THE FEEDING ECOLOGY OF NEOMYSIS MERCEDIS HOLMES IN THE FRASER RIVER ESTUARY by N. THOMAS JOHNSTON B . S c . ( H o n ) , U n i v e r s i t y of Manitoba,1970 M.Sc.,McMaster U n i v e r s i t y , 1 9 7 2 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We ac c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA October 1981 N. Thomas J o h n s t o n , 1981 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f zLoDLDfi-N  The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e V ancouver, Canada V6T 1W5 10 /1Q\ i i ABSTRACT N. me r c e d i s i n the F r a s e r R i v e r e s t u a r y i s a p r e d a t o r on the meiobenthos, e s p e c i a l l y h a r p a c t i c o i d copepods. There a r e no c l e a r s e a s o n a l d i f f e r e n c e s i n i t s u t i l i z a t i o n of food r e s o u r c e s . The consumption r a t e of N. merc e d i s f e e d i n g from n a t u r a l sediments v a r i e s w i t h body s i z e and temperature as C = 3.81*(W**0.782)*(T**0.515) where C i s the consumption r a t e of meiofauna i n ug dry w e i g h t / a n i m a l / h , W i s the mysid s i z e i n mg d r y w e i g h t , and T i s the t e m p e r a t u r e i n degrees C. The weight dependence of the i n g e s t i o n r a t e i s i d e n t i c a l t o t h a t of the m e t a b o l i c r a t e but the temperature dependence i s s i g n i f i c a n t l y l o w e r . The mysid s e l e c t i v e l y f e e ds on the a n i m a l f r a c t i o n of the sediments but o n l y o n e - h a l f of the i n g e s t e d m a t e r i a l (by wei g h t ) i s of b i o l o g i c a l o r i g i n . In m i d - A p r i l , mysid p r e d a t i o n may r e s u l t i n a 12% per day m o r t a l i t y r a t e on h a r p a c t i c o i d copepods. ..Thus, mysid p r e d a t i o n may s t r o n g l y i n f l u e n c e m e i o f a u n a l d e n s i t i e s . The a v a i l a b i l i t y of food r e s o u r c e s may c o n t r i b u t e t o the observed c e s s a t i o n of mysid p o p u l a t i o n growth i n l a t e summer. i i i TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES i v LIST OF FIGURES v ACKNOWLEDGEMENTS v i INTRODUCTION 1 MATERIALS AND METHODS 2 D i e t 2 E g e s t i o n and Consumption 5 RESULTS 11 D i e t 11 E g e s t i o n and Consumption 15 DISCUSSION .22 D i e t 22 I n g e s t i o n 25 M e t h o d o l o g i c a l B i a s e s 27 I m p l i c a t i o n s 29 APPENDIX 38 REFERENCES 41 LIST OF TABLES Tab l e 1. The number of stomachs of l a r g e Neomysis m e r c e d i s c o n t a i n i n g s p e c i f i e d prey c a t e g o r i e s i n monthly samples, 1978 t o 1 979 12 Tab l e 2. The number of stomachs of s m a l l Neomysis merc e d i s c o n t a i n i n g s p e c i f i e d prey c a t e g o r i e s i n monthly samples, 1978 t o 1979 13 Table 3. The number of Neomysis mercedis stomachs c o n t a i n i n g s p e c i f i e d prey c a t e g o r i e s i n n i g h t - c o l l e c t e d samples 14 Tab l e 4. Parameter e s t i m a t e s f o r the e g e s t i o n e q u a t i o n E=aW**b' '. .18 Tab l e 5. Parameter e s t i m a t e s f o r ' t h e e g e s t i o n e q u a t i o n E=a*(W**b)*(T**c) 18 Tab l e 6. A s s i m i l a t i o n e f f i c i e n c i e s of Neomysis mercedis f e e d i n g on S c o t t o l a n a c a n a d e n s i s 19 Tab l e 7. The mean o r g a n i c c o n t e n t of food and f a e c e s 19 Tab l e 8. E s t i m a t e d consumption r a t e s f o r Neomysis merc e d i s f e e d i n g on n a t u r a l sediments 20 T a b l e 9. The weight dependence of i n g e s t i o n and metabolism i n Neomysis mer c e d i s and o t h e r a q u a t i c i n v e r t e b r a t e s ...26 V LIST OF FIGURES F i g u r e 1. Map of the F r a s e r R i v e r e s t u a r y showing the stu d y s i t e 3 F i g u r e 2. The weight dependence of e g e s t i o n r a t e s of Neomysis m e r c e d i s ," f e e d i n g on n a t u r a l s e d i m e n t s , a t d i f f e r e n t t e m p e r a t u r e s 16 F i g u r e 3. The dependence of the e s t i m a t e d r a t i o of consumption : e g e s t i o n on the v a l u e of the a s s i m i l a t i o n ef f i c i e n c y 30 F i g u r e 4. The e s t i m a t e d consumption of meiofauna by Neomysis m e r c e d i s at Woodward I s l a n d i n A p r i l 1978 30 F i g u r e 5. S e a s o n a l v a r i a t i o n i n the r e l a t i v e abundance of meiofauna per mysid f o r mud s u b s r a t e s i n the F r a s e r • R i v e r e s t u a r y 35 F i g u r e 6. S e a s o n a l v a r i a t i o n i n the minimum d a i l y consumption of meiofauna by Neomysis m e r c e d i s i n the F r a s e r R i v e r e s t u a r y 35 v i ACKNOWLEDGEMENTS I am g r a t e f u l t o my s u p e r v i s o r , Tom N o r t h c o t e , f o r h i s encouragement and h e l p d u r i n g the c o u r s e of t h i s work. I thank Brenda H a r r i s o n f o r i d e n t i f y i n g the h a r p a c t i c o i d copepods and Dr. C. Ho l m q u i s t f o r i d e n t i f y i n g the mysid. Dave Lasenby c o l l a b o r a t e d on the f e e d i n g work and a c t e d as a sounding board f o r many s p e c u l a t i v e i d e a s . I am e s p e c i a l l y g r a t e f u l t o Dave Levy, Greg S t e e r , and I t s u o Y e s a k i f o r a s s i s t i n g me i n the f i e l d w o r k . Tom N o r t h c o t e , B i l l N e i l l , Tom C a r e f o o t , and C o l i n L e v i n g s p r o v i d e d h e l p f u l comments on an e a r l i e r d r a f t of the t h e s i s . 1 INTRODUCTION I t has l o n g been known t h a t mysids e x h i b i t two d i s t i n c t f e e d i n g modes, a r a p t o r i a l mode i n which l a r g e prey a re a c t i v e l y s e i z e d w i t h the t h o r a c i c appendages, and a f i l t e r -f e e d i n g mode i n which suspended o r g a n i c matter i s a c q u i r e d (Cannon and Manton 1927). Neomysis i n t e g e r shows both b e h a v i o u r s (Lucas 1936). D e s c r i p t i o n s of both h e r b i v o r o u s (Bowers and G r o s s n i c k l e 1978) and p l a n k t i v o r o u s (Cooper and Goldman 1980) f e e d i n g by M y s i s r e l i c t a suggest such d i e t a r y p l a s t i c i t y may be t y p i c a l i n the M y s i d a c e a . N e v e r t h e l e s s , i t i s c l e a r t h a t mysids, l i k e , o t h e r i n v e r t e b r a t e p r e d a t o r s , can f u n c t i o n e f f e c t i v e l y as c a r n i v o r e s w i t h i mportant e f f e c t s on the abundance and s p e c i e s c o m p o s i t i o n of t h e i r p r e y (Goldman et a l . 1979; Morgan et a l . 1978) There a r e , however, few q u a n t i t a t i v e d a t a on i n g e s t i o n r a t e s or f a c t o r s a f f e c t i n g these (Conover 1978) from which e i t h e r p o t e n t i a l e f f e c t s on prey p o p u l a t i o n s or energy f l o w t o the mysid p o p u l a t i o n can be e s t i m a t e d . F a c t o r s r e g u l a t i n g the r a t e of energy a c q u i s i t i o n a re n e c e s s a r i l y i m p o r t a n t t o the dynamics of both p r e d a t o r and pre y p o p u l a t i o n s . Neomysis mercedis Holmes i s a b r a c k i s h - w a t e r mysid found i n c o a s t a l r i v e r s and l a k e s and ne a r s h o r e marine w a t e r s of western N o r t h America (Holmquist 1973). Other members of the genus a r e u s u a l l y d e s c r i b e d as omnivores f e e d i n g on o r g a n i c m a t e r i a l on or near the sediments (Mauchline 1971; Pechen'-2 Fi n e n k o and P a v l o v s k a y a 1975). Kost and K n i g h t (1975) d e s c r i b e N. m e r c e d i s as f e e d i n g l a r g e l y on d e t r i t u s and d i a t o m s , whereas S i e g f r i e d and Kopache (1980) show N. m e r c e d i s from the same r i v e r system t o be p r i m a r i l y a c a r n i v o r e . The importance of h a r p a c t i c o i d copepods t o the mysid's d i e t i n the l a t t e r study i s of c o n s i d e r a b l e i n t e r e s t s i n c e t h e r e a r e r e l a t i v e l y few d a t a d e m o n s t r a t i n g s i g n i f i c a n t i n v e r t e b r a t e p r e d a t i o n on the meiobenthos. To c l a r i f y the ambiguous t r o p h i c s t a t u s of N. m e r c e d i s , I d e t e r m i n e d i t s d i e t i n the F r a s e r R i v e r e s t u a r y . I a l s o e s t i m a t e d the i n g e s t i o n r a t e of the mysid, f e e d i n g on n a t u r a l sediment-meiofauna assemblages, as a f u n c t i o n of p r e d a t o r s i z e and temperature i n o r d e r t o a s s e s s the p o t e n t i a l impact of mysid p r e d a t i o n on'the meiofauna community. MATERIALS AND METHODS D i e t The d i e t of N. mercedis .was d e t e r m i n e d from the e x a m i n a t i o n of the stomach c o n t e n t s of f i e l d caught a n i m a l s . The mysids were c o l l e c t e d from a l a r g e , d e n d r i t i c , t i d a l d r a i n a g e c h a n n e l on Woodward I s l a n d ( F i g u r e 1 ) , a s m a l l i n t e r t i d a l i s l a n d i n the F r a s e r R i v e r d e l t a (49 6'N, 123 8'W). L i v e a n i m a l s were c o l l e c t e d w i t h an e p i b e n t h i c s l e d (mesh s i z e s of 0.47 or 1.1mm) at daytime low t i d e as the a n i m a l s 3 F i g u r e 1. Map of the F r a s e r R i v e r e s t u a r y showing the study s i t e . The i n s e t map a t the t o p shows the g e n e r a l a r e a w h i l e the lower p a n e l shows the st u d y s i t e ('Stump Channel') i n d e t a i l . R I C H M O N D 4 F R A S E R R 5 con g r e g a t e d i n the lower reaches of the d e w a t e r i n g c h a n n e l . Samples f o r stomach a n a l y s i s were f r o z e n w i t h d r y i c e w i t h i n a few minutes of c a p t u r e . There were no s i g n s of r e g u r g i t a t i o n . The mysids were d i v i d e d i n t o two s i z e c l a s s e s (<1.9mg, >1.9mg), r o u g h l y c o r r e s p o n d i n g t o immature and a d u l t a n i m a l s , and the stomach c o n t e n t s of 10 a n i m a l s from each c l a s s were examined a t monthly i n t e r v a l s between A p r i l 1978 and March 1979. The 10 a n i m a l s were chosen t o span the a v a i l a b l e s i z e range. The w e l l d e f i n e d c r o p was c a r e f u l l y d i s s e c t e d out under low m a g n i f i c a t i o n , broken open on a microscope s l i d e and the c o n t e n t s examined at 100 and 400 X m a g n i f i c a t i o n . The stomach c o n t e n t s were c a t e g o r i z e d as i n T a b l e 1. The fragmented n a t u r e of the stomach c o n t e n t s and the p o s s i b i l i t y of m i s s i n g s o f t -b p d i e d prey made enumeration u n r e l i a b l e ; however, s u b j e c t i v e v i s u a l e s t i m a t e s of the r e l a t i v e abundances of prey t y p e s were made. E g e s t i o n and Consumption D i r e c t measurement of consumption by N. m e r c e d i s was i m p o s s i b l e because the mysid i n g e s t s both o r g a n i c and i n o r g a n i c m a t e r i a l ; I d e t e r m i n e d i n g e s t i o n r a t e s i n d i r e c t l y from measurements of e g e s t i o n r a t e s . E g e s t i o n r a t e s were d e t e r m i n e d on mysids and s u r f a c e sediments c o l l e c t e d a t the Woodward I s l a n d s i t e and h e l d under ambient f i e l d c o n d i t i o n s (10 C, 2 ppt s a l i n i t y , 12 h 6 p h o t o p e r i o d ) . The sediments were o b t a i n e d by removing the t o p 2-3cm from a 30cm by 30cm area a l o n g the c e n t r e of the dewatered p o r t i o n of the c h a n n e l . The sediments were s i e v e d t h r ough a 200um mesh t o remove any mysid f a e c a l p e l l e t s as w e l l as m a c r o i n v e r t e b r a t e s and v e r y l a r g e d e t r i t a l p a r t i c l e s . F i v e or s i x ml of t h i s sediment were p l a c e d i n s m a l l c i r c u l a r c o n t a i n e r s (55 cm2 area) f i l l e d w i t h 250ml of r i v e r water. The meiofauna d e n s i t y , measured i n c o n t r o l chambers, averaged 1470 a n i m a l s (CV = 28%) per c o n t a i n e r . There were 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 meiofauna d e n s i t i e s between c o n t r o l c o n t a i n e r s f o r e x p e r i m e n t s run a t d i f f e r e n t t e m p e r a t u r e s . H a r p a c t i c o i d copepods c o m p r i s e d about 65% of the numbers w i t h nematodes and o s t r a c o d s making up the r e s t . A d u l t Pseudobradya sp. were th e most common copepod, w i t h S c o t t o l a n a c a n a d e n s i s , T a c h i d i u s d i sc i p e s , and T a c h i d i u s sp. a l s o b e i n g p r e s e n t i n much s m a l l e r numbers. The mysids were a c c l i m a t e d t o the t e s t t e m p e r a t u r e s f o r 48 h b e f o r e the e x p e r i m e n t , w i t h no change i n temperature e x c e e d i n g 5 C per 48 h. The c o n t a i n e r s were h e l d o v e r n i g h t a t the t e s t c o n d i t i o n s . Three t o f i v e s i m i l a r s i z e d mysids (average CV = 15.1% f o r dry w e i g h t s ) which had been h e l d o v e r n i g h t i n s i m i l a r c o n t a i n e r s of meiofauna were t r a n s f e r r e d t o each e x p e r i m e n t a l chamber and a l l o w e d t o f e e d on the sediment f o r 5 h between 0700 and 1300 h. P r e l i m i n a r y e x p e r i m e n t s had shown the e x p e c t e d gut passage t i m e s t o be l e s s than t h i s . A l t h o u g h the mysids would have m a t e r i a l i n t h e i r 7 g u t s upon i n t r o d u c t i o n t o the e x p e r i m e n t a l chambers, and t h i s would be e g e s t e d d u r i n g the e x p e r i m e n t , I assumed t h i s compensated f o r the m a t e r i a l i n g e s t e d d u r i n g the experiment and s t i l l r e t a i n e d a t i t s t e r m i n a t i o n . T h i s was r e a s o n a b l e s i n c e the p r e - e x p e r i m e n t a l c o n d i t i o n s were i d e n t i c a l t o the e x p e r i m e n t a l c o n d i t i o n s and the mysids a p p a r e n t l y f e d c o n t i n u o u s l y . I assumed t h a t the f e e d i n g r a t e d i d not v a r y markedly i n t i m e ; the d a t a of S i e g f r i e d and Kopache (1980, F i g u r e 5 and F i g u r e 6 lower p a n e l ) , who found 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 i n g e s t i o n r a t e between l i g h t and dark e x p e r i m e n t s nor marked d i e l v a r i a t i o n i n the t o t a l number of copepods per stomach, support t h i s a s s u m p t i o n . Murtaugh (1981b) a l s o found no d i f f e r e n c e s i n f e e d i n g r a t e s i n the l i g h t and d a r k . Ten d i f f e r e n t s i z e c l a s s e s of mysids were run at each t e m p e r a t u r e . A l l e x p e r i m e n t s were done i n subdued w h i t e ( f l u o r e s c e n t ) l i g h t (0.4-0.9 m i c r o e i n s t e i n s / m 2 / s ) . Two or t h r e e c o n t r o l chambers (no mysids) were run a t each t e m p e r a t u r e . These s e r v e d as checks f o r t h e p r e - e x p e r i m e n t a l removal of f a e c e s , and prey d e n s i t i e s and sediment o r g a n i c c o n t e n t s were measured on t h e s e chambers. Measurements were made at 5, 10, 15, and 21 C, spanning the n a t u r a l s e a s o n a l temperature range. A f t e r 5 h, the mysids were removed and t h e i r f a e c a l p e l l e t s c o l l e c t e d under low m a g n i f i c a t i o n (25 X) w i t h a f i n e w i r e l o o p . The f a e c e s were r i n s e d i n d i s t i l l e d w ater, d r i e d 8 o v e r n i g h t a t 60 C, and weighed t o the n e a r e s t ug. They were then i n c i n e r a t e d at 500 C f o r 2 h and re-weighed. The l o s s of weight on i g n i t i o n was used as a measure of the o r g a n i c c o n t e n t . M a t e r i a l from the c o n t r o l chambers was s i m i l a r l y ashed. E g e s t i o n r a t e s were c a l c u l a t e d as ug/animal/h. The mysids were s i z e d by measuring the l e n g t h of the exopod of the uropod; the r e l a t i o n between the uropod l e n g t h and the d r y weight of the a n i m a l was d e t e r m i n e d on a subsample (n=28) of the a n i m a l s used. The mysid s i z e c l a s s e s ranged from 0.5 t o 9.0 mg d r y weight. I n g e s t i o n r a t e s were e s t i m a t e d (see Appendix) from the measured e g e s t i o n r a t e s a s : [ 1 ] C = F / [1 - A*(1 - p)/(1 - a + a*B - p*A)] where C i s the i n g e s t i o n r a t e i n ug dry w e i g h t / a n i m a l / h ; F i s the measured f a e c a l p r o d u c t i o n r a t e i n ug dry w e i g h t / a n i m a l / h ; A i s the a s s i m i l a t i o n e f f i c i e n c y of the mysid f o r the a n i m a l f r a c t i o n of the i n g e s t e d m a t e r i a l ; B i s the ash a s s i m i l a t i o n e f f i c i e n c y f o r ash from the p r e y ; a i s the p r o p o r t i o n of ash i n the p r e y ; and p i s the p r o p o r t i o n of ash i n the f a e c e s . 9 The p r o p o r t i o n of the i n g e s t e d m a t e r i a l t h a t i s prey i s e s t i m a t e d a s : [2] X = (1 - p ) / ( 1 - a + a*B - p*A) I use the word "prey" f o r a l l i n g e s t e d m a t e r i a l of b i o l o g i c a l o r i g i n i n c l u d i n g both l i v i n g and d e t r i t a l m a t e r i a l . Such b i o l o g i c a l m a tter w i l l c o n s i s t of o r g a n i c and ash f r a c t i o n s ; I use " o r g a n i c " t o r e f e r s o l e y t o the non-ash f r a c t i o n . Because mysids e n c l o s e t h e i r f a e c a l p e l l e t s i n a p e r i t r o p h i c membrane, a p o r t i o n of the measured o r g a n i c c o n t e n t of the f a e c e s i s not d e r i v e d d i r e c t l y from the m a t e r i a l i n g e s t e d d u r i n g the exp e r i m e n t . The p r o p o r t i o n of ash i n the f a e c e s can be c o r r e c t e d a s : [3] p = p'/O - y) where y i s the p r o p o r t i o n of the f a e c e s which i s membrane and p' i s the measured ash f r a c t i o n of the f a e c e s . The a s s i m i l a t i o n e f f i c i e n c y [4] A = (C - E)/C and ash a s s i m i l a t i o n measured w i t h the h a r p a c t i c o i d copepod S c o t t o l a n a c a n a d e n s i s as prey were used as e s t i m a t e s of A and B 10 i n e q u a t i o n 1. Mysids which had been s t a r v e d t o c l e a r t h e i r g u t s c o m p l e t e l y were f e d known numbers of a d u l t S. c a n a d e n s i s . Consumption was e s t i m a t e d as the number i n g e s t e d times the average copepod weight (8.02 ug d r y w e i g h t , n = 49). Faeces were c o l l e c t e d u n t i l the mysid's guts were empty, and were r i n s e d , d r i e d , weighed, ashed, and re-weighed. A s s i m i l a t i o n e f f i c i e n c i e s were determined u s i n g both the g r a v i m e t r i c and c o r r e c t e d a s h - r a t i o methods (Lasenby and L a n g f o r d 1973). The ash c o n t e n t of the copepods was measured as the weight of m a t e r i a l r e m a i n i n g a f t e r i g n i t i o n a t 500 C. To determine the c o n t r i b u t i o n of the p e r i t r o p h i c membrane t o the measured o r g a n i c c o n t e n t of the f a e c e s , s t a r v e d mysids were f e d p r e -ashed ( o r g a n i c f r e e ) sediments and the o r g a n i c c o n t e n t of the r e s u l t i n g f a e c e s d e t e r m i n e d as l o s s on i g n i t i o n . R e s u l t s are e x p r e s s e d as mean v a l u e s and 95% c o n f i d e n c e l i m i t s . A measure of the u n c e r t a i n t y i n d e r i v e d q u a n t i t i e s such as X was e s t i m a t e d as ( B a i r d 1966): [5] sX = f sa2(dX/da)2 + sA2(dX/dA)2 + sB2(dX/dB)2 + sp2(dX/dp)2]**0.5 where sa2 i s the v a r i a n c e of a, e t c . 11 RESULTS D i e t The e x a m i n a t i o n of the stomach c o n t e n t s of monthly samples of N. m e r c e d i s showed t h a t the mysids i n g e s t e d a v a r i e t y of o r g a n i c and i n o r g a n i c m a t e r i a l . The f r e q u e n c i e s of o c c u r r e n c e of food items i n the d i e t of the two s i z e c l a s s e s of the mysid a r e g i v e n i n T a b l e s 1 and 2. D e t r i t u s - l i k e g reenish-brown amorphous o r g a n i c m a t e r i a l and i n o r g a n i c sediment p a r t i c l e s were found i n v i r t u a l l y a l l of the a n i m a l s i n amounts r a n g i n g from a few p a r t i c l e s t o c o m p l e t e l y f i l l i n g the stomach. G e n e r a l l y the amount of such m a t e r i a l was s m a l l . C r u s t a c e a n r emains, p a r t i c u l a r l y those of h a r p a c t i c o i d copepods, o c c u r r e d i n 78% of the l a r g e mysids but were much l e s s f r e q u e n t (35%) i n the s m a l l m y s i d s . Pseudobradya sp. was the o n l y copepod i d e n t i f i e d w i t h any c e r t a i n t y . Much of the m a t e r i a l c l a s s e d as u n i d e n t i f i a b l e c r u s t a c e a n remains was p r o b a b l y a l s o h a r p a c t i c o i d copepods. When p r e s e n t , h a r p a c t i c o i d copepods formed a l a r g e f r a c t i o n of the gut c o n t e n t s , d e t r i t u s and i n o r g a n i c matter b e i n g c o r r e s p o n d i n g l y s p a r s e . A maximum of t h r e e copepods were r e c o g n i z e d i n a s i n g l e stomach but g e n e r a l l y o n l y a s i n g l e h a r p a c t i c o i d was d i s c e r n i b l e ; the enumeration of the m a s t i c a t e d prey i s r a t h e r i m p r e c i s e . H a r p a c t i c o i d copepods were most commonly found i n the l a t e f a l l and w i n t e r samples. Large b e n t h i c organisms, such as p o l y c h a e t e s and e a r l y c h i r o n o m i d i n s t a r s , were o c c a s i o n a l l y 12 T a b l e 1. The number of stomachs of l a r g e Neomysis mer c e d i s c o n t a i n i n g s p e c i f i e d prey c a t e g o r i e s i n monthly samples from A p r i l 1978 t o March 1979. Ten stomachs were examined each month. month food c a t e g o r y Ap My Jn J l Au Se Oc No De Ja Fe Mr unknown Crustacea 5 7 6 6 4 7 4 5 5 - 8 5 h a r p a c t i c o i d copepods 7 4 - 4 3 2 3 7 8 10 3 2 c l a d o c e r a n s - - - 2 - - - - - - - -n a u p l i i 1 1 - - - 1 - - - 1 - -r o t i f e r s - 1 2 - - - - 3 1 - 1 -c h i r o n o m i d l a r v a e - - 2 1 - - - 1 1 1 - 1 polychaetes — 1 — — — 1 — 1 — — — — diatoms - - - 1 2 1 f i l a m e n t o u s a l g a e - - 1 - - - - 2 - - - -v a s c u l a r p l a n t s 1 3 1 2 1 3 2 3 1 6 2 5 d e t r i t u s 10 9 9 10 4 1 0 10 10 10 10 10 8 i n o r g a n i c sediment. 1 0 7 9 1 0 9 1 0 10 10 10 10 10 10 min imum 34 47 24 23 24 25 19 29 36 23 23 36 mysid s i z e (0.1mg) max imum 78 67 69 40 36 42 43 52 63 51 61 81 found but z o o p l a n k t o n such as c l a d o c e r a n s and r o t i f e r s ( K e r a t e l l a , K e l l a c o t i a , N o t h a l e a ) were i n f r e q u e n t l y e n c o u n t e r e d . V a s c u l a r p l a n t m a t e r i a l , a p p a r e n t l y d e t r i t a l , was found i n about 25% of the l a r g e a n i m a l s but was g e n e r a l l y a v e r y s m a l l f r a c t i o n of the c o n t e n t s . Midsummer samples c o n t a i n e d green ( l i v e ? ) v a s c u l a r p l a n t p i e c e s . Diatoms were found i n low abundance i n the l a t e summer samples but seemed t o be an i n s i g n i f i c a n t d i e t a r y i t e m , as d i d o t h e r a l g a e , which were r a r e l y seen. 1 3 Tab l e 2. The number of stomachs of s m a l l Neomysis mercedis c o n t a i n i n g s p e c i f i e d prey c a t e g o r i e s i n monthly samples from A p r i l 1978 t o March 1979. Ten stomachs were examined each month. month food c a t e g o r y Ap My Jn J l Au Se Oc No De J a Fe Mr unknown C r u s t a c e a 4 2 3 5 - 5 2 - 3 2 2 2 -h a r p a c t i c o i d copepods - - - - - - - 5 6 1 - -c l a d o c e r a n s _ _ _ _ _ _ _ _ _ _ _ _ n a u p l i i - - - - - 1 - - - 3 - -r o t i f e r s — — 2 — 1 — — 1 1 - - -c h i r o n o m i d l a r v a e - - - - - - - - 2 1 1 -p o l y c h a e t e s _ _ _ _ _ _ 1 _ _ _ _ _ diatoms _ _ _ _ _ _ _ _ _ _ -| f i l a m e n t o u s a l g a e - - 1 1 - - - - - - - -s i n g l e c e l l e d a l g a e v a s c u l a r p l a n t s 1 9 1 1 - - - - - - 1 d e t r i t u s 10 10 10 10 9 10 10 10 10 9 10 8 i n o r g a n i c sediment . 10 10 9 9 10 10 10 10 10 9 10 9 minimum 8 1 3 2 4 2 3 2 6 6 4 3 mysid s i z e (0.1mg) maximum 19 8 15 11 13 12 15 17 12 15 17 15 For both the l a r g e and s m a l l mysids, the d i f f e r e n c e s i n d i e t between samp l i n g d a t e s were not s t a t i s t c a l l y s i g n i f i c a n t , the rank o r d e r i n g of the prey c a t e g o r i e s i n the monthly samples b e i n g h i g h l y c o r r e l a t e d ( K e n d a l l ' s c o e f f i c i e n t of concordance, p<0.05). Thus t h e r e were 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 s e a s o n a l changes i n the fr e q u e n c y of u t i l i z a t i o n of pr e y t y p e s by the mys i d s , a l t h o u g h minor d i f f e r e n c e s were noted. Q u a n t i t a t i v e l y , h a r p a c t i c o i d copepods, c r u s t a c e a n remains, and d e t r i t u s seemed 1 4 t o form the g r e a t e r p o r t i o n of the d i e t of the l a r g e mysids. T a b l e 3. The number of stomachs of Neomysis m e r c e d i s c o n t a i n i n g s p e c i f i e d prey c a t e g o r i e s i n n i g h t -c o l l e c t e d samples. Ten stomachs were examined on each d a t e . date food c a t e g o r y 17 Nov 1977 12 Jan 1978 0250 h 2010 h unknown c r u s t a c e a n s • 3 2 h a r p a c t i c o i d copepods 10 8 c l a d o c e r a n s - 1 n a u p l i i - -r o t i f e r s - 2 c h i r o n o m i d l a r v a e 1 p o l y c h a e t e s - 1 diatoms 5 1 f i l a m e n t o u s a l g a e 1 1 v a s c u l a r p l a n t - 1 d e t r i t u s 3 1 i n o r g a n i c sediment 8 4 minimum 2.7 1.1 mysid s i z e (mg) maximum 4.4 4.8 The d i e t of the s m a l l mysids i s o b s c u r e , s i n c e d e t r i t u s of unknown o r i g i n and sediment p a r t i c l e s comprised most of the stomach c o n t e n t s . A comparison of the stomach c o n t e n t s of n i g h t - c o l l e c t e d samples ( T a b l e 3) w i t h the above d a t a does not suggest s i g n i f i c a n t changes i n the d i e t between day and n i g h t . 15 E g e s t i o n and Consumption The e g e s t i o n r a t e s measured a t 5, 10, 15, and 21 C a r e shown i n F i g u r e 2. The e g e s t i o n r a t e s f o r the t h r e e lower t e m p e r a t u r e s i n c r e a s e d w i t h i n c r e a s i n g p r e d a t o r s i z e . The 21 C d a t a showed no c l e a r r e l a t i o n t o mysid s i z e . The e r r a t i c response of the l a r g e mysids a t 21 C suggested t h a t they may have been s t r e s s e d ; t h i s was the o n l y experiment i n which m o r t a l i t y o c c u r r e d . W i l s o n (1951) r e p o r t e d the upper l e t h a l t e m perature f o r nearby N. m e r c e d i s p o p u l a t i o n s t o be near 23 C. The e g e s t i o n d a t a a t 5, 10, and 15 C were s e p a r a t e l y f i t t e d d i r e c t l y t o the f u n c t i o n E = aW**b u s i n g a n o n - l i n e a r l e a s t squares parameter e s t i m a t i o n program (Dixon and Brown 1979). The e r r o r s t r u c t u r e of the d a t a ( F i g u r e 2) d i d not suggest t h a t a t r a n s f o r m a t i o n was r e q u i r e d as t h e r e was no c l e a r e v i d e n c e t h a t the v a r i a n c e changed w i t h the independent v a r i a b l e . S i l v e r t (1979) advanced arguments why d i r e c t f i t t i n g may be p r e f e r r a b l e t o f i t t i n g parameters t o t r a n s f o r m e d d a t a u s i n g s t a n d a r d l i n e a r r e g r e s s i o n t e c h n i q u e s . The best f i t parameters a r e g i v e n i n T a b l e 4. The weight exponents d i d not d i f f e r s i g n i f i c a n t l y between t e m p e r a t u r e s ( p > 0 . l ) . C o n s e q u e n t l y , the d a t a were f i t t e d t o the model E = a(W**b)*(T**c) t o y i e l d the parameters g i v e n i n T a b l e 5. I n s p e c t i o n of the 16 F i g u r e 2. The weight dependence of e g e s t i o n r a t e s of Neomysis mer c e d i s , f e e d i n g on n a t u r a l s ediments, d i f f e r e n t t e m p e r a t u r e s . 100 80 60 40 ca .1 20 c co w 0 (1) §100 c o 80 w <D cn ^ 6 0 5°C J L J 1 i I 40 20 15 C J i i 10°C • / / / J L J 1 1 I 21 C J L 4 6 8 0 2 Dry Weight (mg) 18 Ta b l e 4. Parameter e s t i m a t e s f o r the e g e s t i o n e q u a t i o n E=aW**b where E i s the e g e s t i o n r a t e i n ug/animal/h and W i s the dry weight i n mg. temperature (C) a b r2 n 5 12.71+-2.10 0.723+-0.103 0.82 10 10 17.01+-4.04 0.795+-0.136 0.81 10 15 20.82+-4.76 0.777+-0.144 0.71 10 Ta b l e 5. Parameter e s t i m a t e s f o r the e g e s t i o n e q u a t i o n E=a(W**b)*(T**c) where E i s the e g e s t i o n r a t e i n ug dry w e i g h t / a n i m a l / h , W i s the d r y weight i n mg, and T i s the temperature i n C. a b c r2 n 5.17+-0.75 0.782+-0.041 • 0.515+-0.052 0.80 30 r e s i d u a l s d i d not suggest any s y s t e m a t i c d e v i a t i o n from the s t r u c t u r e of the model. A s s i m i l a t i o n e f f i c i e n c i e s o b t a i n e d f o r N. mercedis a d u l t s f e e d i n g on S c o t t o l a n a c a n a d e n s i s u s i n g the g r a v i m e t r i c and c o r r e c t e d a s h - r a t i o t e c h n i q u e s a r e g i v e n i n Table 6. The a s s i m i l a t i o n e f f i c i e n c i e s e s t i m a t e d by the two t e c h n i q u e s d i d not d i f f e r s i g n i f i c a n t l y (p>0.1). The p o o l e d e s t i m a t e was 82.2(+4.1,-4.5)%. Consumption was e s t i m a t e d from the above e g e s t i o n d a t a 19 T a b l e 6. A s s i m i l a t i o n e f f i c i e n c i e s of Neomysis merc e d i s a d u l t s f e e d i n g on S c o t t o l a n a c a n a d e n s i s . food f a e c e s d r y weight %ash (mg) 0.995 10.42 0.822 11.11 ash d r y weight weight (mg) (mg) 0.104 0.186 0.091 0.173 ash %ash weight (mg) 30.11 0.056 27.75 0.048 % a s s i m i l a t i o n c o r r ash grav ash r a t i o 46.0 81.3 85.4 47.4 79.0 82.9 Ta b l e 7. The mean o r g a n i c c o n t e n t of food and f a e c e s i n s e v e r a l e x p e r i m e n t s . experiment nominal food f a e c e s food % o r g a n i c n % o r g a n i c n 5 C fee d n a t u r a l 3.86 3 11.78 10 sediment (+0.81,-0.73) (+2.1,-1.9) 10 C fee d n a t u r a l 3.21 4 11.24 9 sediment (+0.47,-0.45) (+1.3,-1.2) 21 C f e e d n a t u r a l 3.32 2 13.02 9 sediment (+1.85,-1.45) (+1.7,-1.6) a s s i m i l a t i o n S. c a n a d e n s i s 89.4 4 71.1 2 (+-1.2) (+13.6,-16.0) p e r i t r o p h i c pre-ashed 0.49 5 2.89 2 membrane sediments (+0.06,-0.05) (+5.7,-2.6) assuming t h a t the a s s i m i l a t i o n e f f i c i e n c y , ash c o n c e n t r a t i o n , and ash a s s i m i l a t i o n f o r S c o t t o l a n a c a n a d e n s i s were r e p r e s e n t a t i v e of the prey i n g e s t e d by N. m e r c e d i s . The 20 parameter v a l u e s used were: a=0.l076, A=0.822, B=0.467, and y=0.0289. The r e s u l t i n g consumption e s t i m a t e s a r e g i v e n i n Tab l e 8. The c o n f i d e n c e l i m i t s f o r the i n d i v i d u a l e s t i m a t e s were c a l c u l a t e d from e q u a t i o n 5. Tab l e 8. E s t i m a t e d consumption r a t e s (+-2SE) f o r Neomysis  mercedis f e e d i n g on n a t u r a l s e d i m e n t s , p' i s the measured p r o p o r t i o n of i n o r g a n i c m a t e r i a l i n the f a e c e s , X i s the p r o p o r t i o n (by we i g h t ) of the i n g e s t e d m a t e r i a l t h a t i s p r e y , and C/F i s the r a t i o of consumption t o e g e s t i o n . t emperature ( C) p' X C/F 5 0.8822 0.467+-0.086 1.62+-0.22 10 0.8841 0.461+-0.095 1.61+-0.23 15 0.9014 0.400+-0.065 1.49+-0.15 21 0.8698 0.506+-0.075 1.71+-0.22 N e i t h e r the r a t i o of i n g e s t i o n t o d e f e c a t i o n nor the an i m a l f r a c t i o n of the m a t e r i a l i n g e s t e d d i f f e r e d between e x p e r i m e n t s done a t d i f f e r e n t t e m p e r a t u r e s u s i n g the same sediments. I e s t i m a t e d t h a t the m a t e r i a l i n g e s t e d was 1.61+-0.14 ti m e s t h a t d e f e c a t e d , and t h a t about 0.458+-0.069 of t h i s i n g e s t e d m a t e r i a l (by weight) were prey organisms or o t h e r o r g a n i c m a t e r i a l . The r e s u l t i n g e x p r e s s i o n f o r the consumption of prey was: [6] C = 3.81*(W**0.782)*(T**0.515) where C i s i n ug d r y weight of p r e y / a n i m a l / h . A l t h o u g h o n l y 21 o n e - h a l f (by we i g h t ) of the m a t e r i a l i n g e s t e d was p r e y , t h i s was a 1 5 - f o l d i n c r e a s e over the average o r g a n i c c o n t e n t of the sediments (Table 7 ) . Thus, i t i s c l e a r t h a t N. merce d i s s e l e c t i v e l y i n g e s t s the o r g a n i c f r a c t i o n of the sed i m e n t s . However, a p o r t i o n of the o r g a n i c c o n t e n t s of the f a e c e s c o n s i s t s of the p e r i t r o p h i c membrane. I e s t i m a t e d (Table 7) t h a t about 2.9% of the f a e c e s weight i s p e r i t r o p h i c membrane. The e g e s t i o n r a t e of the pre-ashed ( o r g a n i c f r e e ) sediment was v e r y much l e s s than t h a t observed f o r n a t u r a l sediments. The r a t e was 5.3ug/animal/h f o r 6.29mg a n i m a l s a t 10 C, as compared t o an e x p e c t e d r a t e of 73.4ug/animal/h. The reduced f e e d i n g a c t i v i t y i n o r g a n i c f r e e sediment a g a i n suggested t h a t the mysids p r e f e r e n t i a l l y i n g e s t e d the o r g a n i c f r a c t i o n of the sedi m e n t s . L a b o r a t o r y o b s e r v a t i o n s of the f e e d i n g b e h a v i o r of the mysids f u r t h e r s u g g ested t h a t they f e d on meiofauna. F o r a g i n g mysids swam s l o w l y j u s t above the sediment s u r f a c e , i n t e r m i t t a n t l y d e s c e n d i n g t o the sediment s u r f a c e i n a "pouncing" motion t o d i g a c t i v e l y a t the sediment s u r f a c e w i t h the t h o r a c i c appendages f o r s h o r t p e r i o d s . I t was u n c l e a r whether the appendages or the c u r r e n t s they s e t up removed the sediment. The mysid e x c a v a t e d s e v e r a l m i l l i m e t e r s of sediment i n t h i s b e h a v i o r and then resumed an a l t e r n a t i n g swimming and r e s t i n g p a t t e r n . I b e l i e v e t h i s b e h a v i o r r e p r e s e n t e d the d e t e c t i o n and p u r s u i t of a c t i v e p r e y . The i n o r g a n i c sediment 22 commonly found i n the stomachs may be i n c i d e n t a l l y i n g e s t e d w h i l e c a p t u r i n g or m a n i p u l a t i n g prey a t the sediment s u r f a c e , a l t h o u g h the a t t a c h e d m i c r o f l o r a or adsorbed o r g a n i c c o a t i n g s c o u l d p r o v i d e a food s o u r c e . DISCUSSION D i e t Large Neomysis mercedis i n t i d a l c h a n n e l s of the F r a s e r R i v e r e s t u a r y seem t o be l a r g e l y a p r e d a t o r on the meiobenthos, a l t h o u g h o t h e r food items were i n g e s t e d . The h i g h f r e q u e n c i e s of o c c u r r e n c e of m i c r o c r u s t a c e a n remains, e s p e c i a l l y h a r p a c t i c o i d copepods, i n the l a r g e mysids and the r e l a t i v e l y l a r g e r amounts of such items' s t r o n g l y suggest c a r n i v o r y . The presence of macrobenthos, s u c h as c h i r o n o m i d l a r v a e , and the r e l a t i v e absence of r e c o g n i z a b l e p l a n k t o n i c C r u s t a c e a f u r t h e r imply t h a t f e e d i n g o c c u r r e d a t the sediment s u r f a c e . The i n c r e a s e i n the o r g a n i c c o n t e n t of the f a e c e s r e l a t i v e t o t h a t of the sediment and the presence of h a r p a c t i c o i d copepods i n the stomachs of the mysids used i n the f e e d i n g e x p e r i m e n t s a l s o imply t h a t N. m e r c e d i s s e l e c t i v e l y i n g e s t e d the meiofauna. These o b s e r v a t i o n are i n c o n t r a s t t o the f e e d i n g of N. m e r c e d i s i n the Sacramento-San J o a q u i n R i v e r e s t u a r y where Kost and K n i g h t (1975) found d e t r i t u s and p l a n k t o n i c diatoms t o be the p r i n c i p a l d i e t a r y i t e m s . C r u s t a c e a n remains were found 23 i n the l a r g e r mysids but much l e s s commonly than i n the F r a s e r R i v e r specimens. More r e c e n t l y , however, S i e g f i e d and Kopache (1980) have re-examined the f e e d i n g of N. m e r c e d i s i n the Sacramento R i v e r e s t u a r y and c o n c l u d e d t h a t the mysid i s a c a r n i v o r e on z o o p l a n k t o n ( i n c l u d i n g r o t i f e r s , h a r p a c t i c o i d copepods, and Eurytemora h i r u n d o i d e s ). In b o t h t h e s e C a l i f o r n i a s t u d i e s , the abundance of d i e t a r y items b r o a d l y r e f l e c t e d t h e i r a v a i l a b i l i t y i n the environment. My d a t a showed no c l e a r s e a s o n a l i t y i n prey u t i l i z a t i o n , perhaps because of the extreme t u r b i d i t y and the low abundances of p l a n k t o n i c z o o p l a n k t o n and p h y t o p l a n k t o n i n t h e F r a s e r R i v e r e s t u a r y ( N o r t h c o t e e t a l . 1975; N o r t h c o t e , u n p u b l i s h e d ) . Both s m a l l diatoms a n d - v a s c u l a r p l a n t m a t e r i a l o c c u r r e d i n 85-100% of a s m a l l sample of N. m e r c e d i s from the nearby Nico m e k l R i v e r , B.C. w h i l e a n i m a l remains (copepods and mysids) were found i n a l l t h e s e mysids ( W i l s o n 1951). However, a l l d i r e c t enumeration t e c h n i q u e s a r e p o t e n t i a l l y b i a s e d by the d i f f e r e n t i a l r e t e n t i o n of l a r g e r p a r t i c l e s i n the c a r d i a c stomach. R e c o g n i z a b l e p l a n k t o n i c m i c r o c r u s t a c e a n s were v e r y r a r e i n the stomach c o n t e n t s of N. m e r c e d i s from the F r a s e r R i v e r . D e s p i t e a pronounced s e a s o n a l i t y i n the abundance of z o o p l a n k t o n , t h e r e was no c l e a r s e a s o n a l i n c r e a s e i n the f r e q u e n c y of u n i d e n t i f i e d m i c r o c r u s t a c e a n remains t h a t would suggest a s h i f t t o p l a n k t o n i c forms. T h i s r e s u l t i s somewhat s u r p r i s i n g s i n c e , i n l a b o r a t o r y s t u d i e s N. m e r c e d i s was an e f f i c i e n t p r e d a t o r on a v a r i e t y of c l a d o c e r a n and copepod 24 z o o p l a n k t o n ( S i e g f r i e d and Kopache 1980; p e r s o n a l o b s e r v a t i o n ) . F u r t h e r m o r e , some p o p u l a t i o n s a r e p l a n k t i v o r o u s ( S i e g f r i e d and Kopache 1980; Murtaugh 1981a). The r e l a t i v e absence of r e c o g n i z a b l e z o o p l a n k t o n i n the stomach c o n t e n t s of mysids from the F r a s e r R i v e r e s t u a r y c o u l d r e s u l t from my sa m p l i n g regime (daytime low t i d e ) . S i e g f r i e d and Kopache (1980, F i g u r e 6) p r e s e n t e d d a t a which i n d i c a t e d a d i e l change i n the d i e t of N. m e r c e d i s i n the Sacramento-San J o a q u i n e s t u a r y , from s i m i l a r numbers of h a r p a c t i c o i d copepods and p l a n k t o n i c copepods d u r i n g the day t o p r i m a r i l y z o o p l a n k t o n a t n i g h t , presumably i n c o n c e r t w i t h the n o c t u r n a l a s c e n t r e p o r t e d f o r t h a t p o p u l a t i o n (Heubach 1969). N i g h t - c o l l e c t e d samples from the F r a s e r R i v e r ( T a b l e 3)- showed no such change, h a r p a c t i c o i d copepods and i n o r g a n i c sediment r e m a i n i n g the most common d i e t a r y i t e m s . D i f f e r e n c e s i n the h a b i t a t s s t u d i e d may account f o r some of the d i f f e r e n c e s i n d i e t between the two p o p u l a t i o n s ; I sampled i n s h a l l o w i n t e r t i d a l d r a i n a g e c h a n n e l s i n which t h e r e was l i t t l e e v i d e n c e t h a t the mysids ascend from the bottom w h i l e the C a l i f o r n i a work was done i n deeper c h a n n e l s i n which a p o r t i o n of the mysid p o p u l a t i o n was p l a n k t o n i c . A l t h o u g h z o o p l a n k t o n and s o f t - b o d i e d prey may be u n d e r e s t i m a t e d i n my s t u d y , n e v e r t h e l e s s the data suggest t h a t t h i s r i v e r i n e p o p u l a t i o n feeds on b e n t h i c meiofauna. The d i e t of Neomysis m e r c e d i s i n the F r a s e r R i v e r was s i m i l a r t o those d e c r i b e d f o r o t h e r n e a r s h o r e s p e c i e s of Neomysis . M a u c h l i n e (1971, p.353) d e s c r i b e d Neomysis i n t e g e r 25 as an omnivore f e e d i n g "on o r g a n i c d e b r i s on the bottom and i n s u s p e n s i o n " as w e l l as b e i n g "an a c t i v e c a r n i v o r e f e e d i n g on s m a l l c r u s t a c e a n s " , n o t a b l y h a r p a c t i c o i d copepods. Kinne (1955) a l s o c o n c l u d e d t h a t N. i n t e g e r i s an omnivore. Neomysis  i n t e r m e d i a f e d on "the remains of p l a n k t o n organisms, d e t r i t u s , and o t h e r s u b s t a n c e s which f a l l t o the bottom as sediment" (Murano 1966, p.66) w h i l e Neomysis m i r a b i l i s f e d on d e t r i t u s , a l g a e , and a n i m a l food (Pechen'-Finenko and P a v l o v s k a y a 1975). I t seems c l e a r t h a t Neomysis i s c a p a b l e o f , e x p l o i t i n g a v a r i e t y of f o o d s . I n g e s t i o n The consumption of prey by N. m e r c e d i s i s e s t i m a t e d as C = 3.81*(W**0.782)*(T**0.515) T h i s consumption r a t e i s the same (0.97 t i m e s ) as t h a t o b t a i n e d by Cammen (1980) f o r a q u a t i c d e t r i t i v o r e s . The e g e s t i o n r a t e s , and c o n s e q u e n t l y the e s t i m a t e d consumption r a t e s f o r N. m e r c e d i s f e e d i n g on n a t u r a l sediments v a r i e d w i t h mysid s i z e and w i t h t e m p e r a t u r e . My e s t i m a t e of the weight dependence of the i n g e s t i o n r a t e was i n d i s t i n g u i s h a b l e (p>0.l) from t h a t of the oxygen consumption r a t e ( T a b l e 9 ) , s u g g e s t i n g t h a t i n N. m e r c e d i s the i n g e s t i o n r a t e was c o u p l e d t o the m e t a b o l i c r a t e . A s i m i l a r c o rrespondance of the weight exponents of i n g e s t i o n and m e t a b o l i c r a t e s has been noted i n s e v e r a l o t h e r 26 organisms (Conover 1978), a l t h o u g h not u n i v e r s a l l y . The weight Table 9. The weight dependence of i n g e s t i o n and metabolism i n Neomysis m e r c e d i s and o t h e r a q u a t i c i n v e r t e b r a t e s . weight exponent source organism N. merc e d i s aquat i c c r u s t a c e a aquat i c d e t r i t i v o r e s r a t e i n g e s t i o n 02 consumption 02 c o n s u p t i o n 0.782+-0.041 0.836+-0.026 0.774 t h i s study u n p u b l i shed Simmons and K n i g h t 1975 i n g e s t i o n 0.800+-0.032 Sushchenya and Khmeleva 1967 i n g e s t i o n 0.742+-0.088 Cammen 1980 exponent of i n g e s t i o n was a l s o the same 'as t h a t found f o r s e v e r a l a q u a t i c c r u s t a c e a of v a r i o u s f e e d i n g modes (Sushchenya and Khmeleva 1967) as w e l l as t h a t found f o r o r g a n i c matter consumption by a q u a t i c d e t r i t i v o r e s (Cammen 1980). At a g i v e n t e m p e r a t u r e , the r a t i o of i n g e s t i o n t o metabolism i n N. merc e d i s i s thus independent of body s i z e a l t h o u g h the r a t i o n i s a d e c r e a s i n g p r o p o r t i o n of i n c r e a s i n g body w e i g h t . The exponent of the tem p e r a t u r e dependence of the i n g e s t i o n r a t e was 0.515+-0.052, which d i f f e r e d (p<0.0l) from t h a t of the oxygen consumption r a t e (0.738+-0.081). The r a t i o of i n g e s t i o n t o m e t a b o l i s m thus d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e . 27 M e t h o d o l o g i c a l B i a s e s My e g e s t i o n measurements were p o t e n t i a l l y s u s c e p t i b l e t o s e v e r a l m e t h o d o l o g i c a l b i a s e s i n c l u d i n g the e f f e c t s of prey d e n s i t y , p r e d a t o r d e n s i t y , c o n t a i n e r s i z e , the d u r a t i o n of the e x p e r i m e n t , and d i e l f e e d i n g rhythms. Most of t h e s e f a c t o r s may i n f l u e n c e the magnitude of the consumption r a t e s but i t i s l e s s c l e a r t h a t they would a l t e r the f u n c t i o n a l form of the weight and t emperature dependence of the r a t e s . Consumption r a t e s of i n v e r t e b r a t e p r e d a t o r s o f t e n show a t ype I I f u n c t i o n a l response t o prey d e n s i t y , w i t h the r a t e r e m a i n i n g c o n s t a n t above some p r e y d e n s i t y . N. m e r c e d i s f e e d i n g on A r t e m i a n a u p l i i shows sucrh a response ( u n p u b l i s h e d d a t a ) . My e x p e r i m e n t s were done at the n a t u r a l prey d e n s i t y (about 27/cm2) a t the time of c o l l e c t i o n (mid A p r i l ) ; the r e s u l t s are r e f e r a b l e t o t h a t d e n s i t y but e x t r a p o l a t i o n t o o t h e r d e n s i t i e s would r e q u i r e a d d i t i o n a l assumptions about the l o c a t i o n of the s t a b l e response r e g i o n . Prey d e n s i t y i n the e x p e r i m e n t a l c o n t a i n e r s would v a r y as prey were consumed; i n the worst c a s e , I e s t i m a t e 86% of the prey c o u l d have been e a t e n . Meiofauna d e n s i t i e s on mud s u b s t r a t e s i n the F r a s e r R i v e r e s t u a r y f l u c t u a t e s e a s o n a l l y between a p p p r o x i m a t e l y 10-300/cm2 ( B . J . H a r r i s o n , Oceanography, UBC, p e r s o n a l c ommunication). Prey d e n s i t i e s were thus w i t h i n the normal range of v a r i a b i l i t y , as were the p r e d a t o r d e n s i t i e s which v a r y s e a s o n a l l y from 20 t o 1400/m2. I n t e r f e r e n c e between mysids was 28 not o b s e r v e d and no d i f f e r e n c e s were found between c o n t a i n e r s w i t h 4 or 5 s i m i l a r mysids. C o n t a i n e r s i z e may a l t e r the normal s e a r c h b e h a v i o r of the p r e d a t o r or a l l o w the prey t o be t r a p p e d a g a i n s t u n n a t u r a l b o u n d a r i e s . The c o n t a i n e r s used i n my ex p e r i m e n t s p e r m i t t e d a seemingly normal p a t t e r n of a l t e r n a t e f r e e swimming and r e s t i n g on the s u b s t r a t e . The d u r a t i o n of the ex p e r i m e n t s were l o n g e r than the l o n g e s t gut-passage time e s t i m a t e d from p r e l i m i n a r y e x p e r i m e n t s . P r e - f e e d i n g the mysids on the e x p e r i m e n t a l sediments reduced the problem of d i f f e r e n t i a l r e t e n t i o n of l a r g e p a r t i c l e s . The consumption e s t i m a t e s depend upon the v a l u e s a s s i g n e d the parameters i n e q u a t i o n s 1-3. I assumed t h a t the measured a s s i m i l a t i o n e f f i c i e n c i e s , ash a s s i m i l a t i o n , and ash c o n t e n t r e p r e s e n t e d those of i n g e s t e d p r e y . The ash c o n t e n t of i n g e s t e d m a t e r i a l may be p o o r l y a p p r o x i m a t e d by the v a l u e s measured f o r S. c a n a d e n s i s . The ash c o n t e n t of Pseudobradya (6.83% + 0 .84,-0.78%) was c o n s i d e r a b l y l e s s than t h a t of S c o t t o l a n a . Lasenby and L a n g f o r d (1973) found the ash c o n t e n t of Daphnia t o be 21% and t h a t of c h i r o n o m i d l a r v a e t o be 12%. Thus the ash c o n t e n t of prey may v a r y c o n s i d e r a b l y . Lasenby and L a n g f o r d a l s o d e t e r m i n e d an ash a s s i m i l a t i o n e f f i c i e n c y of 74% f o r M y s i s  r e l i c t a f e e d i n g on Daphnia , a v a l u e much l a r g e r than the 46.7% I found. However, t h e r e were no d i f f e r e n c e s (p>0.l) between the f o u r s e t s of e s t i m a t e s of (C/F) or X o b t a i n e d u s i n g the ash c o n t e n t s of the two copepods and the two ash a s s i m i l a t i o n v a l u e s . 29 A l t h o u g h the consumption e s t i m a t e s a re r e l a t i v e l y i n s e n s i t i v e t o the v a l u e s of the ash c o n t e n t and ash a s s i m i l a t i o n e f f i c i e n c y f o r the p r e y , they a re r a t h e r s e n s i t i v e t o the a s s i m i l a t i o n e f f i c i e n c y ( F i g u r e 3 ) . However, the v a l u e d e t e r m i n e d f o r the a s s i m i l a t i o n e f f i c i e n c y (82.2%) i s the same as those o b t a i n e d f o r M. r e l i c t a f e e d i n g on Daphnia 'or c h i r o n o m i d l a r v a e (Lasenby and L a n g f o r d 1973) and f o r N. m i r a b i l i s f e e d i n g on a l g a e or a l g a l d e t r i t u s (Pechen'-F i n e n k o and P a v l o v s k a y a 1975). I t i s a l s o the same as v a l u e s (79.7+-8.6%) which I measured u s i n g A r t e m i a n a u p l i i as p r e y . A l t h o u g h F o u l d s and Mann (1978) found much lower a s s i m i l a t i o n e f f i c i e n c i e s (minimum v a l u e s of 30-50%) f o r M y s i s s t e n o l e p i s f e e d i n g on c e l l u l o s e , the v a l u e I used i n the c a l c u l a t i o n s seems r e a s o n a b l e f o r a v a r i e t y of n a t u r a l f o o d s . The l o s s of d i s s o l v e d o r g a n i c m a t t e r d u r i n g the i n g e s t i o n of prey would reduce the t r u e a s s i m i l a t i o n e f f i c i e n c y below the v a l u e s d e t e r m i n e d i n most of the above s t u d i e s , but s l i g h t r e d u c t i o n s i n the a s s s i m i l a t i o n e f f i c i e n c y would not g r e a t l y a f f e c t my e s t i m a t e s of consumption ( F i g u r e 3 ) . There a r e few da t a t o suggest t h a t a s s i m i l a t i o n e f f i c i e n c y v a r i e s w i t h temperature (Conover 1978). I m p l i c a t i o n s The p r e d a t i o n r a t e on the meiofauna can be c r u d e l y e s t i m a t e d u s i n g the mean d r y weight of an a d u l t Pseudobradya 30 F i g u r e 3. The dependence of the e s t i m a t e d r a t i o of consumption : e g e s t i o n on the v a l u e of the a s s i m i l a t i o n ef f i c i e n c y . F i g u r e 4. The e s t i m a t e d consumption of meiofauna by Neomysis  m e r c e d i s at Woodward I s l a n d i n A p r i l 1978. The lower p a n e l g i v e s the s i z e d i s t r i b u t i o n of the mysids w h i l e the upper p a n e l i s the e s t i m a t e d consumption by mysid s i z e c l a s s . 31 32 sp. (1.38+-0.29ug) as a r e p r e s e n t a t i v e prey w e i g h t . T h i s p r e d a t i o n r a t e w i l l depend on the d e n s i t y and s i z e s t r u c t u r e of the mysid p o p u l a t i o n as w e l l as the ambient temperature and p r o b a b l y the meiofauna d e n s i t y . The p r e d a t i o n r a t e on the meiofauna was e s t i m a t e d from the mysid d e n s i t i e s and s i z e c o m p o s i t i o n observed i n mid and l a t e A p r i l 1978 ( F i g u r e 4 ) . A l t h o u g h the s i z e s t r u c t u r e v a r i e d somewhat between the two d a t e s , the mysid p o p u l a t i o n c o n s i s t e d p r i m a r i l y of l a r g e a d u l t s from 5-7mg dry w e i g h t . M y s i d d e n s i t y d i d not d i f f e r s i g n i f i c a n t l y between dates and averaged 4 9 ( * / 2.1) per m2. The temperature was 10 C and I assumed t h a t the meiofauna d e n s i t y was 2 7 ( * / 1.3) per cm2 as I o b s e r v e d i n A p r i l 1980. With .continuous f e e d i n g on the meiofauna and an average prey weight of 1.38ug, the r e s u l t i n g consumption r a t e s ( F i g u r e 4) a r e 1340 and 1440/m2/h, or 12.1 and 13.0% of the meiofauna per day. T h i s p r e d i c t i o n was p r o b a b l y an o v e r e s t i m a t e s i n c e t i d a l regimes i n the c h a n n e l may not p e r m i t c o n t i n u o u s f e e d i n g . N e v e r t h e l e s s , mysid p r e d a t i o n may be of c o n s i d e r a b l e importance i n d e t e r m i n i n g m e i o f a u n a l d e n s i t i e s . There a r e few o t h e r d a t a t o a s s e s s the r o l e of i n v e r t e b r a t e p r e d a t i o n on the s t r u c t u r e and dynamics of meiofauna communities. M c l n t y r e (1969, p.283) argued t h a t "a l a r g e p r o p o r t i o n of meiofauna i n d i v i d u a l s appear t o be f r e e from p r e d a t i o n by a n i m a l s of h i g h e r t r o p h i c l e v e l s " . R e i s e (1979, p.453) found t h a t m a c r o b e n t h i c p r e d a t o r e x c l o s u r e had 33 l i t t l e e f f e c t on the permanent meiofauna and c o n c l u d e d t h a t "the abundance of the permanent meiofauna i s o n l y l o c a l l y or t e m p o r a r i l y r e g u l a t e d by ma c r o b e n t h i c p r e d a t o r s " . However, B e l l and C o u l l (1978) found c l e a r d i f f e r e n c e s i n the abundance and c o m p o s i t i o n of the meiofauna between e n c l o s u r e s s u b j e c t t o shrimp ( Palaemontes pugio ) p r e d a t i o n and c o n t r o l e n c l o s u r e s , but c o u l d not n e c e s s a r i l y a t t r i b u t e t h e s e d i f f e r e n c e s t o p r e d a t i o n . The 12% per day m o r t a l i t y r a t e s e s t i m a t e d from my mysid consumption d a t a imply t h a t i n v e r t e b r a t e p r e d a t i o n may e x e r t a s t r o n g i n f l u e n c e on the abundance of the meiofauna, e s p e c i a l l y a t times when the r a t e of i n c r e a s e of the prey p o p u l a t i o n i s r e s t r i c t e d by tem p e r a t u r e or food c o n d i t i o n s . C e r t a i n l y the r o l e of such p r e d a t o r s as mysids i n the dynamics of meiofauna communities needs c a r e f u l r e - e v a l u a t i o n . The abundance of the meiofauna community may i t s e l f i n f l u e n c e the r a t e of growth of the mysid p o p u l a t i o n by l i m i t i n g the r a t e of energy a q u i s i t i o n . A crude measure of the r e l a t i v e a v a i l a b i l i t y of food r e s o u r c e s per mysid was o b t a i n e d as the r a t i o of t o t a l meiofauna abundance t o mysid abundance on c o r r e s p o n d i n g s a m p l i n g d a t e s , u s i n g the meiofauna d a t a of H a r r i s o n (1981) f o r mud s u b s t r a t e s a t a s i t e s e v e r a l m i l e s d i s t a n t . A l t h o u g h the abundances a t her s i t e may not c o r r e s p o n d e x a c t l y t o those a t Woodward I s l a n d , the s e a s o n a l trend, s h o u l d be s i m i l a r . Food r e s o u r c e s were h i g h e s t i n A p r i l - May when the s m a l l p o p u l a t i o n of l a r g e o v e r w i n t e r i n g mysids r e p r o d u c e d , d e c l i n e d t h r o u g h e a r l y summer, and s t a b i l i z e d a t lower v a l u e s 34 i n l a t e summer ( F i g u r e 5 ) . The abundance of N. mercedis s t a b i l i z e d a t h i g h v a l u e s i n l a t e summer when the r e l a t i v e a v a i l a b i l i t y of food was l o w e s t , s u g g e s t i n g t h a t food may be a f a c t o r i n determing mysid abundance. In the Sacramento - San J o a q u i n e s t u a r y , y e a r - t o - y e a r v a r i a t i o n i n mysid abundance i s s t r o n g l y c o r r e l a t e d w i t h c h l o r o p h y l l a c o n c e n t r a t i o n , a measure of food a v a i l a b i l i t y ( C a l i f o r n i a Game and F i s h 1978), which a g a i n s u g g e s t s mysid abundance may g e n e r a l l y be i n f l u e n c e d by food l e v e l s . E s t i m a t e s of mysid food r e q u i r e m e n t s made u s i n g the above consumption e q u a t i o n , the observed mysid abundance and s i z e s t r u c t u r e , and the f i e l d t e m perature regimes i n d i c a t e d t h a t demand peaked i n m i d - l a t e summer ( F i g u r e 6 ) . These e s t i m a t e s assumed t h a t the mysids f e d c o n t i n u a l l y and t h a t the consumption r a t e d i d not v a r y w i t h meiofauna abundance; s i n c e the prey d e n s i t y a t which the consumption e q u a t i o n was measured may f a l l on the a s c e n d i n g arm of the f u n c t i o n a l response c u r v e , the demand e s t i m a t e s a r e p r o b a b l y minimum e s t i m a t e s . Maximum consumption by the mysid p o p u l a t i o n o c c u r r e d a t the p e r i o d of lo w e s t per mysid food a v a i l a b l i t y . The mid summer consumption r e p r e s e n t e d about 11% of the meiofauna d a i l y , i f H a r r i s o n ' s (1981) t o t a l meiofana abundance d a t a may be e x t r a p o l a t e d t o my sa m p l i n g s i t e . These d a t a i n d i c a t e t h a t food per mysid was l o w e s t , t o t a l d a i l y consumption of meiofauna h i g h e s t , and the p r o p o r t i o n of the meiofauna consumed per day h i g h e s t i n the p e r i o d when the p r e v i o u s l y expanding mysid p o p u l a t i o n 35 F i g u r e 5. S e a s o n a l v a r i a t i o n i n the r e l a t i v e abundance of meiofauna per mysid f o r mud s u b s r a t e s i n the F r a s e r R i v e r e s t u a r y . F i g u r e 6. S e a s o n a l v a r i a t i o n i n the minimum d a i l y consumption of meiofauna by Neomysis mercedis i n the F r a s e r R i v e r e s t u a r y . TOTAL KEIS3FALNA F B R MY5JD CA S C 8 0 ) ±978 1979 •ATE MYSID CDNELM^TICN or MEIOFAUNA 550. _ 37 s t a b i l i z e d i n abundance. Food l i m i t a t i o n may be i n v o l v e d i n the s t a b i l i z a t i o n of mysid abundance i n l a t e summer. 38 APPENDIX The r e l a t i o n s h i p between the measured e g e s t i o n r a t e and the i n g e s t i o n r a t e can be d e r i v e d from mass-balance c o n s i d e r a t i o n s f o r the o r g a n i c and i n o r g a n i c f r a c t i o n s of the i n g e s t e d m a t e r i a l s . S i n c e the mysids i n g e s t m i n e r a l i z e d sediment as w e l l as food of b i o l o g i c a l o r i g i n , the consumption of i n o r g a n i c m atter w i l l i n c l u d e b oth these i n o r g a n i c sediments and the ash f r a c t i o n of the p r e y , w h i l e the consumption of o r g a n i c m a t t e r w i l l o n l y be the o r g a n i c f r a c t i o n of the p r e y . L e t C be the i n g e s t i o n r a t e of a l l m a t e r i a l s and l e t F be the r e s u l t i n g d e f a e c a t i o n r a t e . I f X i s the p r o p o r t i o n by weight of the i n g e s t e d m a t e r i a l t h a t i s p r e y , then (1 - X) w i l l be the p r o p o r t i o n t h a t i s sediment. L e t A be the a s s i m i l a t i o n e f f i c i e n c y of the mysid f o r the whole p r e y , l e t a be the p r o p o r t i o n of ash i n the p r e y , and l e t B be the mysid's a s s i m i l a t i o n e f f i c i e n c y f o r ash from the p r e y . I assume t h a t the i n o r g a n i c sediment i s not a s s i m i l a t e d . The e g e s t e d m a t e r i a l can be p a r t i t i o n e d i n t o components d e r i v e d from the a n i m a l and sediment f r a c t i o n s i n g e s t e d , F = C * X * (1 - A ) + C * (1 -X) whence, 39 [1] C = F / (1 - A *X) I f f i s the weight of i n o r g a n i c matter i n the f a e c e s , and p (=f/F) i s the p r o p o r t i o n of i n o r g a n i c matter i n the f a e c e s , then [2] f = C * X * a * (1 - B) + C * (1 - X) S u b s t i t u t i n g from ( 1 ) , e q u a t i o n (2) becomes p * F = F * ( 1 ' - X + a * X - a * X * B)/(1 - A * X) whence, [3] X = (1 - p ) / ( 1 - a + a * B - p * A ) and [4] C = F/[1 - A * (1 - p ) / ( l - a + a * B - p * A ) ] The consumption i s e s t i m a t e d as C * X. A p o r t i o n of the measured o r g a n i c c o n t e n t of the f a e c e s i s the p e r i t r o p h i c membrane s e c r e t e d around the f a e c a l p e l l e t , r a t h e r than b e i n g d e r i v e d d i r e c t l y from prey i n g e s t e d d u r i n g the e x p e r i m e n t . I f m i s the weight of the membrane and F i s the weight of the f a e c e s d e r i v e d from the i n g e s t e d m a t e r i a l s , the 40 t o t a l f a e c e s weight w i l l be m + F. I assume t h a t the weight of ash from the membrane i s n e g l i g i b l e compared t o t h a t the f a e c e s . L e t y be the p r o p o r t i o n of the t o t a l f a e c e s weight c o n t r i b u t e d by the membrane and l e t p' be the measured ash c o n t e n t of the f a e c e s from the e x p e r i m e n t . 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