t u d i e s on a q u a t i c organisms o f t e n seem to d e f i n e the niche i n terms of food, r a t h e r than other environmental parameters. There has been c o n s i d e r a b l e work on competition i n b i r d s , s i n c e Lack's support of Gause's views at the B r i t i s h E c o l o g i c a l S o c i e t y ' s meeting of 1944, d e a l i n g with the b i o l o g y of c l o s e l y r e l a t e d s p e c i e s (Hardin, 1960). For i n s t a n c e , Orians and Horn (1969) s t u d i e d s i m i l a r - s i z e d b l a c k b i r d s p e c i e s i n sympatry around potholes i n sagebrush country, reasoning t h a t i n f l u c t u a t i n g environments l i t t l e i s known about how much o v e r l a p i s r e g u i r e d before c o m p e t i t i v e e x c l u s i o n need be expected, although the p r i n c i p l e i s f a i r l y w e l l understood i n the l a b o r a t o r y . O v e r a l l food o v e r l a p was high a t a l l times f o r three of the f o u r s p e c i e s , enough to favour the development of i n t e r s p e c i f i c i n t e r f e r e n c e c o m p e t i t i o n ( t e r r i t o r i a l behaviour) between two of these. Thus, when f o r a g i n g i n the same h a b i t a t at the same time, they obtained approximately the same d i e t , and could be c l a s s e d as ' f i n e - g r a i n e d ' i n the sense of MacArthur and Levi n s (1964). The data supported the suggestion t h a t under co m p e t i t i o n i n a patchy environment a s p e c i e s w i l l decrease the number of patches foraged i n , r a t h e r than the number of prey 238 types taken (HacArthur and Pianka, 1966). Here again, the fundamental and r e a l i s e d n iches were c l e a r l y d efined i n terms of h a b i t a t r a t h e r than r o l e . In a r a t h e r s i m i l a r study Catchpole (1973) looked at two warblers and c l a r i f i e d the i d e a s o f 'sympatry 1 i n t h e i r c o n t e x t ; he i d e n t i f i e d h o r i z o n t a l , s p a t i a l and seasonal s e g r e g a t i o n i n breeding and p a r t l y i n f e e d i n g . The s p e c i e s were e f f e c t i v e l y l a r g e l y a l l o p a t r i c , although both i n h a b i t e d the f r i n g e s of ponds. with such a r e s u l t , i t i s tempting to p o s t u l a t e past c o m p e t i t i o n . In a re c e n t i n v e s t i g a t i o n , Enders (1974) found t h a t orb-web s p i d e r s were segregated v e r t i c a l l y i n s p r i n g and e a r l y summer, but when the p o p u l a t i o n was lowered by p r e d a t i o n , the s p e c i e s of Arcjiope were able to c o e x i s t i n l a t e r summer. Competition was both by e x p l o i t a t i o n and i n t e r f e r e n c e i n the e a r l i e r stages. Z a r e t and Rand (1971), p o i n t i n g out t h a t c o m p e t i t i v e e x c l u s i o n was most o f t e n used as an e x p l a n a t i o n of present niche s e p a r a t i o n of sympatric s p e c i e s and that t h i s was impossible to d i s p r o v e , c o n s i d e r e d i t best s t u d i e d by c o l l e c t i n g , as d i d Orians and Horn (1969), data on niche o v e r l a p i n the absence of i n f o r m a t i o n on, or preconceived i d e a s o f , com p e t i t i o n . They found t h a t t r o p i c a l stream f i s h e s s h a r i n g s i m i l a r h a b i t a t s had most d i s t i n c t food p r e f e r e n c e s , and found a l s o a greater niche o v e r l a p i n the wet season when food was abundant, than i n the dry season when s p e c i e s and numbers of preys were both fewer. T h i s i s i n c o n f l i c t with some other f i n d i n g s , but more resembles Reynoldson and Bellamy's (1971) concepts of refuge prey s p e c i e s , 239 whereas Emlen (1966) p r e d i c t e d mathematically t h a t p r e d a t o r s would switch from s p e c i a l i s e d t o g e n e r a l i s e d f e e d i n g i n times of s c a r c i t y (confirmed by S p r u l e s i n 1970) . Most of these s t u d i e s p r e d i c t e d h a b i t a t s from stomach contents - the r e a l i s e d niche evidence - without an independent measure of what food organisms may c o n s t i t u t e the fundamental niche of the s p e c i e s . S i m i l a r i t y i n d i e t need not n e c e s s a r i l y mean s i m i l a r f e e d i n g - p l a c e s or behaviour; thus, as the f u n c t i o n a l and d i s t r i b u t i o n a l n i ches may not o v e r l a p , c o m p e t i t i o n need not n e c e s s a r i l y occur. T h e r e f o r e independent f e e d i n g experiments, d e s p i t e t h e i r obvious disadvantages and a r t i f i c i a l i t y , are of t e n necessary to d i s c e r n the fundamental niche and to i n t e r p r e t the f i e l d r e s u l t s b e h a v i o u r a l l y and i n terms of p r e f e r e n c e . In t h i s way, l a b o r a t o r y s t u d i e s of Kear (1962) on f i n c h f e e d i n g behaviour and i t s e v o l u t i o n with development, throw l i g h t on many f i e l d study s i t u a t i o n s . In c o n t r a s t to the r a t h e r c l e a r - c u t s i t u a t i o n s c o n s i d e r e d above, there are o b s e r v a t i o n s of c e r t a i n i n s e c t groups where s e v e r a l s p e c i e s may c o e x i s t , a p p a r e n t l y i n complete sympatry. DeBach (1966) and M i l l e r (1967) reviewed d e t a i l e d work by Dempster and c o l l e a g u e s on c o e x i s t e n c e of broom fauna. Ross (1957) d e s c r i b e s another such s i t u a t i o n where s i b l i n g s p e c i e s of le a f - h o p p e r s (Erythroneura lawsoni s . l . ) c o e x i s t e d a p p a r e n t l y without competition on sycamore t r e e s . T r e e s with few hoppers d i d not show g r e a t e r s p e c i e s d i v e r s i t y than those supporting high p o p u l a t i o n s , i n d i c a t i n g a g e n e r a l l a c k o f co m p e t i t i o n . Perhaps each s p e c i e s has a s l i g h t l y d i f f e r e n t e c o l o g i c a l 240 optimum, but f l u c t u a t i o n s i n weather and other f a c t o r s tend to favour each i n t u r n . Thus d e s p i t e synchronous breeding p a t t e r n s and i d e n t i c a l f e e d i n g h a b i t s i n these l e a f - h o p p e r s , the s i t u a t i o n s appear not to be r e s o u r c e - l i m i t e d ; each t r e e r e p r e s e n t s an ecosystem with s u p e r f l u o u s food. Perhaps breeding i s inadequate to e x p l o i t the s i t u a t i o n i n the time p e r i o d when le a v e s are on the t r e e s . These and other f i e l d examples i n d i c a t e t h at c ompetition may occur between s i b l i n g s , but may not n e c e s s a r i l y reach a r a p i d e x c l u s i o n end p o i n t ; t h e r e i s thus f i e l d evidence f o r the mathematically p r e d i c t e d c o e x i s t e n c e ( M i l l e r , 1967). COMPETITION IN FLUCTUATING ENVIRONMENTS Some e c o l o g i c a l s t u d i e s suggest that many s p e c i e s may have very p l a s t i c g e n e t i c s , to enable them to adapt r a p i d l y to a changing environment (Bi r c h and E h r l i c h , 1967). Although the id e a of com p e t i t i v e e x c l u s i o n i s fundamentally an e v o l u t i o n a r y concept, i n s t u d i e s of com p e t i t i o n there i s o f t e n a tendency to look f o r an end-point s i t u a t i o n - the e x c l u s i o n o f one or other s p e c i e s - without i n v e s t i g a t i n g the p o s s i b i l i t y of ongoing c o m p e t i t i o n . Weatherley (1963) suggests t h a t among the i n d i c a t o r s of ongoing c o m p e t i t i o n are f a c t o r s such as depressed growth and r e p r o d u c t i o n which may be observed on experimental r e d u c t i o n of the p o p u l a t i o n d e n s i t y . However, t h i s i s a p p l i c a b l e t o i n t e r s p e c i f i c and i n t r a s p e c i f i c competition a l i k e , as many s t u d i e s on t r o u t show ( i . e . Reynolds et a l . , 1964). 241 In any environment, c o m p e t i t i v e displacement may s t i l l a c t i v e l y be i n progress (DeBach, 1966). In a f l u c t u a t i n g environment the outcome of c o m p e t i t i o n may be much delayed, or even r e v e r s e d from time to time; c o m p e t i t i v e e x c l u s i o n may o f t e n depend on r e p r o d u c t i v e i n e q u a l i t i e s ( M i l l e r , 1967), and changes i n environmental s i t u a t i o n s may r e v e r s e the advantages. Ideas of a time s c a l e f o r such a process, while i n f l u e n c e d by the r e p r o d u c t i v e p a t t e r n s and l i f e spans of the s p e c i e s under c o n s i d e r a t i o n , remain vague. In temperate areas, l a r g e - s c a l e p h y s i c a l and c l i m a t i c changes have occurred w i t h i n the past 20,000 years and there i s evidence of c o n t i n u i n g c l i m a t i c change, e i t h e r s e m i - c y c l i c a l , or now f o l l o w i n g an i r r e v e r s i b l e d i r e c t i o n (Bryson, 1974). Thus the s t r e s s e s i n a changing h a b i t a t w i l l c o n t i n u a l l y be a l t e r i n g , and a f f e c t i n g the i n h a b i t a n t s d i f f e r e n t l y . On Becher's P r a i r i e , Lake Lye has o c c a s i o n a l l y separated i n t o two p a r t s i n e x c e p t i o n a l l y dry years; other l a k e s have dim i n i s h e d i n volume and area, and c o n d u c t i v i t y has f l u c t u a t e d g r e a t l y (Scudder, 1969b). There seems a l s o to have been a g e n e r a l r i s e i n water l e v e l i n many lakes about 20 years ago, seen by the stands of dead aspens i n many lake shallows (Cannings, 1973)..The e f f e c t s of c l i m a t i c change would normally be more marked i n c l o s e d systems, such as the Becher's P r a i r i e l a k e s , than i n l a k e s or other a q u a t i c h a b i t a t s l i n k e d by streams to other systems. Further, i n a c o n t i n e n t a l c l i m a t e such as t h a t of the c e n t r a l i n t e r i o r of B r i t i s h Columbia, there i s a pronounced winter break s e p a r a t i n g r e l a t i v e l y s h o r t growing seasons. T h i s 242 w i l l not only cause great changes i n c o n d i t i o n s i n the l a k e s as a r e s u l t of f r e e z i n g over, i . e . changes i n s a l i n i t y , oxygen content and so on, but i t may a l s o l e a d to the components of co m p e t i t i v e e x c l u s i o n being reduced i n e f f e c t i v e n e s s , e s p e c i a l l y where the l i f e span of the organisms i n v o l v e d i s s h o r t e r than the p e r i o d i c i t y of environmental v a r i a t i o n . The process of comp e t i t i o n may e f f e c t i v e l y s t a r t anew each year with many groups of i n v e r t e b r a t e s , i n c l u d i n g c o r i x i d s , which u s u a l l y overwinter as a d u l t s (Martin, 1972) and eat l i t t l e or nothing below 5 C (Jansson and Scudder, 1972). 243 SUMMARY OF THE FINDINGS FOR C. BIFIDA AND C. EXPLETA The present study was c e n t r e d around the problem of c o e x i s t e n c e i n c l o s e l y r e l a t e d c o r i x i d s . The aim of the study was b a s i c a l l y to e s t a b l i s h fundamental and r e a l i s e d niches f o r £• b i f i d a and C. e x p l e t a . A r e d u c t i o n of the fundamental niche i n sympatry i s an e s s e n t i a l f o r the demonstration of a c o m p e t i t i v e s i t u a t i o n ( M i l l e r , 1967), but i t need not be an i n d i c a t o r of c o m p e t i t i o n . C a r e f u l methodology i s needed i n f i e l d s t u d i e s to show t h a t two s p e c i e s are e c o l o g i c a l homologues i n the f i r s t p l a c e , a p r e r e q u i s i t e f o r an e x c l u s i o n outcome (Connell, 1961) . Lake LB 2 can e s s e n t i a l l y be c o n s i d e r e d as a l l o p a t r i c f o r C. e x p l e t a s i n c e no C. b i f i d a were detected there during the present study, and the Becher's P r a i r i e l a k e s Greer, Rock and East are s i m i l a r l y a l l o p a t r i c f o r C. b i f i d a . The l i f e of the c o r i x i d s i n these l a k e s may thus be s t u d i e d and compared (a) with where they occur i n sympatry on Becher's P r a i r i e (Lakes Barnes, Lye and Round-up), and (b) with l a b o r a t o r y r e s u l t s , to determine f i r s t whether such data i n d i c a t e e c o l o g i c a l i d e n t i t y between the s p e c i e s , and second, the d i f f e r e n c e s between fundamental and r e a l i s e d n i c h e s , i f any, f o r each s p e c i e s . T h i s d i s c u s s i o n w i l l analyse the major t o p i c s s t u d i e d , i n terms of p h y s i o l o g y , h a b i t a t and f e e d i n g p r e f e r e n c e s of each s p e c i e s . F i r s t , i n terms of t h e i r osmotic and i o n i c p h y s i o l o g y , C. e x p l e t a , which has not been found i n l a k e s i n c e n t r a l B r i t i s h Columbia below 5000 umhos cm - 1 c o n d u c t i v i t y (Scudder, 244 1969) was e s s e n t i a l l y a l l o p a t r i c i n the h i g h e s t s a l i n i t y s t u d i e d (Lake LB 2). C. b i f i d a was a l l o p a t r i c i n lak e s below 2000 umhos cm - 1, while both s p e c i e s occurred s y m p a t r i c a l l y on Becher's P r a i r i e l a k e s with waters of from 6000 to 12,000 umhos cm - 1 mean c o n d u c t i v i t y . Other c o r i x i d s p e c i e s occurred i n smal l numbers i n most l a k e s , but bred i n sympatry with C. b i f i d a o nly. The l a k e s i n v e s t i g a t e d were g e n e r a l l y s i m i l a r i n i o n i c content, but d i f f e r e d markedly i n c o n d u c t i v i t y . Topping (1969), however, p o i n t s out that i n each lake there i s a s i g n i f i c a n t seasonal v a r i a t i o n i n c o n d u c t i v i t y . The d i f f e r e n t d i s t r i b u t i o n of the Cenocorixa s p e c i e s i n the upper s a l i n i t y ranges appears t o be a t l e a s t p a r t l y due to d i f f e r e n c e s i n p h y s i o l o g i c a l t o l e r a n c e . Scudder et a l . , (1972) have shown that both s p e c i e s of Cenocorixa have t y p i c a l l y freshwater i o n i c - o s m o t i c balances, and l i v e e q u a l l y well i n experimental s i t u a t i o n s up to 20,000 umhos cm - 1. However, C. e x p l e t a can s u r v i v e i n up to 30,000 umhos c o n d u c t i v i t y , whereas C. b i f i d a cannot. Savage (1971) has d e s c r i b e d the s u c c e s s i o n of c o r i x i d s with i n c r e a s i n g s a l i n i t y i n s a l i n e l a k e s i n Cheshire, with a s i m i l a r c o n d u c t i v i t y range to those s t u d i e d here, of 500 to 22,000 umhos cm- 1, but he d i d not adduce reasons why s a l i n i t y per se should be the c o n t r o l l i n g f a c t o r . By c o n t r a s t , Macan (1938, 1962) has shown h a b i t a t p r e f e r e n c e s i n c o r i x i d s which do not appear to r e l a t e to c o n d u c t i v i t y or s a l i n i t y , but h i s lake s e r i e s d i d not extend i n t o the range of s a l i n i t i e s covered here. Istock (1973) remarks t h a t f o r t h i s very reason of h a b i t a t p r e f e r e n c e s . 245 c o r i x i d s are i n t e r e s t i n g f o r s t u d i e s of s p e c i e s i n t e r a c t i o n s . While C. e x p l e t a does not n a t u r a l l y occur i n the low s a l i n i t y l a k e s , experiments by Scudder et a l . (1972) suggest t h a t a d u l t s have the p h y s i o l o g i c a l c a p a c i t y to do so. In the present study, C. e x p l e t a o v i p o s i t e d i n l a b o r a t o r y c o n d i t i o n s i n water of around 1100 umhos cm - 1, and the eggs hatched and development proceeded s u c c e s s f u l l y to the second i n s t a r . Scudder £t a l . (1972) thus suggest t h a t e c o l o g i c a l i n t e r a c t i o n s may be among the f a c t o r s t h a t exclude t h i s s p e c i e s from the most freshwater h a b i t a t s . In summary, with r e s p e c t to p h y s i o l o g i c a l a s p e c t s of the n i c h e , there i s evidence t h a t the fundamental n i c h e of both s p e c i e s should encompass the e n t i r e range of the l a k e s s t u d i e d . However, the r e a l i s e d n i ches i n d i c a t e a d i s t i n c t r e s t r i c t i o n of t h i s , with absence of C, b i f i d a from h i g h e s t s a l i n i t i e s , and of £• e x p l e t a from the lower s a l i n i t y water bodies. When s e l e c t i n g the study area on Becher's P r a i r i e , care was taken to choose lakes with s i m i l a r depth, area and shore development and major i o n i c composition. Thus t h e r e were minimal d i f f e r e n c e s i n s u b s t r a t e and temperature among the p h y s i c a l parameters. However, h a b i t a t s were r a t h e r d i v e r s e , as submerged macrophytes were absent from l a k e s of 6000 umhos cm - 1 and higher on Becher's P r a i r i e , namely l a k e s Barnes, Lye and Bound-up, although a l l l a k e s on Becher's P r a i r i e had o c c a s i o n a l f r i n g i n g patches of S c i r p u s and Juncus. The l a c k o f submerged v e g e t a t i o n i n the s a l i n e l a k e s produces a simpler, l e s s heterogenous environment with fewer h i d i n g - p l a c e s and probably l e s s s p a t i a l 246 s e p a r a t i o n of c o r i x i d s . T h i s may permit p o t e n t i a l l y more i n t e r a c t i o n s between s p e c i e s and i n s t a r s , but i t was j u s t i n these more s a l i n e l a k e s t h a t the s p e c i e s were sympatric. Macrophytes were seen i n Lake LB 2, but here they formed i s o l a t e d clumps r a t h e r than a continuous mat. Since the presence of a q u a t i c p l a n t s apparently depends upon both s a l i n i t y and i o n i c content (Seddon, 1972), s a l i n i t y may thus be i n d i r e c t l y r e s p o n s i b l e f o r major a l t e r a t i o n s of the environment, making i t l e s s complex i n the more s a l i n e l a k e s . Should pr e f e r e n c e s of the Cenocorixa s p e c i e s f o r weedy or bare s u b s t r a t e s d i f f e r , t here might be an i n d i r e c t as well as a d i r e c t i n f l u e n c e of a l k a l i n i t y upon t h e i r d i s t r i b u t i o n . In the i n v e s t i g a t i o n of h a b i t a t p r e f e r e n c e s , Scudder (1966) has shown t h a t C. e x p l e t a o v i p o s i t e d p r e f e r e n t i a l l y on s o l i d s u b s t r a t e s i n c a p t i v i t y and i n the f i e l d , whereas C. b i f i d a seemed to p r e f e r to l a y eggs on s u r f a c e s such as p l a n t stems. Thus C. e x p l e t a may be absent from weedy areas and l a k e s because the p r e f e r r e d o v i p o s i t i o n s i t e i s absent or covered over. S u b s t r a t e . However, l a b o r a t o r y evidence i n the present study i n d i c a t e d that the d i f f e r e n c e i s not c l e a r - c u t , f o r most £• b i f i d a , whether from weedy or weedless l a k e s , o v i p o s i t e d on the bottom of the c o n t a i n e r s r a t h e r than on the p l a s t i c s c r e e n i n g p r o v i d e d . Walton (1943) has shown t h a t d i f f e r e n c e s i n o v i p o s i t i o n s i t e may i n f l u e n c e development r a t e s through micro-environmental d i f f e r e n c e s , eggs l a i d on the bottom developing r e l a t i v e l y slower than those l a i d higher i n the water column. However, more d e f i n i t e experiments on the o v i p o s i t i o n a l aspects 247 of the fundamental niches of C. b i f i d a and C. e x p l e t a need to be c a r r i e d out. H a b i t a t p r e f e r e n c e s f o r each s p e c i e s of Cenocorixa were i n v e s t i g a t e d i n the higher s a l i n i t y l a k e s on Becher's P r a i r i e where both Cenocorixa s p e c i e s were sympatric. A h i g h e r p r o p o r t i o n of both a d u l t s and nymphal stages of C. e x p l e t a was recorded midlake than i n s h o r e . S i m i l a r l y i n Lake LB 2, £• ®2El§£§ appears to occur both i n s h o r e and i n deeper waters (Scudder, persomal communication). However, C. b i f i d a i n the more freshwater l a k e s was r a r e l y taken midlake, and most surveys suggest i t i s c h i e f l y c o n f i n e d to i n s h o r e areas. D i s p e r s a l between and i n water hodies might account f o r some of the midlake a d u l t s i g h t i n g s , but would not a f f e c t o b s e r v a t i o n s of j u v e n i l e s i n midwaters, nor should i t a f f e c t r a t i o s of the s p e c i e s i f e q u i v a l e n t d i s p e r s a l can be assumed f o r each. Thus the p a t t e r n seen may i n d i c a t e a more widespread h a b i t a t f o r C. e x p l e t a than f o r C. b i f i d a . The fundamental niches i n terms of h a b i t a t of the two s p e c i e s were not d i r e c t l y s t u d i e d here i n the l a b o r a t o r y , but by e x t r a p o l a t i o n from the evidence i n d i f f e r e n t l a k e s , i t would appear that f o r C. e x p l e t a the fundamental niche i s wider than t h a t f o r C. b i f i d a , and f u r t h e r , t h a t the r e a l i s e d n iches are not markedly d i f f e r e n t i n a l l o p a t r y and sympatry. In Lakes Barnes and Lye t h e r e was a g e n e r a l l y g r e a t e r abundance of C. e x p l e t a on bare shores with f i r m s i l t or g r a v e l , and o f f s h o r e . On the other hand, C. b i f i d a was more abundant over s o f t mud, among Juncus or f l o o d e d twigs. In sympatry. 248 t h e r e f o r e , C. e x p l e t a p r e f e r e d the l e s s d i v e r s e h a b i t a t and somewhat deeper water, although these sympatric pr e f e r e n c e s were by no means a b s o l u t e . In summary, a l l h a b i t a t s t u d i e s i n d i c a t e d evidence f o r a wider range of h a b i t a t and water depth pr e f e r e n c e s i n C. e x p l e t a than i n C. b i f i d a , but there i s apparently l i t t l e or no r e d u c t i o n of the fundamental niche i n sympatry. Feeding requirements may a l s o i n f l u e n c e h a b i t a t s e l e c t i o n by d i f f e r e n t c o r i x i d s p e c i e s . Benthic organisms are g e n e r a l l y r e s t r i c t e d to the l i t t o r a l r e g i o n s , where c o r i x i d s a l s o predominate, but i n t h i s area p l a n k t o n i c organisms v a r i e d c o n s i d e r a b l y i n abundance , l a r g e l y as a r e s u l t of wind and wave a c t i o n . While biomass of i n v e r t e b r a t e organisms appeared r e l a t i v e l y heavy and s u s t a i n e d i n each l a k e , t h e r e were d i f f e r e n c e s i n f a u n a l composition. Jansson (1971) has suggested, however, t h a t d i e t a r y d e f i c i e n c i e s may occur at c e r t a i n times i n the l a k e s , and b r i n g about the o v a r i a n a r r e s t t h a t i s observed i n midsummer. Fo l l o w i n g Rawson and Moore (1944) he c o r r e l a t e d primary p r o d u c t i v i t y with c o n d u c t i v i t y i n s a l i n e l a k e s , and suggested t h a t i n c r e a s e d production of prey organisms might allow more c o r i x i d g e n e r a t i o n s i n the more s a l i n e l a k e s , where ov a r i a n a r r e s t occurred i n August r a t h e r than i n e a r l y J u l y as i n the more freshwater l a k e s . However, the present s t u d i e s i n d i c a t e t h a t , i f any t h i n g , there was more p h y t o p l a n k t o n i c primary p r o d u c t i v i t y i n the more freshwater lakes than i n the s a l i n e l a k e s . Much of the primary p r o d u c t i v i t y i n the higher s a l i n i t y 249 l a k e s may come from e p i p e l i c and b e n t h i c blue-green a l g a e , and i n the more freshwater l a k e s from submerged a q u a t i c macrophytes and t h e i r a s s o c i a t e d green fil a m e n t o u s a l g a e . C l e a r l y , the c o r r e l a t i o n of c o n d u c t i v i t y and p r o d u c t i v i t y i s not simple i n lakes of the study area. Much rec e n t work, notably t h a t of Parsons and c o l l e a g u e s (Parsons et a l . , 1972; LeBrasseur and Kennedy, 1972; Barraclough and Robinson, 1972) has shown t h a t there may be no c o r r e l a t i o n between primary p r o d u c t i o n and standing crop. Greater a l g a l p r o d u c t i o n f o l l o w i n g enrichment of a water body may not show a s i g n i f i c a n t i n c r e a s e i n s t a n d i n g crop s i n c e t h e r e i s o f t e n a compensatory i n c r e a s e i n consumers, e s p e c i a l l y among the f a s t e r -d eveloping cladocerans and r o t i f e r s . Thus, i f standing crop i s to be used to assess p r o d u c t i v i t y , i t i s best t o c h a r a c t e r i s e a l a k e by repeated samples of z o o p l a n k t o n i c biomass, e s p e c i a l l y i n the absence of higher p r e d a t o r s . Volume measurements (Chapter II) suggest an e q u i v a l e n t l e v e l of zooplankton biomass averaged over the season f o r a l l l a k e s , so t h a t p r o d u c t i v i t y i n terms of food abundance i s a p p a r e n t l y not l i m i t i n g . T h i s i s f u r t h e r i n d i c a t e d by comparing Round-up Lake and Lake Lye, where d e s p i t e g r e a t d i f f e r e n c e s i n plankton and c o r i x i d biomass, development of s i m i l a r numbers of c o r i x i d g e n e r a t i o n s occurred, and a t a s i m i l a r r a t e (Chapter I I I ) . Q u a l i t y of food i s more l i k e l y to be a s i g n i f i c a n t f a c t o r a f f e c t i n g c o r i x i d d i s t r i b u t i o n and r e p r o d u c t i v e success. Thus C. b i f i d a i s able to s u r v i v e and breed both i n a l l o p a t r y and when sympatric with C. e x p l e t a , and i t may be 250 t h a t the q u a l i t y of food i s g r e a t e r i n the sympatric l a k e s , r e s u l t i n g i n more extended breeding. I f the absence of C. e x p l e t a from the low s a l i n i t y l a k e s i s not owing to s a l i n i t y or o t h e r f a c t o r s so f a r c o n s i d e r e d , i t i s p o s s i b l e that some aspect of the f e e d i n g requirements i s r e s p o n s i b l e . The major p a r t of t h i s study was i n f a c t concerned with i n v e s t i g a t i n g what organisms both s i b l i n g s p e c i e s would eat, as a measure of t h e i r fundamental niches, and what they had eaten i n the f i e l d , using evidence from s e r o l o g i c a l examination of guts, as an i n d i c a t i o n of the r e a l i s e d niche of each s p e c i e s . The present study r e p r e s e n t s the f i r s t s o p h i s t i c a t e d demonstration t h a t many of the C o r i x i d a e are i n f a c t c a r n i v o r o u s . S e r o l o g i c a l analyses of gut contents of s i x s p e c i e s i n f i v e genera, supplemented by data from e x t e n s i v e f e e d i n g experiments using preys s i n g l y or i n combination, showed the predatory and scavenging h a b i t s t o dominate. L e v e l s of f e e d i n g on p l a n t m a t e r i a l were at best low, and over h a l f the specimens t e s t e d from each s p e c i e s contained evidence of chironomid and zygopteran remains i n the guts, with s m a l l e r amounts of other l a k e i n v e r t e b r a t e s . T h i s i s a new f i n d i n g f o r Cenocorixa , s i n c e p r e v i o u s workers (Jansson and Scudder, 1972) have noted f e e d i n g to be c h i e f l y on diaptomids. The r e s u l t s could i n d i c a t e food l i m i t a t i o n i n the la k e s s t u d i e d , f o r c i n g the c o r i x i d s to t u r n to a c a r n i v o r o u s d i e t ; but i f so, one would expect p l a n t f e e d i n g to be more important i n c o r i x i d s from the more freshwater l a k e s where p l a n t m a t e r i a l was abundant and where s e v e r a l s p e c i e s of c o r i x i d s c o e x i s t . That the 251 f e e d i n g p a t t e r n i s not p e c u l i a r to a l k a l i n e l a k e s of c e n t r a l B r i t i s h Columbia i s i n d i c a t e d by the f a c t t h a t Cenocorixa b l a i s d e l l i {Hungerford) c o l l e c t e d i n the moist c o a s t a l c l i m a t e again took animal food i n experiments. Despite high d e n s i t i e s of c o r i x i d s i n a l l la k e s , mud p o p u l a t i o n s and emergences of chironomids remained high (Cannings, 1973) , s u g g e s t i n g t h a t these foods, l i k e the p l a n k t e r s , were not l i m i t i n g . R e s u l t s suggest that there i s no shortage of p o t e n t i a l food i n the lak e s s t u d i e d . In f e e d i n g experiments C. e x p l e t a markedly p r e f e r r e d dead over l i v e foods, a f a c t o r not demonstrated i n C. b i f i d a (Figure 15). However, l i v e p l a n k t e r s and chironomid l a r v a e were more oft e n taken by C. b i f i d a than by C. e x p l e t a . Although i n a sense a r t i f i c i a l , f e e d i n g experiments help to d e l i m i t the d i e t a r y components of the fundamental ni c h e of each s p e c i e s . However, the u l t i m a t e t e s t of the e f f e c t i v e n e s s of a l a b o r a t o r y -determined d i e t must come through s u c c e s s f u l r e a r i n g of the s p e c i e s on i t f o r s e v e r a l generations, as was done by Jansson and Scudder (1972) and P e t e r s (1962). In the experiments of Hungerford (1917) and G r i f f i t h (1945) l a b o r a t o r y f e e d i n g was l e s s than s u c c e s s f u l l y c o n t r o l l e d through the very nature of the t e s t e d d i e t s , which may have contained many types of organisms, a l l p o t e n t i a l l y a v a i l a b l e to the c o r i x i d s . For long-term r e a r i n g experiments with a s i n g l e d i e t axenic c u l t u r e s of food s p e c i e s must be used to o b t a i n unequivocal r e s u l t s . P e t e r s and U l b r i c h (1973) autoclaved mud and f e d t h i s p l u s branched algae to Trichocorixe11a, but no 252 i n f o r m a t i o n i s given on the a l g a l source, nor of the p o s s i b l e dead organisms contaminating the mud. In a s y n t h e s i s of the f i n d i n g s of the l a s t two chapters, Table L I I I compares r e s u l t s of f e e d i n g experiments and s e r o l o g i c a l gut a n a l y s e s of the f e e d i n g p a t t e r n s of each SSHQSOEiX-S s p e c i e s . The major d i f f e r e n c e between these r e s u l t s i s i n the high p r o p o r t i o n of guts r e a c t i n g p o s i t i v e l y f o r chironomids and zygopterans compared with importance of these items i n f e e d i n g experiments. To account f o r t h i s , major p r e d a t i o n may occur on zygopteran eggs which are i n s e r t e d i n submerged p l a n t stems; t h e r e may s i m i l a r l y be much pr e d a t i o n on very young stages of chironomids which are extremely abundant i n a l l l a k e s s t u d i e d . Further, the adjuvant-produced chironomid antiserum i s l i k e l y to r e a c t with Aedes l a r v a e , which were a l s o abundant i n the s t u d i e d l a k e s . Experiments o f f e r i n g a c h o i c e of preys suggested a l s o that capture success of chironomids was g r e a t e r than t h a t of p l a n k t e r s , although both e l i c i t e d a s t r o n g predatory response. Whatever the e x p l a n a t i o n , there i s a p p a r e n t l y a s t r o n g emphasis on chironomids and zygopterans i n the f i e l d . L a boratory and f i e l d data show f a i r l y c l o s e correspondence i n the order of p r e f e r e n c e of other food items, except f o r those foods o f f e r e d e x p e r i m e n t a l l y but not normally a v a i l a b l e i n the f i e l d . Both i n d i c a t e high acceptance of p l a n k t o n i c organisms i n both s p e c i e s , and g e n e r a l l y more food d i v e r s i t y i n the d i e t of C- b i f i d a than i n C. e x p l e t a . T h i s g r e a t e r food d i v e r s i t y i s i n d i c a t e d by the higher v a l u e s of ' a l l foods' (not l i s t e d ) by 253 Table L I I I . Feeding p r e f e r e n c e s of Cenocorixa s p e c i e s i n experiments and from s e r o l o g i c a l l y determined gut c o n t e n t s . R e s u l t s expressed as percentages f e e d i n g (experiments) and percentages of guts c o n t a i n i n g the f o o d s t u f f ( s e r o l o g y ) . For f e e d i n g r e s u l t s , unweighted means were taken of dead and l i v e organisms r e s u l t s . FOOD ITEM Diaptomids Daphniids Amphipods Chaoborids Chironomid s Ephemeropterans Zygopterans Others: C. b i f i d a FEEDING (%) 23.8 20.0 13.9 3.0 18.1 3.6 11.6 6.0 SEROLOGY (*) 6.6 12.6 1.8 1. 1 33.5 10.5 25.1 8. 8 C. e FEEDING 18. 0 17.1 13. 4 11. 9 16.7 11.6 11.3 0. 0 SEROLOGY 11.4 14. 3 0.0 0.7 34.8 10.5 28.0 0.3 254 both methods. T h i s may be r e l a t e d to prey organism d i s t r i b u t i o n , as C. b i f i d a has a wider d i s t r i b u t i o n p a t t e r n on Becher's P r a i r i e than C. e x p l e t a . The l a t t e r may then be s a i d to f a l l i n t o an i n c l u d e d niche w i t h i n the wider one of C. b i f i d a , and, i f c o m p e t i t i o n i s t o r e s u l t i n c o e x i s t e n c e r a t h e r than e x c l u s i o n of one s p e c i e s , i n theory the resource of the i n c l u d e d s p e c i e s should be more s p e c i a l i s e d . To a v o i d c o m p l i c a t i o n of the f e e d i n g p a t t e r n through p o o l i n g a l l r e s u l t s , s e r o l o g i c a l data were subdivided so that the sympatric l a k e s , i n which h a b i t a t s were r e l a t i v e l y uniform and p l a n t masses were absent, were s t u d i e d s e p a r a t e l y from the a l l o p a t r i c l a k e s . S i g n i f i c a n t l y more C, b i f i d a and C. e x p l e t a took daphniids and ephemeropterans i n sympatry than i n a l l o p a t r y , and s i g n i f i c a n t l y more C. e x p l e t a contained diaptomids and zygopterans i n a l l o p a t r y than i n sympatry. I t thus seems that both s p e c i e s concentrated on c e r t a i n key food items when i n sympatry. T h i s does not a p p a r e n t l y e n t i r e l y r e f l e c t f a u n a l d i f f e r e n c e s between lakes, as d a p h n i i d s d i d not appear to d i f f e r markedly i n abundance between l a k e s , although ephemeropterans and zygopterans were perhaps more p r e v a l e n t i n the t h r e e higher and the three lower s a l i n i t y l a k e s r e s p e c t i v e l y (Chapter I I ) . R e s u l t s of s e r o l o g i c a l experiments are summarised, and c o n t r a s t e d with d i s t r i b u t i o n of major p o t e n t i a l foods, i n F i g u r e 20. In summary, the d i s t r i b u t i o n p a t t e r n s of prey organisms are u n l i k e l y to account f o r the preference of C. e x p l e t a f o r diaptomids i n a l l o p a t r y , and that f o r both s p e c i e s f o r daphniids 2 5 5 Figure 20. Summary o f occurrence of food items i n the f i e l d , and percentage of guts of C. b i f i d a and C. e x p l e t a c o n t a i n i n g s e r o l o g i c a l evidence of items. A= occurrence i n low s a l i n i t y l akes ( C. b i f i d a o n l y ) , b= occurrence i n moderate s a l i n i t i e s ( C. b i f i d a and C. e x p l e t a s y m p a t r i c ) , c= occurence i n high s a l i n i t y ( C. e x p l e t a c n l y ) . D i a p t o m i d s a C. bifida D a p h n i i d s Amphipods C h a o b o r i d s D C h i r o n o m i d s Ephemeropterans x Z y g o p t e r a n s Pu lmonates B l u e - g r e e n s Macrophy tes 0 50 ro VJl Ch 257 i n sympatry; i t i s a l s o d o u b t f u l i f f e e d i n g pressure on ephemeropterans and zygopterans c o u l d be e x p l a i n e d s o l e l y by h a b i t a t p r e f e r e n c e s . The r e s u l t s from the study of the s p e c i e s of Cenocorixa i n sympatry a f f o r d e d the best evidence f o r the e x i s t e n c e of d i f f e r e n c e s i n the f e e d i n g n i c h e . Thus, i n s p r i n g (Tables XLVII and XLVIII) more C. e x p l e t a than C. b i f i d a contained d a p h n i i d s , whereas i n midsummer the r e v e r s e was t r u e . T h i s change was not immediately e x p l i c a b l e i n terms of prey abundance. Perhaps copepodites and n a u p l i i of diaptomids were most abundant by midseason, p r o v i d i n g an a l t e r n a t i v e foodsource. S i m i l a r l y , C. b i f i d a took more chironomids and ephemeropterans than £• S i f i i S i S i n summer, but the r e v e r s e was seen i n f a l l , i n d i c a t i n g another s e a s o n a l s h i f t i n emphasis on f o o d s t u f f s . T h i s might be because of the d i f f e r e n t p r o p o r t i o n s of i n s t a r s of each s p e c i e s present at d i f f e r e n t times of the year, r e s u l t i n g i n the changing e x p r e s s i o n of o v e r a l l food p r e f e r e n c e s f o r each s p e c i e s at d i f f e r e n t seasons. Changes i n the day to day dynamics of the f i e l d environment a f f e c t e d prey d e n s i t i e s i n s h o r e and p r o p o r t i o n s of developmental stages present at any one time. However ex p l a i n e d , these d i f f e r e n t and changing p r e f e r e n c e s of each s p e c i e s f o r d i f f e r e n t preys i n d i c a t e the d i f f i c u l t i e s of summarising f e e d i n g p a t t e r n s . Nonetheless, the r e s u l t s of f e e d i n g s t u d i e s i n d i c a t e s i m i l a r , although not i d e n t i c a l , fundamental niches f o r the Cenocorixa s p e c i e s , and there i s some evidence f o r a more r e s t r i c t e d , or s p e c i a l i s e d , r e a l i s e d niche of each i n sympatry. 258 When l i f e h i s t o r i e s a r e c o n s i d e r e d , i n t h e more f r e s h w a t e r l a k e s C . b i f i d a , H. l a e v i g a t a , C . a u d e n i , C . a m e r i c a n a , and S i g a r a s p . b r e e d i n s y m p a t r y . Of t h e s e , C . a u d e n i i s s m a l l e r t h a n t h e n u m e r i c a l l y d o m i n a n t C . b i f i d a , w h i l e C . a m e r i c a n a i s somewhat l a r g e r , and H. l a e v i g a t a i s m a r k e d l y l a r g e r and more r o b u s t . A l l p r o d u c e one g e n e r a t i o n p e r y e a r i n t h e s e l a k e s . H u t c h i n s o n (1959) p o i n t s o u t t h a t i n c o r i x i d s w i t h s i m i l a r l i f e h i s t o r i e s , t h e young o f l a r g e r s p e c i e s p o t e n t i a l l y c o m p e t e w i t h t h e a d u l t s o f s m a l l e r f o r m s , whe ther f o r f o o d , s p a c e o r o t h e r f a c t o r s . M a r t i n (1970) s u g g e s t s t h a t i n s y m p a t r y l a r g e r s p e c i e s u s u a l l y d e v e l o p s l o w e r , b u t s t a r t e a r l i e r . H o w e v e r , t h i s wou ld a p p e a r t o m i n i m i s e a n y s e l e c t i v e a d v a n t a g e t h a t m i g h t be o b t a i n e d , and one w o u l d e x p e c t r a t h e r t h e o p p o s i t e , t h a t t h e a d v a n t a g e s o f s i z e d i f f e r e n c e s w o u l d be e n h a n c e d b y s i m u l t a n e o u s b r e e d i n g , o r e v e n by e a r l i e r b r e e d i n g o f t h e l a r g e r f o r m . D i f f e r e n c e s i n t i m i n g o f b r e e d i n g s e a s o n would most l o g i c a l l y be e x p e c t e d i n t h e c a s e s o f s i m i l a r l y - s i z e d , s y m p a t r i c s p e c i e s , and i n t h e more f r e s h w a t e r l a k e s on B e c h e ^ s P r a i r i e t h i s was i n f a c t s e e n , f o r S i g a r a s p . , w h i c h was v e r y s i m i l a r i n s i z e and a p p e a r a n c e a t a l l i n s t a r s t o C . b i f i d a , s t a r t e d t o b r e e d m a r k e d l y e a r l i e r t h a n t h e l a t t e r . H o w e v e r , i n the more s a l i n e l a k e s where C . b i f i d a a n d C . e x p l e t a c o e x i s t , s u c h a s t a g g e r i n g o f b r e e d i n g p a t t e r n was n o t o b v i o u s , a l t h o u g h J a n s s o n a n d S c u d d e r (1974) f o u n d a t i m e - l a g o f a b o u t one week be tween t h e s p e c i e s a t t h e b e g i n n i n g o f t h e s e a s o n . T h u s , t h e r e i s no e v i d e n c e f o r c o m p e t i t i v e i n t e r a c t i o n h a v i n g g r e a t l y i n f l u e n c e d t h e t i m i n g o f b r e e d i n g b e t w e e n t h e C e n o c o r i x a s p e c i e s i n 259 sympatry. DeBach (1966) suggests t h a t non-competing animal s p e c i e s should not show d i f f e r e n c e s i n abundance between a l l o p a t r y and sympatry, other f a c t o r s being equal. However, t h i s d i d not prove t o be a u s e f u l c r i t e r i o n i n the present study, s i n c e the abundance of the c o r i x i d s v a r i e d enormously i n and between l a k e s , and i n no r e g u l a r manner. T h i s was e s p e c i a l l y obvious when p a i r s of l a k e s with s i m i l a r chemistry (Rock and Greer; Lye and Round-up) were compared. C e r t a i n l y , numbers of C. e x p l e t a were ap p a r e n t l y much lower on Becher's P r a i r i e where they are sympatric with C. b i f i d a than i n Lake LB 2, where C, b i f i d a cannot s u r v i v e a l l through the year. However, C. b i f i d a was no l e s s abundant i n sympatric l a k e s i n g e n e r a l than when not c o e x i s t i n g with C. e x p l e t a . In the freshwater l a k e s the numbers of C. b i f i d a f l u c t u a t e d g r e a t l y (Chapter I I I ) . Therefore these abundance data give no c l e a r i n d i c a t i o n of any c o m p e t i t i v e s i t u a t i o n i n sympatry. The evidence so f a r amassed on the e c o l o g i c a l r e l a t i o n s h i p s of C. b i f i d a and C. e x p l e t a may now be summarised. As f a r as t h e i r fundamental n i c h e s are concerned, C. b i f i d a and C. e x p l e t a have been shown to d i f f e r i n osmotic physiology ( C. e x p l e t a i s able to t o l e r a t e higher s a l i n i t i e s ) ; h a b i t a t ( C. e x p l e t a g e n e r a l l y ranging wider i n depth, s u b s t r a t e and v e g e t a t i o n than C. b i f i d a ) and to some extent i n f e e d i n g p a t t e r n s ( C. e x p l e t a but not C. b i f i d a p r e f e r r i n g dead to l i v e f o o d s ) . The s t u d i e s thus show t h a t the s p e c i e s are not e c o l o g i c a l homologues: they do not occupy the same niche . 260 A complex p a t t e r n of v a r i a t i o n , p a r t i c u l a r l y i n f e e d i n g behaviour, has thus emerged f o r the s i b l i n g Cenocorixa s p e c i e s i n the l a k e s s t u d i e d ; t h i s i s t o be expected as prey abundance and a v a i l a b i l i t y a l s o f l u c t u a t e , with marked peaks of plankton abundance and d e f i n i t e periods, of l a r v a l i n s e c t a v a i l a b i l i t y . The experimental work provided evidence .that, although the same prey items were accepted by both C. b i f i d a and C. e x p l e t a , the s p e c i e s o f t e n d i f f e r e d i n t h e i r r a t e s of acceptance of many preys. Jansson (1971) and Jansson and Scudder (1974) have suggested that d i e t a r y f a c t o r s may r e g u l a t e o v a r i a n a r r e s t and hence p o p u l a t i o n growth i n Cenocorixa . Since a l l l a k e s s t u d i e d are e u t r o p h i c i n b i o t i c p o t e n t i a l i f not i n chemical data (Cannings, 1973) and l i m i t a t i o n s i n abundance of food organisms are not apparent, q u a l i t y r a t h e r than q u a n t i t y of food may indeed be important. In the sympatric l a k e s the plankton i s c h a r a c t e r i s e d by diaptomids, which have been shown to be a r e l a t i v e l y favoured item of d i e t of both s p e c i e s o f c o r i x i d . These food organisms have a r e l a t i v e l y higher c a l o r i f i c value (on average, 5500 c a l g _ l dry weight) than daphniids (4500 c a l g~ l) (Cummins, 1967). They may thus be b e t t e r foods f o r a micropredator such as a c o r i x i d , than d a p h n i i d s . F u r t h e r , Wissing and Hasler (1972) i n d i c a t e that d a p h n i i d c a l o r i c content decreases i n e a r l y summer. The data suggest t h a t there i s ample food a v a i l a b l e t o both s p e c i e s i n the freshwater l a k e s , and i f C. e x p l e t a i s not excluded from low s a l i n i t y waters because of s a l i n i t y or owing 261 to h a b i t a t e x c l u s i o n , then i t might be absent because of a need f o r a more s p e c i a l d i e t than C. b i f i d a . Thus, although demonstrably not e c o l o g i c a l homologues, the e c o l o g i c a l p r e f e r e n c e s of C. b i f i d a and C. e x p l e t a are s t i l l q u i t e s i m i l a r . K i l l e r (1967) suggested a r e s t r i c t i o n of the r e a l i s e d n i c hes i n sympatry may a t times be the r e s u l t of a c o m p e t i t i v e s i t u a t i o n . In the two Cenocorixa s p e c i e s , no apparent r e s t r i c t i o n i n r e a l i s e d niche from a l l o p a t r y to sympatry was seen i n terms of h a b i t a t , but t h e r e was only s l i g h t evidence of a displacement of d i e t a r y t h r u s t i n sympatry. The present work seems to i n d i c a t e l i t t l e niche d i f f e r e n t i a t i o n , a f i n d i n g s i m i l a r to that of Goulden (1966). However, most s t u d i e s of c l o s e l y r e l a t e d s p e c i e s i n sympatry have detected d e f i n i t e d i f f e r e n c e s i n the e c o l o g i c a l niches ( i . e . , Catchpole, 1973; Sandercock, 1967). Such d i f f e r e n c e s are u s u a l l y i n t e r p r e t e d to be a r e s u l t of c o m p e t i t i v e i n t e r a c t i o n , e i t h e r present or i n the past. S t a h l (1966) concluded t h a t the r e s o u r c e s i n h i s study were e s s e n t i a l l y u n l i m i t e d , and t h i s may occur i n the lakes under study. However, although food seemed to be i n abundance, some e f f e c t of food q u a l i t y should not be r u l e d out. Jansson and Scudder (1974) have suggested t h a t food q u a l i t y r a t h e r than food q u a n t i t y perhaps i s i n v o l v e d i n the r e g u l a t i o n of o v a r i a n diapause i n Cenocorixa. The experimental r e s u l t s with C. b i f i d a and C. e x p l e t a show t h a t there i s some r e s t r i c t i o n of the fundamental niche i n 262 sympatry. However, at t h i s time i t has not been p o s s i b l e to c o n c l u s i v e l y demonstrate c l e a r c o m p e t i t i o n between the s p e c i e s , or the l a c k of i t . The d i e t a r y requirements of the two s p e c i e s are extremely s i m i l a r and c e r t a i n l y the absence of C. e x p l e t a from the freshwater l a k e s i s not a p p a r e n t l y owing to absence of s p e c i f i c food items. Thus, although s e v e r a l f a c e t s of the ecology of Cenocorixa p e r m i t t i n g the two s p e c i e s t o c o e x i s t have been i n v e s t i g a t e d , there i s s t i l l no d e f i n i t e answer to the i n i t i a l q u e s t i o n encountered, the reasons f o r the e x c l u s i o n of C. e x p l e t a from the most freshwater l a k e s . I t seems l i k e l y t h a t other, as yet undetermined, f a c t o r s must be i n v o l v e d . The p h y s i o l o g y of the s p e c t e s i s s t i l l l i t t l e understood, and may warrant f u r t h e r i n v e s t i g a t i o n . There i s a need f o r t r a n s p l a n t a t i o n and d e t a i l e d r e a r i n g experiments. Thus i n s e t t i n g out to s o l v e one problem, the present study has succeeded i n e l u c i d a t i n g some aspects of the b i o l o g y of the s p e c i e s , but i t has emphasised a need f o r f u r t h e r study of t h i s s p e c i e s p a i r . 263 REFERENCES Ancona, L. H. 1933. 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