UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Comparative study of the life history tactics of two species of marine intertidal isopods Buskey, Edward Joseph 1976

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1976_A6_7 B88.pdf [ 3.35MB ]
Metadata
JSON: 831-1.0093822.json
JSON-LD: 831-1.0093822-ld.json
RDF/XML (Pretty): 831-1.0093822-rdf.xml
RDF/JSON: 831-1.0093822-rdf.json
Turtle: 831-1.0093822-turtle.txt
N-Triples: 831-1.0093822-rdf-ntriples.txt
Original Record: 831-1.0093822-source.json
Full Text
831-1.0093822-fulltext.txt
Citation
831-1.0093822.ris

Full Text

A COMPARATIVE STUDY OF THE LIFE HISTORY TACTICS OF TWO SPECIES OF MARINE INTERTIDAL ISOPODS by EDWARD JOSEPH BUSKEY A.B., Brown University, 1974 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the DEPARTMENT OF ZOOLOGY We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA July, 1976 (§) Edward Joseph Buskey, 1976 In presenting th i s thes is in pa r t i a 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 that the L ibrary sha l l make it f ree ly ava i l ab le for reference and study. 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 i ca t ion of th i s thes is fo r f i nanc ia l gain sha l l not be allowed without my writ ten permission. Department of The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 A b s t r a c t A d v o cates o f the t h e o r y o f r-K s e l e c t i o n c l a i m t h a t h i g h l e v e l s o f d e n s i t y - i n d e p e n d e n t m o r t a l i t y s e l e c t f o r a h i g h i n t r i n s i c r a t e o f n a t u r a l i n c r e a s e ( r - s t r a t e g i s t ) , w h i l e d e n s i t y - d e p e n d e n t m o r t a l i t y s e l e c t s f o r g r e a t e r c o m p e t i t i v e a b i l i t y and a h i g h e r c a r r y i n g c a p a c i t y (K- s t r a t e g i s t ) . F urthermore, e n v i r o n m e n t a l s t a b i l i t y i s thought t o a f f e c t t h e amount o f d e n s i t y - i n d e p e n d e n t o r d e n s i t y - d e p e n d e n t m o r t a l i t y t h a t a s p e c i e s e n c o u n t e r s w i t h u n p r e d i c t a b l e and h i g h l y v a r i a b l e environments l e a d i n g p r i m a r i l y t o d e n s i t y - i n d e p e n d e n t m o r t a l i t y , and more p r e d i c t a b l e , l e s s v a r i a b l e e n v i r o n -ments l e a d i n g p r i m a r i l y t o d e n s i t y - d e p e n d e n t m o r t a l i t y . S p e c i e s can be p o s i t i o n e d on an r-K continuum on the b a s i s o f s e v e r a l l i f e h i s t o r y c h a r a c t e r i s t i c s which a r e c o r r e l a t e d t o t h e i n t r i n s i c r a t e o f n a t u r a l i n c r e a s e . Thus i t i s s u g g e s t e d t h a t s e l e c t i o n s h o u l d f a v o u r r a p i d development, s i n g l e r e p r o d u c t i o n , e a r l y m a t u r i t y , s m a l l e r body s i z e , and h i g h e r b i r t h r a t e f o r r - s t r a t e g i s t s , and s l o w e r development, r e p e a t e d r e p r o d u c t i o n , l a t e r m a t u r i t y , l a r g e r body s i z e and lower b i r t h r a t e s f o r K-s t r a t e g i s t s . I compared two s p e c i e s o f i s o p o d s from the upper and lower i n t e r t i d a l zone f o r the r e p r o d u c t i v e parameters c o r r e l a t e d w i t h h i g h i n t r i n s i c r a t e o f i n c r e a s e . I c o n s i d e r e d the upper i n t e r t i d a l environment t o be more v a r i a b l e and u n p r e d i c t a b l e because o f the i n c r e a s e d time o f exposure t o a i r , and t h e r e s u l t i n g changes i n temperature and h u m i d i t y , compared t o the more s t a b l e c o n d i t i o n s which e x i s t when submerged. Thus the s p e c i e s from the upper i n t e r t i d a l , I d o t e a w o s n e s e n s k i i , was e x p e c t e d t o have l i f e h i s t o r y c h a r a c t e r i s t i c s which were r - s e l e c t e d w i t h r e s p e c t t o t h o s e o f I d o t e a m o n t e r e y e n s i s , from t h e lower i n t e r t i d a l . I found t h e d i f f e r e n c e s i n two o f t h e c o r r e l a t e s t o be i n t h e d i r e c t i o n p r e d i c t e d by r - and K- s e l e c t i o n , two o t h e r c o r r e l a t e s t o be i n t h e o p p o s i t e d i r e c t i o n , and a f i f t h n o t t o d i f f e r between the two s p e c i e s . By r a n k i n g t h e s e parameters w i t h r e s p e c t t o t h e i r i n f l u e n c e on t h e i n t r i n s i c r a t e o f p o p u l a t i o n i n c r e a s e , I found t h e two most i m p o r t a n t p a r a -meters, age o f f i r s t r e p r o d u c t i o n and b i r t h r a t e , t o s u p p o r t the p r e d i c t i o n s o f r-K t h e o r y , and I c o n c l u d e d t h a t 1_. w o s n e s e n s k i i was an r - s t r a t e g i s t r e l a t i v e t o JE. m o n t e r e y e n s i s . R e p r o d u c t i v e e f f o r t , t h e p r o p o r t i o n o f f o o d energy r e s o u r c e s a l l o c a t e d t o r e p r o d u c t i o n , was a l s o used as a measure o f a s p e c i e s ' p o s i t i o n on the r-K continuum. Three d i f f e r e n t measures o f r e p r o d u c t i v e e f f o r t s u p p o r t e d t h e p r e d i c t i o n o f r-K t h e o r y , and showed t h a t I_. w o s n e s e n s k i i had a h i g h e r r e p r o d u c t i v e e f f o r t t h a t d i d I_. m o n t e r e y e n s i s . i i i T a b l e o f C o n t e n t s Page i A b s t r a c t i T a b l e o f C o n t e n t s 1 1 1 L i s t o f T a b l e s ' 1 V L i s t o f F i g u r e s v Acknowledgements V 1 I n t r o d u c t i o n 1 M a t e r i a l s and Methods ^.i H a b i t a t and D i s t r i b u t i o n 2.1 Study S i t e 11 I n f o r m a t i o n Needed f o r P l a c i n g a S p e c i e s on an r-K Continuum 15 P o p u l a t i o n Samples 15 E m i g r a t i o n and I mmigration 17 Growth Rates and B r o o d i n g Time 17 Brood S i z e 19 M o r t a l i t y 19 Dry Weight-Length R e l a t i o n s h i p 19 Foods and F e e d i n g 19 A b s o r p t i o n E f f i c i e n c y . .: 20 C a l o r i m e t r y 21 R e s u l t s 22 P o p u l a t i o n Samples 22 E m i g r a t i o n and I m m i g r a t i o n 30 Growth Rates and B r o o d i n g P e r i o d 30 Brood S i z e 37 M o r t a l i t y 37 Dry Weight-Length R e l a t i o n s h i p 37 Food and F e e d i n g 44 C a l o r i m e t r y 44 A b s o r p t i o n E f f i c i e n c y 44 D i s c u s s i o n 4 7 Summary 65 L i t e r a t u r e C i t e d 6 7 i v L i s t o f T a b l e s T a b l e Page I P e r c e n t a g e o f f e m a l e s , g r a v i d females, and newly r e l e a s e d young i n samples o f I. w o s n e s e n s k i i c o l l e c t e d from u n d e r - r o c k and Fucus h a b i t a t s 31 I I C a l o r i f i c v a l u e s o f i s o p o d t i s s u e s , f o o d and f e c e s used t o e s t i m a t e r e p r o d u c t i v e e f f o r t 45 I I I Some o f the c o r r e l a t e s o f r-K s e l e c t i o n . • 48 IV V a l u e s used i n t h e c a l c u l a t i o n o f r e p r o d u c t i v e e f f o r t , and the t h r e e measures o f r e p r o d u c t i v e e f f o r t 62 V L i s t o f F i g u r e s F i g u r e Page 1 L o c a t i o n o f f i e l d s t u d y s i t e s 13 2 S i z e f r e q u e n c y h i s t o g r a m s f o r monthly samples o f I_. m o n t e r e y e n s i s 23 3 S i z e f r e q u e n c y h i s t o g r a m s f o r monthly samples o f 1^ . w o s n e s e n s k i i 25 4 The p e r c e n t a g e o f r e p r o d u c t i v e - s i z e d females t h a t were found t o be c a r r y i n g broods each month 27 5 The s i m u l a t e d growth r a t e o f I_. m o n t e r e y e n s i s 33 6 The s i m u l a t e d growth r a t e o f JE. w o s n e s e n s k i i 35 7 The r e l a t i o n s h i p o f the mean number o f g ( l ) embryos p e r b r o o d t o female body l e n g t h ' 38 8 The r e l a t i o n s h i p o f dry w e i g h t o f I_. m o n t e r e y e n s i s t o body l e n g t h 40 9 The r e l a t i o n s h i p o f d r y w e i g h t o f I. w o s n e s e n s k i i t o body l e n g t h 42 v i Acknowledgements I wish to thank my research supervisor, Dr. T. H. Carefoot, f o r his guidance and f i n a n c i a l assistance throughout the course of t h i s study, and f o r h i s h e l p f u l c r i t i c i s m of the e a r l i e r d r a f t s of t h i s t h e s i s . I also wish to thank Dr. P. A. Dehnel f o r h i s constructive c r i t i c i s m and for the use of equipment, and Dr. W. E. N e i l l f o r h i s encouragement and enlightening discussions on l i f e h i s t o r y t a c t i c s . Among my fellow graduate students, I wish to thank John Himmelman, Barbara Moon, and Dave Z i t t o n f o r t h e i r encouragement and assistance i n various aspects of t h i s study. My s p e c i a l thanks go to George L i l l y , whose constructive discussions and encouragement helped me throughout my f i r s t year of study, and who was l a r g e l y responsible f o r stimulating my i n t e r e s t i n l i f e h i s t o r y t a c t i c s . 1 I n t r o d u c t i o n The l i f e h i s t o r y p a t t e r n o f an o r g a n i s m c o n s i s t s o f a s e t o f coadapted t r a i t s which have e v o l v e d t o form a " t a c t i c " which l e a v e s t h e o r g a n i s m w i t h descendants i n f u t u r e g e n e r a t i o n s . Those s p e c i e s which have n o t e v o l v e d s u c c e s s f u l l i f e h i s t o r y t a c t i c s f a c e e x t i n c t i o n ; t h e r e f o r e a l l e x i s t i n g l i f e h i s t o r y p a t t e r n s r e p r e s e n t , a t l e a s t u n t i l the p r e s e n t , s u c c e s s f u l t a c t i c s . The r e p r o d u c t i v e t r a i t s t h a t comprise t h i s t a c t i c i n c l u d e t h o se which d e f i n e the r e p r o d u c t i v e p o t e n t i a l o f t h e organism: age s p e c i f i c growth and m o r t a l i t y r a t e s , age a t f i r s t r e p r o d u c t i o n , number o f young p e r brood, and number o f b r oods. The c o m b i n a t i o n o f t h e s e t r a i t s must a l l o w the o r ganism a t l e a s t t o r e p l a c e i t s e l f i n f u t u r e g e n e r a t i o n s d e s p i t e t h e c o m b i n a t i o n o f b i o l o g i c a l and p h y s i c a l f a c t o r s which t e n d t o d r i v e t h e p o p u l a t i o n toward e x t i n c t i o n (e.g. p r e d a t i o n , s t a r v a t i o n , e n v i r o n m e n t a l s t r e s s e s ) . The e v o l u -t i o n o f l i f e h i s t o r y t a c t i c s i s o b v i o u s l y a complex p r o c e s s , w i t h numerous p o s s i b l e t a c t i c s f o r c o n f r o n t i n g v a r i o u s e n v i r o n m e n t a l c o n d i t i o n s . In o r d e r t o make p r e d i c t i o n s about the e v o l u t i o n o f l i f e h i s t o r y t a c t i c s , a s i m p l i f y -i n g assumption i s n e c e s s a r y . Such an assumption i s t o d i v i d e a l l the e n v i r o n -m ental c o n d i t i o n s i n t o t h o se t h a t l e a d t o m o r t a l i t y i n d e n s i t y - d e p e n d e n t (DD) o r d e n s i t y - i n d e p e n d e n t (DI) manners, and t h e n t o c o n s i d e r t h e e v o l u t i o n o f l i f e h i s t o r y t a c t i c s i n s t a b l e and f l u c t u a t i n g environments, where DD and DI m o r t a l i t y s o u r c e s , r e s p e c t i v e l y , a r e assumed t o predominate. Dobzhansky (1950) was the f i r s t t o base a comprehensive t h e o r y o f n a t u r a l s e l e c t i o n on t h e e f f e c t s o f e n v i r o n m e n t a l s t a b i l i t y . He h y p o t h e s i z e d t h a t n a t u r a l s e l e c t i o n a c t e d t h r o u g h f u n d a m e n t a l l y d i s t i n c t mechanisms i n temperate and t r o p i c a l a r e a s , w i t h p h y s i c a l f a c t o r s , o p e r a t i n g i n p r i m a r i l y DI ways, 2 most f r e q u e n t l y l i m i t i n g p o p u l a t i o n s i z e i n temperate a r e a s , and w i t h b i o l o g i -c a l i n t e r a c t i o n s , o p e r a t i n g i n p r i m a r i l y DD ways, most f r e q u e n t l y l i m i t i n g p o p u l a t i o n s i z e i n t h e t r o p i c s . MacArthur (1962) l e n t s u p p o r t t o t h i s con-c e p t by s u g g e s t i n g t h a t n a t u r a l s e l e c t i o n would f a v o u r a genotype which a l l o w e d f o r g r e a t e r c o m p e t i t i v e a b i l i t y under c o n d i t i o n s o f h i g h DD m o r t a l i t y , and t h a t a b i l i t y t o compete f o r l i m i t e d r e s o u r c e s would come t o r e p l a c e p o t e n t i a l f o r p o p u l a t i o n i n c r e a s e as a measure o f f i t n e s s . The f o l l o w i n g d e s c r i p t i o n w i l l i l l u s t r a t e t h i s p o i n t . In an environment o f u n l i m i t e d r e s o u r c e s , t h e r a t e o f i n c r e a s e o f a p o p u l a t i o n , w i t h r e s p e c t t o time can be r e p r e s e n t e d by an e x p o n e n t i a l growth f u n c t i o n (dN/dt = r N ) . Gi v e n two s p e c i e s i n an u n l i m i t e d environment, d i f f e r -i n g o n l y i n t h e i r i n t r i n s i c r a t e o f i n c r e a s e r , the s p e c i e s w i t h t h e h i g h e r r w i l l e v e n t u a l l y come t o dominate t h e environment n u m e r i c a l l y . I n t h i s c a s e, " f i t n e s s " w i l l be measured as an i n c r e a s e i n r . In a r e s o u r c e - l i m i t e d environment, the p o p u l a t i o n would i n c r e a s e exponen-t i a l l y u n t i l t h e l i m i t i n g r e s o u r c e becomes i n s h o r t s u p p l y . S u r v i v a l o f i n d i v i d u a l s e n t e r i n g t h e p o p u l a t i o n would t h e n be p o o r e r , and t h e r a t e o f p o p u l a t i o n growth would b e g i n t o d e c r e a s e as the p o p u l a t i o n s i z e approached the c a r r y i n g c a p a c i t y o f t h e environment. T h i s l e a d s t o t h e f a m i l i a r l o g i s t i c e q u a t i o n ( P e a r l and Reed, 1920): dN/dt = rN(K-N/K) In t h i s model o f p o p u l a t i o n growth, f i t n e s s i s n o t n e c e s s a r i l y e x p r e s s e d s i m p l y as t h e g r e a t e s t r a t e o f i n c r e a s e , b u t depends on how e f f i c i e n t l y one p o p u l a t i o n c a n u t i l i z e t h e l i m i t e d r e s o u r c e s i n comparison t o a n o t h e r . Thus the s p e c i e s which i n c r e a s e s i t s r e l a t i v e c a r r y i n g c a p a c i t y w i l l be most f i t i n an environment t h a t i s r e s o u r c e - l i m i t e d . 3 Thus, i n an environment i n which r e s o u r c e s do n o t l i m i t the growth o f a p o p u l a t i o n , organisms w i t h genotypes w h i c h a l l o w them t o e a t t h e most f o o d , and r a i s e the most young, w i l l be b e s t r e p r e s e n t e d i n f u t u r e g e n e r a t i o n s . T h i s i s most l i k e l y t o o c c u r when t h e environment i s u n s t a b l e , and DI m o r t a l -i t y keeps t h e p o p u l a t i o n w e l l below the c a r r y i n g c a p a c i t y (and thus i n the e x p o n e n t i a l growth p h a s e ) . T h i s t y p e o f s p e c i e s i s r e f e r r e d t o as an " r -s t r a t e g i s t " . An a l t e r n a t i v e t y p e o f t a c t i c o c c u r s i n a crowded environment when the s p e c i e s e x i s t s n e a r i t s c a r r y i n g c a p a c i t y . Here organisms which can most e f f i c i e n t l y make use o f l i m i t e d r e s o u r c e s , and not waste energy on ex-c e s s i v e r e p r o d u c t i o n when l i m i t e d r e s o u r c e s reduce chance o f s u r v i v a l o f o f f s p r i n g , w i l l t e n d t o be most f i t . T h i s t y p e o f s p e c i e s i s r e f e r r e d t o as a "K- s t r a t e g i s t " (MacArthur and W i l s o n , 1967). I n o r d e r t o p r e d i c t t h e c o m b i n a t i o n o f l i f e h i s t o r y p arameters e x p e c t e d under c o n d i t i o n s o f r - and K- s e l e c t i o n , i t i s n e c e s s a r y t o determine how t h e s e parameters a f f e c t f i t n e s s measured as r o r K, r e s p e c t i v e l y . C o l e (1954) uses n u m e r i c a l a n a l y s i s t o compare w i d e l y d i f f e r i n g h y p o t h e t i c a l p o p u l a t i o n s , a s s e s s i n g t h e r e l a t i v e importance o f a g i v e n amount o f change i n a l i f e h i s -t o r y parameter on the i n t r i n s i c r a t e o f i n c r e a s e , r . He f i n d s t h a t the e f f e c t o f v a r i a t i o n o f b r o o d s i z e on r d e c r e a s e s as age a t f i r s t r e p r o d u c t i o n (a) i n c r e a s e s , and t h a t t h e c o n t r i b u t i o n o f a t o change i n r i s h i g h e s t when a i s low. Furthermore, r i s most r e s p o n s i v e t o changes i n a when l i t t e r s i z e i s l a r g e . Thus the d i f f i c u l t i e s i n v o l v e d w i t h c o n s i d e r i n g r e p r o d u c t i v e p a r a -meters i n i s o l a t i o n a r e app a r e n t ; t h e e f f e c t o f change i n one parameter on r w i l l depend on how t h e o t h e r parameters d e f i n e the framework o f t h e t a c t i c . Such t r a i t s a r e coadapted; t h e y do n o t e v o l v e i n i s o l a t i o n . A n o t h e r way t h a t v a r i o u s l i f e h i s t o r y t a c t i c s d i f f e r i s i n t h e number o f 4 times t h a t r e p r o d u c t i o n o c c u r s . A s p e c i e s which r e p r o d u c e s o n l y once i n a l i f e t i m e i s known as a semelparous s p e c i e s ; one which r e p r o d u c e s r e p e a t e d l y i s known as an i t e r o p a r o u s s p e c i e s ( C o l e , 1954). I n t u i t i v e l y , i t might be e x p e c t e d t h a t g i v e n t h e same age o f f i r s t r e p r o d u c t i o n , r e p e a t e d r e p r o d u c t i o n would be s t r o n g l y s e l e c t e d f o r i n s p e c i e s l i v i n g i n h a b i t a t s where f i t n e s s i s measured as t h e a b i l i t y t o maximize r . However, C o l e (1954) shows t h a t f o r a semelparous p o p u l a t i o n w i t h no j u v e n i l e m o r t a l i t y which d i e a f t e r r e p r o d u c -i n g : r = I n (B ) ci where B i s the b i r t h r a t e p e r female. F o r an i t e r o p a r o u s s p e c i e s , on the o t h e r hand, when the l i m i t i n g c ase o f i n f i n i t e i t e r o p a r i t y i s c o n s i d e r e d (each i n d i v i d u a l p r o d u c i n g B o f f s p r i n g e v e r y y e a r w i t h no m o r t a l i t y ) : r = In (B + 1) T h e r e f o r e , i f t h e semelparous and i t e r o p a r o u s p o p u l a t i o n s a r e t o have t h e same r a t e o f i n c r e a s e , r : l n ( B ) = r = l n ( B + 1 ) and a p B = B + 1 a p Thus f o r semelparous p o p u l a t i o n s t o grow a t the same r a t e as an i n f i n i t e l y i t e r o p a r o u s p o p u l a t i o n , the semelparous p o p u l a t i o n must o n l y i n c r e a s e i t s l i t t e r s i z e by one. On t h e b a s i s o f t h i s model i t seems t h a t i f a s p e c i e s l i v e s i n an e n v i r -onment i n which f i t n e s s can be measured i n terms o f maximal p o t e n t i a l r a t e o f p o p u l a t i o n i n c r e a s e , i t would be e x p e c t e d t h a t a change i n l i f e h i s t o r y which would add one i n d i v i d u a l t o the l i t t e r s i z e would be more l i k e l y t o e v o l v e t h a n a change t o r e p e a t e d r e p r o d u c t i o n . Thus, u s i n g i n s e c t s w i t h t h e i r h i g h f e c u n d i t y as an example, one might e x p e c t t h a t any s e l e c t i v e p r e s s u r e f o r 5 r e p e a t e d r e p r o d u c t i o n as a means o f i n c r e a s i n g r must be n e g l i g i b l e . Based on " C o l e ' s r e s u l t " , t h e q u e s t i o n a r i s e s as t o why i t e r o p a r i t y has e v o l v e d i n t h e f i r s t p l a c e . The main p r o b l e m w i t h C o l e ' s model o f semelparous and i t e r o p a r o u s r e p r o -d u c t i v e s t r a t e g i e s i s h i s u n r e a l i s t i c assumption o f a c o n s t a n t age o f s u r v i v a l f o r a l l i n d i v i d u a l s i n the p o p u l a t i o n (Charnov and S c h a f f e r , 1973). By modi-f y i n g C o l e ' s e q u a t i o n s w i t h two f a c t o r s , C, t h e p r o p o r t i o n o f o f f s p r i n g s u r -v i v i n g t h e f i r s t y e a r , and P, the a d u l t s u r v i v a l r a t e o f t h e i t e r o p a r o u s s p e c i e s , Charnov and S c h a f f e r d e r i v e t h e r a t e o f i n c r e a s e f o r t h e semelparous s p e c i e s as: r = l n ( B -C) a and t h a t o f t h e i t e r o p a r o u s s p e c i e s as: and thus: r = l n ( B -C+P) B = B + P/C a . p Charnov and S c h a f f e r r e v i s e " C o l e ' s r e s u l t " such t h a t a semelparous p o p u l a t i o n must i n c r e a s e i t s l i t t e r s i z e by P/C i n d i v i d u a l s t o grow a t the same r a t e as an i n f i n i t e l y i t e r o p a r o u s s p e c i e s . T h i s r e s u l t s u g g e s t s t h a t i t e r o p a r i t y s h o u l d be f a v o u r e d i n environments w i t h r e l a t i v e l y low j u v e n i l e s u r v i v a l . I t does n o t m a t t e r whether the m o r t a l -i t y i s DD o r DI as l o n g as i t i s age s p e c i f i c , making P/C l a r g e . I n t h i s c a s e , a semelparous s p e c i e s may need t o add"a c o n s i d e r a b l e number o f o f f s p r i n g t o i t s l i t t e r s i z e t o g e t t h e same i n t r i n s i c r a t e o f i n c r e a s e as an i t e r o p a r -ous s p e c i e s o f t h e same l i t t e r s i z e . I t i s a l s o a p p a r e n t t h a t C o l e ' s d e r i v a -t i o n i s s i m p l y a s p e c i a l c ase o f the Charnov and S c h a f f e r model, where P=C=1. 6 S i n c e m o r t a l i t y i s more l i k e l y t o be s e v e r e on the young o f a p o p u l a t i o n than on t h e s t r o n g e r , more c o m p e t i t i v e a d u l t s , we might e x p e c t t h a t i t e r o p a r i t y would be a K- s e l e c t e d t r a i t . The p r e c e d i n g d i s c u s s i o n o f the e v o l u t i o n o f l i f e h i s t o r y t a c t i c s has been m a i n l y c o n c e r n e d w i t h t h e f o r c e s i n f l u e n c i n g e v o l u t i o n w i t h i n a s i n g l e s p e c i e s . However, P i a n k a (1970) s u g g e s t s t h a t t h e c o ncept o f r - and K-s e l e c t i o n a p p l i e s n o t o n l y t o i n t r a s p e c i f i c p o p u l a t i o n s t u d i e s , b u t t h a t i t s h o u l d a l s o i n c l u d e comparisons between i n d i v i d u a l s o f d i f f e r e n t s p e c i e s . He n o t e s t h a t c l a s s i f i c a t i o n o f a s p e c i e s as an r - s t r a t e g i s t o r a K- s t r a t e g i s t i s m e a n i n g f u l o n l y when comparing i t t o a n o t h e r s p e c i e s ; i t i s a r e l a t i v e q u a l i t y . P i a n k a s u g g e s t s t h a t s p e c i e s can be o r d e r e d on an "r-K continuum" based on t h e comparison o f l i f e h i s t o r y t a c t i c s , and he has i d e n t i f i e d a s e t o f c h a r a c t e r i s t i c s which r e l a t e t o the l i f e h i s t o r y c h a r a c t e r i s t i c s o f s p e c i e s o c c u p y i n g t h e e n d p o i n t s o f t h e continuum. He p r e d i c t s t h a t r - s e l e c t e d s p e c i e s s h o u l d have r a p i d development, s i n g l e r e p r o d u c t i o n , e a r l i e r age o f f i r s t r e p r o d u c t i o n , s m a l l e r body s i z e , and h i g h e r b i r t h r a t e s , t h a n K-s e l e c t e d s p e c i e s . A n o t h e r approach f o r d e t e r m i n i n g i f a s p e c i e s i s a r e l a t i v e r - o r K-s t r a t e g i s t i s t h a t o f G a d g i l and S o l b r i g (1972) who p ropose t h a t comparison o f r e p r o d u c t i v e e f f o r t , d e f i n e d as t h e t o t a l r e s o u r c e s o b t a i n e d by the o r g a n -ism m u l t i p l i e d by t h e f r a c t i o n d e v o t e d t o r e p r o d u c t i o n , i s more m e a n i n g f u l than the measure o f r e p r o d u c t i v e parameters such as b i r t h r a t e f o r comparative s t u d i e s o f l i f e h i s t o r y t a c t i c s . They contend t h a t an o rganism i s under p o s i t i v e s e l e c t i o n t o a l l o c a t e a g r e a t e r p r o p o r t i o n o f i t s r e s o u r c e s t o r e p r o -d u c t i v e a c t i v i t i e s under c o n d i t i o n s o f h i g h DI m o r t a l i t y , s i n c e non-reproduc-t i v e a c t i v i t i e s a r e f a v o u r e d o v e r r e p r o d u c t i v e a c t i v i t i e s o n l y when they 7 enhance chances o f r e p r o d u c t i o n l a t e r on i n l i f e , and thus maximize o v e r a l l f i t n e s s . S i n c e DI m o r t a l i t y s o u r c e s a r e c o n s i d e r e d t o be u n p r e d i c t a b l e , b o t h i n o c c u r r e n c e and s e v e r i t y , a d a p t a t i o n s which d i v e r t r e s o u r c e s from r e p r o d u c -t i o n t o reduce chances o f m o r t a l i t y under t h e s e c i r c u m s t a n c e s may o f t e n be u n s u c c e s s f u l . T h e r e f o r e , t h e amount o f r e s o u r c e s d i v e r t e d t o n o n - r e p r o d u c t i v e a c t i v i t i e s w i l l depend on the r e l a t i o n s h i p between t h e c o s t o f such a d i v e r -s i o n i n terms o f a d e c r e a s e i n r and the b e n e f i t g a i n e d from i t ; t h e l e s s p r e d i c t a b l e t h e sou r c e o f m o r t a l i t y , the l e s s b e n e f i t t h a t i s t o be g a i n e d by d i v e r t i n g energy t o combat t h e s o u r c e . I t might be c o n c l u d e d from t h e above argument t h a t an r - s t r a t e g i s t would have a h i g h e r b i r t h r a t e t h a n a K- s t r a t e g i s t . G a d g i l and S o l b r i g c o n tend t h a t i n c r e a s e d b i r t h r a t e a l o n e i s n o t s u f f i c i e n t e v i d e n c e f o r the e x i s t e n c e o f an r - s t r a t e g y f o r s p e c i e s under c o n d i t i o n s o f h i g h DI m o r t a l i t y . S i n c e even r - s t r a t e g i s t s may be r e s o u r c e l i m i t e d , i n t e n s e DI m o r t a l i t y would r e l e a s e t h e p o p u l a t i o n from some o f t h i s r e s o u r c e l i m i t a t i o n . I f t h i s l e a d s t o i n c r e a s e d f o o d energy i n t a k e by t h e organism, and assuming t h a t t h e same p r o p o r t i o n o f t h e organism's a b s o r b e d energy i s a l l o c a t e d t o r e p r o d u c t i o n under the- new c o n d i t i o n s , more energy w i l l be a v a i l a b l e f o r r e p r o d u c t i o n , and b i r t h and d e a t h r a t e s w i l l i n c r e a s e t o r e a c h a new e q u i l i b r i u m . Thus G a d g i l and S o l b r i g argue t h a t a change i n l i f e h i s t o r y o c c u r s o n l y when energy a l l o c a t i o n , n o t s i m p l y b i r t h r a t e , change. Based on t h e s e p r e m i s e s , G a d g i l and S o l b r i g make t h e f o l l o w i n g q u a l i t a -t i v e p r e d i c t i o n s c o n c e r n i n g the l i f e h i s t o r y a t t r i b u t e s o f r - and K- s t r a t e -g i s t s : 1) r - s t r a t e g i s t s w i l l produce a l a r g e r t o t a l mass o f o f f s p r i n g f o r any g i v e n s i z e o f p a r e n t ; 2) r - s t r a t e g i s t s w i l l d i v e r t a g r e a t e r p r o p o r t i o n o f a b s o r b e d f o o d energy t o r e p r o d u c t i o n t h r o u g h o u t t h e l i f e h i s t o r y . 8 The m a t h e m a t i c a l models from which the t h e o r y o f r - and K- s e l e c t i o n a r e d e r i v e d a r e b a s e d on the assumption t h a t t h e most f i t p o p u l a t i o n o r s p e c i e s i s t h e one which can maximize i t s r e p r e s e n t a t i o n i n f u t u r e g e n e r a t i o n s . An a l t e r n a t i v e approach i s t h a t o f Murphy (1968) and S c h a f f e r (1974 ) who d e f i n e f i t n e s s i n l i f e h i s t o r y s t r a t e g i e s as the m i n i m i z a t i o n o f t h e p r o b a b i l i t y t h a t a p o p u l a t i o n w i l l go t o e x t i n c t i o n . Murphy (1968) p r e s e n t e d a s i m u l a t i o n model based on two s p e c i e s d i f f e r i n g o n l y i n i n t r i n s i c r a t e o f i n c r e a s e and age o f f i r s t r e p r o d u c t i o n . Under s t a b l e c o n d i t i o n s , where no v a r i a t i o n i n m o r t a l i t y between p r e - and p o s t - r e p r o d u c t i v e organisms i s assumed t o e x i s t , t h e r - s t r a t e g i s t s ( h i g h r , e a r l y age o f f i r s t r e p r o d u c t i o n ) i n c r e a s e d a n number more r a p i d l y t h a n d i d t h e K- s t r a t e g i s t s (lower r , l a t e r age o f f i r s t r e p r o d u c t i o n ) . However, r e s o u r c e s became l i m i t i n g when the K- s t r a t e g i s t s approached t h e i r c a r r y i n g c a p a c i t y , and t h e r - s t r a t e g i s t s went t o e x t i n c t i o n . A l t e r n a t i v e l y , when temporal v a r i a t i o n i n t h e p r o b a b i l i t y o f s u r v i v a l o f p r e -r e p r o d u c t i v e a n i m a l s was added t o the s i m u l a t i o n , t h e K- s t r a t e g i s t s i n c r e a s e d and dominated the p o p u l a t i o n , w h i l e t h e r - s t r a t e g i s t s d e c r e a s e d . S i n c e the m o r t a l i t y o f young was independent o f p o p u l a t i o n d e n s i t y , t h i s r e s u l t i s t h e o p p o s i t e o f what might be p r e d i c t e d on t h e b a s i s o f r-K t h e o r y . Murphy s u g g e s t s t h a t u n c e r t a i n t y i n s u r v i v a l from zygote t o f i r s t r e p r o d u c t i o n would p r o v i d e s e l e c t i v e p r e s s u r e f o r i t e r o p a r i t y and f o r r e d u c e d a l l o c a t i o n o f energy t o r e p r o d u c t i o n i n o r d e r t o ensure l o n g e r l i f e . I f r educed r e p r o d u c -t i v e e f f o r t enhanced chances o f s u r v i v a l f o r t h e p a r e n t , t h e o rganism s h o u l d "hedge i t s b e t " ( S t e a r n s , 1975) on t h e s i d e o f a s m a l l e r c l u t c h i n t h e f a c e o f e n v i r o n m e n t a l i n s t a b i l i t y . Through the use o f a s i m u l a t i o n model, S c h a f f e r (1974 ) c o n s i d e r e d t h e e f f e c t s o f e n v i r o n m e n t a l i n s t a b i l i t y , measured as the d e p a r t u r e from t h e mean 9 i n s u r v i v o r s h i p f o r good and bad years, on the s u r v i v a l of both o f f s p r i n g and the female parent. His model i n d i c a t e d that when increased environmental f l u c t u ations a f f e c t e d the s u r v i v a l of pre-reproductives but not.parents, reduced reprp ductive e f f o r t (expressed as smaller clutches and l a t e r age of f i r s t reproduc-tion) would be selected f o r , but when t h i s f l u c t u a t i o n a f f e c t e d the s u r v i v a l of parents rather than young, s e l e c t i o n would favour increased reproductive e f f o r t , i n c l u d i n g l a r g e r clutches and e a r l i e r reproduction. Thus Schaffer's model makes the same pre d i c t i o n s as r-K theory when mo r t a l i t y has a greater e f f e c t on adults than young, but opposes i t when the reverse i s true (higher mortality of young than a d u l t s ) . Few i n t e r s p e c i f i c comparisons of l i f e h i s t o r y t a c t i c s have been attempted, perhaps mainly because o f the d i f f i c u l t i e s involved with comparing species with widely d i f f e r i n g modes of reproduction. Menge (1974), for example, compared the l i f e h i s t o r y c h a r a c t e r i s t i c s of the sea stars P i s a s t e r ochraceus and Leptasterias hexactis i n the San Juan Islands. He concluded that Lepas- t e r i a s was an r - s t r a t e g i s t s r e l a t i v e to P i s a s t e r , based on Lep t a s t e r i a s ' smaller body s i z e , apparent e a r l i e r age at f i r s t reproduction (based on s i z e ) , and higher reproductive e f f o r t . Menge noted that the great d i f f e r e n c e s between the modes of reproduction of these two sea stars (Pisaster by broad-cast f e r t i l i z a t i o n , Leptasterias by brooding) made the use of fecundity as a measure of t h e i r r e l a t i v e p o s i t i o n s on an r-K continuum unsuitable, and, because of t h i s d i f f i c u l t y , c r i t i c i z e d i n t e r s p e c i f i c comparisons of l i f e h i s t o r y t a c t i c s as unsuited f o r t e s t i n g the basic concepts of r-K theory at the present l e v e l of understanding. In order to t e s t further the pr e d i c t i o n s of r-K theory, and the Pianka continuum, I intend to compare i n t h i s study the l i f e h i s t o r y c h a r a c t e r i s t i c s of two congeneric species of i n t e r t i d a l isopods. Through comparison of 10 c o n g e n e r i c s p e c i e s , r a t h e r t h a n more d i s t a n t l y r e l a t e d s p e c i e s , I hope t o a v o i d t h e p r o b l e m o f d i f f e r i n g modes o f r e p r o d u c t i o n which l e a d t o d i f f e r e n c e s i n f e c u n d i t y o p p o s i t e t o t h a t p r e d i c t e d by r-K t h e o r y i n Menge's s t u d y . P r e -sumably, c o n g e n e r i c s p e c i e s have e v o l v e d from some common a n c e s t o r r e l a t i v e l y r e c e n t l y i n e v o l u t i o n a r y time, and have s i n c e e v o l v e d t o occupy d i f f e r e n t h a b i t a t s t h r o u g h a d a p t a t i o n i n t h e i r m o r p h o l o g i e s , b e h a v i o u r , and l i f e h i s t o r i e s . S i n c e t h e g e n e r a l method o f r e p r o d u c t i o n i s t h e same f o r b o t h s p e c i e s o f i s o p o d ( i n t e r n a l f e r t i l i z a t i o n and b r o o d i n g o f young), p o s s i b l e c o m p l i c a t i o n s i n v o l v e d i n making d i r e c t comparisons o f b r o o d s i z e and j u v e n i l e m o r t a l i t y a r e e l i m i n a t e d . Two s p e c i e s o f i n t e r t i d a l i s o p o d s , I d o t e a w o s n e s e n s k i i and I d o t e a  m o n t e r e y e n s i s , were chosen f o r t h i s s t u d y . I d o t e a w o s n e s e n s k i i i s found i n a s s o c i a t i o n w i t h t h e a l g a Fucus d i s t i c h u s i n t h e mid- and upper i n t e r t i d a l zones, w h i l e 1^. m o n t e r e y e n s i s i s found i n the lower i n t e r t i d a l zone i n a s s o c i -a t i o n w i t h t h e s u r f g r a s s , P h y l l o s p a d i x s c o u l e r i . Because o f the d i f f e r e n c e i n i n t e r t i d a l d i s t r i b u t i o n s , I_. w o s n e s e n s k i i i s exposed t o a g r e a t e r degree o f e n v i r o n m e n t a l f l u c t u a t i o n i n such p h y s i c a l parameters as temperature and h u m i d i t y d u r i n g i t s l o n g exposure t o a i r , t h a n i s I_. m o n t e r e y e n s i s which spends more time submerged. On t h i s b a s i s , I would p r e d i c t t h a t I_. wosnesen- s k i i has e v o l v e d a l i f e h i s t o r y t a c t i c which i s r - s e l e c t e d i n comparison t o t h a t o f I_. m o n t e r e y e n s i s , and s h o u l d t h e r e f o r e have more r a p i d development o f young, s i n g l e r e p r o d u c t i o n , an e a r l i e r age o f f i r s t r e p r o d u c t i o n , s m a l l e r body s i z e , l a r g e r b r o o d s i z e , and a h i g h e r r e p r o d u c t i v e e f f o r t . I f m o r t a l i t y i s found t o have i t s g r e a t e s t e f f e c t on p r e - r e p r o d u c t i v e s , however, S c h a f f e r ' s model would p r e d i c t the o p p o s i t e , t h a t I_. m o n t e r e y e n s i s i s th e r - s t r a t e g i s t r e l a t i v e t o I . w o s n e s e n s k i i . 11 M a t e r i a l s and Methods H a b i t a t and D i s t r i b u t i o n I d o t e a m o n t e r e y e n s i s i s d e s c r i b e d by Menzies (1950) as r a n g i n g from Seabeck, West S e a t t l e , Washington, t o E s t e r o Bay, San L u i s Obispo County, C a l i f o r n i a . Thus i t s o c c u r r e n c e i n B a r k l e y Sound r e p r e s e n t s a s l i g h t e x t e n -s i o n i n the known range o f t h i s s p e c i e s . I n B a r k l e y Sound, 1_. m o n t e r e y e n s i s i s found most commonly i n the low i n t e r t i d a l (0.0-0.7 m) o f exposed r o c k y s h o r e s on t h e s u r f g r a s s P h y l l o s p a d i x . I n d i v i d u a l s a r e o c c a s i o n a l l y found on o t h e r s p e c i e s o f a l g a e , b u t i n such s m a l l numbers t h a t i t seems l i k e l y t h a t t h e y have been washed t h e r e from nearby P h y l l o s p a d i x beds. A s i m i l a r i n t e r t i d a l d i s t r i b u t i o n and s u b s t r a t e p r e f e r e n c e i s r e p o r t e d by Lee (1966) f o r I_. m o n t e r e y e n s i s a t D i l l o n Beach, C a l i f o r n i a . I d o t e a w o s n e s e n s k i i i s d e s c r i b e d by Menzies (1950) as r a n g i n g from the Okhotsk Sea t o E s t e r o Bay, San L u i s Obispo County, C a l i f o r n i a . On Haines I s l a n d , I_. w o s n e s e n s k i i was found most a b u n d a n t l y i n a s s o c i a t i o n w i t h t h e a l g a Fucus d i s t i c h u s i n the upper i n t e r t i d a l zone (1.2-2.3 m) o r under l o o s e l y p i l e d r o c k s i n t h i s same zone, a l t h o u g h t h e y were o c c a s i o n a l l y found i n th e lower i n t e r t i d a l zone o(0.0-1.2 m) e i t h e r i n a s s o c i a t i o n w i t h t h e a l g a U l v a o r t h e mussel M y t i l u s c a l i f o r n i a n u s . Menzies (1950) d e s c r i b e s the h a b i t a t o f I_. w o s n e s e n s k i i i n C a l i f o r n i a as b e i n g m a i n l y among the mussel, M y t i l u s  c a l i f o r n i a n u s , w i t h some i n d i v i d u a l s l i v i n g a s s o c i a t e d w i t h t h e U l v a o u t s i d e o f t h e M y t i l u s beds and l e s s f r e q u e n t l y on o t h e r s p e c i e s o f a l g a e i n the mid-d l e and upper i n t e r t i d a l zones. Study S i t e F i e l d d a t a were c o l l e c t e d a t f o u r i n t e r t i d a l s i t e s on H a i n e s I s l a n d , 12 l o c a t e d i n t h e Deer Group o f i s l a n d s i n B a r k l e y Sound (Vancouver I s l a n d , B r i t i s h C o l u m b i a ) . The two s i t e s f o r t h e I d o t e a m o n t e r e y e n s i s s t u d i e s were on the s o u t h s i d e o f t h e i s l a n d , f a c i n g T r e v o r c h a n n e l , and s u b j e c t t o i n -t e n s e wave a c t i o n from the nearby open P a c i f i c Ocean. The two c o l l e c t i o n s i t e s f o r I d o t e a w o s n e s e n s k i i were on the a d j a c e n t e a s t e r n s i d e o f the i s l a n d , f a c i n g Dodger c h a n n e l , an a r e a p r o t e c t e d from wave surge d u r i n g even the most s e v e r e storms ( F i g u r e 1 ) . S i n c e t h e p r e s e n c e o f I d o t e a m o n t e r e y e n s i s and I d o t e a w o s n e s e n s k i i i s most commonly a s s o c i a t e d w i t h t h e p l a n t s P h y l l o s p a d i x and Fucus, r e s p e c t i v e l y , s t u d y s i t e s were s e l e c t e d which r e p r e s e n t e d s u b s t a n t i a l , y e t i s o l a t e d , s t a n d s o f t h e r e s p e c t i v e p l a n t t y p e s , such t h a t minimal i m m i g r a t i o n and e m i g r a t i o n o f i s o p o d s might be a n t i c i p a t e d . The two I d o t e a m o n t e r e y e n s i s s t u d y s i t e s a r e l o n g narrow surge c h a n n e l s which c o n t a i n a l u x u r i a n t growth o f P h y l l o s p a d i x . The two c h a n n e l s a r e s e p a r a t e d by a p p r o x i m a t e l y 50 m o f s t e e p g r a n i t i c s h o r e -l i n e , u n i n t e r r u p t e d by o t h e r c h a n n e l s . The I d o t e a w o s n e s e n s k i i s i t e s are b o t h i s o l a t e d o u t c r o p p i n g s o f g r a n i t e , a p p r o x i m a t e l y 30 m a p a r t , s u r r o u n d e d by ex-t e n s i v e a r e a s o f sand and mud. One s i t e f o r each s p e c i e s was used e x c l u s i v e l y f o r the c o l l e c t i o n o f p o p u l a t i o n d a t a . A l l i s o p o d s c o l l e c t e d were r e t u r n e d unharmed w i t h i n 24 h r , and minimal impact on t h e p o p u l a t i o n a t t h e s e s i t e s i s assumed. These s i t e s w i l l be r e f e r r e d t o as s i t e I f o r t h e I d o t e a w o s n e s e n s k i i s i t e , and I f o r w m t h e I d o t e a m o n t e r e y e n s i s s i t e ( F i g u r e 1 ) . The o t h e r study s i t e s were used f o r t h e c o n s t r u c t i o n o f f i e l d e n c l o s u r e s and f o r c o l l e c t i o n s o f a n i m a l s which c o u l d n o t be r e t u r n e d (e.g. t h o s e used i n l a b e x p e r i m e n t s , o r t h o s e k i l l e d t o determine l e n g t h - d r y weight r e l a t i o n s h i p s ) . These a r e r e f e r r e d t o as s i t e s 11^ and 11^ f o r the I_. w o s n e s e n s k i i and I_. m o n t e r e y e n s i s s i t e s , r e s p e c t i v e l y . 13 F i g u r e 1. L o c a t i o n o f f i e l d s t u d y s i t e s , l o c a t e d on Haines I s l a n d , B a r k l e y Sound, B r i t i s h Columbia. I and I I were t h e f i e l d s t u d y s i t e s m m f o r I d o t e a m o n t e r e y e n s i s , and I and I I were the f i e l d s t u d y w w s i t e s f o r I d o t e a w o s n e s e n s k i i . 15 Information Needed for Placing a_ Species on an r-K Continuum The placement of these two isopods on an r-K continuum w i l l be done on the basis o f the q u a l i t a t i v e p r e d i c t i o n s of Pianka's (1970) co r r e l a t e s of r-K se l e c t i o n , and on the basis of Gadgil and Solbrig's (1972) predictions of the r e l a t i v e reproductive e f f o r t of r - and K- s t r a t e g i s t s . In order to compare the two species on the basis of Pianka's c o r r e l a t e s , data were c o l l e c t e d on the brood period, number of broods, s i z e structure of the population, the rate of growth i n the f i e l d , and the brood s i z e of gravid female isopods. In order to compare the two species on the basis of Gadgil and Solbrig's measure of reproductive e f f o r t , data were c o l l e c t e d on the r e l a t i o n s h i p between length and dry weight of the isopods, the c a l o r i c content of the body t i s s u e s , the feeding rates, and the absorption e f f i c i e n c i e s of the isopods. A d e t a i l e d explanation of the importance of these various measures w i l l follow i n the Discussion. Population Samples C o l l e c t i o n s of 200-600 isopods were made at montly i n t e r v a l s i n order to determine the demographic structure of the populations, and t h e i r change with time. Isopods were c o l l e c t e d by hand, i n a narrow (<lm) v e r t i c a l s t r i p of suit a b l e substrate, from the lowest point of the t i d e to the upper i n t e r t i d a l . This procedure was intended to compensate for any bias i n the samples due to d i f f e r e n t sexes or age classes seeking out d i f f e r e n t microhabitats. Since the handling and transport of gravid females often i n i t i a t e d the release of young isopods, small, newly released isopods were t a l l i e d i n the f i e l d and not c o l l e c t e d ; t h e i r uniform s i z e made measurement unnecessary. Thus, the isopods born i n t r a n s i t could be distin g u i s h e d from newly released isopods from the f i e l d populations. For four of the monthly samples (June, September 1975 and January, March, 1976) I_. wosnesenskii c o l l e c t e d from under rocks 16 were k e p t s e p a r a t e from i n d i v i d u a l s c o l l e c t e d among t h e Fucus, and the p r o p o r -t i o n s o f g r a v i d females and newly r e l e a s e d young i n each sample were compared. A l l c o l l e c t i o n s were br o u g h t back t o the l a b o r a t o r y , and each i s o p o d was measured and i t s sex i d e n t i f i e d , and t h e r e p r o d u c t i v e s t a t e o f the females was n o t e d . A l l i s o p o d s were th e n k e p t i n r u n n i n g seawater u n t i l t h e low t i d e o f t h e f o l l o w i n g day when th e y were r e l e a s e d i n t h e o r i g i n a l a r e a o f c o l l e c t i o n . A l l measurements o f body l e n g t h were made a l o n g t h e m i d - d o r s a l a x i s , from the a n t e r i o r edge o f t h e c e p h a l o n t o t h e p o s t e r i o r edge o f t h e t e l s o n . I s o -pods were measured t o t h e n e a r e s t mm by h o l d i n g them f l a t a g a i n s t a p l a s t i c m e t r i c r u l e r . The sex o f i n d i v i d u a l i s o p o d s was i d e n t i f i e d as f o l l o w s : L a r g e r i n d i v i d -u a l s (>10 mm i n l e n g t h ) were sexed by examining f o r the p r e s e n c e o f p a i r e d penes a r i s i n g from the f i r s t abdominal segment, as w e l l as f o r t h e appendix m a s c u l i n u s , t h e i n n e r ramus o f the second p l e o p o d o f a d u l t males. Females l a c k e d b o t h o f t h e above s t r u c t u r e s . R e p r o d u c t i v e - s i z e d i s o p o d s c o u l d o f t e n be i d e n t i f i e d by t h e p r e s e n c e o f secondary s e x u a l c h a r a c t e r i s t i c s , i n c l u d i n g the p r e s e n c e o f o o s t e g i t e s , o r f u l l b r o o d pouches f o r fe m a l e s , and f o r male I_. w o s n e s e n s k i i , t h e p r e s e n c e o f l a r g e t u f t s o f s e t a e on the i n s i d e o f the p e r i -opods. S m a l l i n d i v i d u a l s (<10 mm i n l e n g t h ) were c l a s s i f i e d as b e i n g o f i n -d e t e r m i n a t e sex, because o f t h e l a c k o f e x t e r n a l g e n i t a l i a . The r e p r o d u c t i v e s t a t e o f each female was n o t e d f o r each p o p u l a t i o n sam-p l e . S i n c e the o o s t e g i t e s o f most females were n e a r l y t r a n s p a r e n t , i t was u s u a l l y p o s s i b l e t o i d e n t i f y the r e p r o d u c t i v e s t a g e o f g r a v i d females through s i m p l e o b s e r v a t i o n . When opaque o o s t e g i t e s were p r e s e n t , one o f t h e p l a t e s was g e n t l y l i f t e d w i t h a probe, and t h e embryos were o b s e r v e d . The s t a t e o f each female was c l a s s i f i e d as f o l l o w s : Not G r a v i d - Females l a c k b o t h o o s t e g i t e s and embryos. Empty Marsupium - Females p o s s e s s o o s t e g i t e s , b u t l a c k embryos; o c c u r s f o r a s h o r t p e r i o d p r i o r t o t h e d e p o s i t i o n o f eggs i n the b r o o d pouch and f o r a l o n g e r p e r i o d f o l l o w i n g r e l e a s e o f t h e bro o d . G r a v i d : g(1) Females p o s s e s s o o s t e g i t e s and c o n t a i n s p h e r i c a l eggs o r young embryos, b u t p r i o r t o t h e r u p t u r e o f the egg membrane. Females p o s s e s s o o s t e g i t e s and c o n t a i n e l o n g a t e embryos. The embryos may have r u d i m e n t a r y eyes, segments, and appendages. Females p o s s e s s o o s t e g i t e s and c o n t a i n embryos w i t h f u l l y formed appendages, r e s e m b l i n g the a d u l t s u p e r -f i c i a l l y , b u t w i t h a d i s p r o p o r t i o n a t e l y l a r g e head and a r e d u c e d s e v e n t h segment b e a r i n g no l e g s . g(2) g(3) 17 E m i g r a t i o n and Immigration E m i g r a t i o n and i m m i g r a t i o n were measured by c a p t u r e - r e c a p t u r e t e c h n i q u e s . A p p r o x i m a t e l y 200 i s o p o d s o f each s p e c i e s were marked by immersion i n 0.5% N i l e B u l e A f o r 24 h r , and t h e n r e l e a s e d a t t h e p o p u l a t i o n sampling s i t e f o r each s p e c i e s ( s i t e s 1^ and I ). S i m i l a r l y , 200 i s o p o d s o f each s p e c i e s were marked by immersion i n 0.5% Orange G f o r 24 h r , and t h e n r e l e a s e d i n two s i t e s a d j a c e n t t o and s i m i l a r t o , s i t e s I and I ( i . e . i s o l a t e d s t a n d s o f Fucus  J m w o r P h y l l o s p a d i x ) . C o l l e c t i o n s o f 100-200 i s o p o d s were made a t s i t e s I and I and t h e i r a d j a c e n t h a b i t a t s , one, two, and seven days a f t e r the r e l e a s e o f w t h e marked i s o p o d s . Each a n i m a l from the c o l l e c t i o n s was measured, sexed, and examined f o r th e p r e s e n c e o f dye, and r e l e a s e d t h e f o l l o w i n g day a t the same s i t e o f c o l l e c t i o n . Thus, the d i s c o v e r y o f dyed i s o p o d s a t a s i t e o t h e r t h a n where th e y were r e l e a s e d would i n d i c a t e t h a t e m i g r a t i o n o r i m m i g r a t i o n had o c c u r r e d . Growth Rates and B r o o d i n g Time F i e l d e n c l o s u r e s were used t o measure growth and b r o o d i n g time i n t h e f i e l d . Twelve cages were c o n s t r u c t e d f o r each s p e c i e s a t s i t e s I I and I I » f o u r a t each o f t h r e e i n t e r t i d a l h e i g h t s (mean h e i g h t s : 1.3, 2.0, and 2.4 m f o r X. w o s n e s e n s k i i , and 0.4, 0.6, and 0.8 m f o r I_. m o n t ereyensis) . The t i d a l h e i g h t o f the cages was d e t e r m i n e d by s u r v e y i n g methods. In each c a s e , the cages were b u i l t w i t h i n t h e normal v e r t i c a l d i s t r i b u t i o n s o f t h e s p e c i e s . The cages were c o n s t r u c t e d o f s t a i n l e s s s t e e l s c r e e n (5 meshes p e r cm) and were 10 x 10 x 3 cm i n dimension. The cages were b o l t e d t o t h e r o c k substratum, and t h e edges o f the e n c l o s u r e s were s e a l e d t o t h e r o c k u s i n g A l l - C r e t e , a f a s t - s e t t i n g cement which hardened under water. A s m a l l door, a p p r o x i m a t e l y 6 x 4 cm, was c u t t h r o u g h t h e t o p o f t h e cage, f o r a d d i n g and removing i s o p o d s . 18 F i e l d determinations of growth rate were made by pl a c i n g groups of 6-12 isopods of uniform length (3, 6, and 12 mm for I_. montereyensis, and 3, 6, and 16 mm for I_. wosnesenskii) i n t o the s t e e l enclosures with an excess supply of food (Phyllospadix for I_. montereyensis, and Fucus for I_. wosnesenskii) . The enclosures were checked at i n t e r v a l s of two or four weeks, at which time the isopods were measured and new food provided. In the f i e l d growth experi-ment using newly released isopods (3 mm) for each species, doorless cages were used to prevent the escape of the small animals. The cages were b u i l t around these t i n y isopods a f t e r wrapping them i n a prot e c t i v e sheet of Ulva. Growth rates of i n d i v i d u a l isopods were also measured i n the laboratory. Newly released young, intermediate si z e d pre-reproductive young and adults were kept i n small finger bowls for as long as four months. Food was replaced and the water i n the bowls changed at regular i n t e r v a l s . The time between molts was recorded, and the increment i n length following the molt was measur-ed two days a f t e r the molt had been completed. Animals were kept i n fresh running seawater while at Bamfield, with a temperature of 12±3°C, and i n re-fr i g e r a t e d seawater while at the University of B r i t i s h Columbia, with a tem-perature of 11±1°C. To determine the brooding time: i n the f i e l d , newly gravid females were placed i n enclosures at three t i d a l heights, and checked at each t i d e . Since the cages i n t h i s experiment were checked only at two-week i n t e r v a l s , these measures of brooding time provide only an upper and lower l i m i t , and serve as a check on the brooding period determinations made i n the laboratory. Deter-minations of brooding time were made i n the laboratory by pl a c i n g newly gravid females i n small fi n g e r bowls and recording the number of days u n t i l the release of the brood, and u n t i l the next (non-reproductive) molt. 19 Brood S i z e In o r d e r t o determine the average b r o o d s i z e o f each s p e c i e s , and t o f i n d the r e l a t i o n s h i p between female body 'length and b r o o d s i z e , a l a r g e num-b e r o f g r a v i d females was c o l l e c t e d from s i t e s 11^ and I n the l a b o r a t o r y t h e o o s t e g i t e s o f f r e s h i s o p o d s were c u t away and t h e embryos were removed and counted under a d i s s e c t i n g scope. I f new embryos (stage g(D) were p r e s -o e n t , a known number o f embryos were d r i e d a t 75 C f o r 24 h r and weighed t o ±0.01 mg on a M e t t l e r H20 b a l a n c e . M o r t a l i t y The m o r t a l i t y r a t e o f newly r e l e a s e d i s o p o d s d u r i n g the f i r s t two months o f t h e i r l i v e s was e s t i m a t e d by f i r s t d e t e r m i n i n g the number o f young t h a t would be r e l e a s e d by a c o h o r t o f 1000 i s o p o d s each month (based on the p r o p o r -t i o n o f g r a v i d females o f v a r i o u s l e n g t h s , t h e i r r e p r o d u c t i v e s t a t e s , and t h e b r o o d i n g time f o r each s p e c i e s ) . The d e c r e a s e i n t h i s c o h o r t o f newly r e l e a s -ed i s o p o d s o v e r two months was found by comparing i t t o t h e a c t u a l r e p r e s e n t a -t i o n o f i s o p o d s i n the a p p r o p r i a t e s i z e c l a s s e s i n the p o p u l a t i o n sample t a k e n two months l a t e r , b a s e d on the 95% c o n f i d e n c e l i m i t s on growth o f newly r e l e a s e d i s o p o d s i n t h e f i e l d . Dry Weight-Length R e l a t i o n s h i p The r e l a t i o n s h i p between l e n g t h and dry weight o f th e i s o p o d s was d e t e r -mined f o r males and n o n - g r a v i d females o f b o t h s p e c i e s . The i s o p o d s were oven d r i e d a t 75°C f o r 24 h r and weighed t o ±0.01 mg on a M e t t l e r H20 b a l a n c e . Foods and F e e d i n g Rate The major f o o d o f a n i m a l s taken from s i t e s I I and I I was a s c e r t a i n e d m w by comparing the g u t c o n t e n t s and f e c e s o f t h e s e a n i m a l s w i t h t h o s e o f a n i m a l s k e p t i n t h e l a b o r a t o r y f o r s e v e r a l weeks and f e d o n l y a s i n g l e s p e c i e s o f a l g a e (e.g. Fucus o r U l v a f o r I . w o s n e s e n s k i i ) . 20 The f e e d i n g r a t e o f each i s o p o d s p e c i e s was d e t e r m i n e d i n the f i e l d by p l a c i n g 6-12 female i s o p o d s o f t h e same l e n g t h and r e p r o d u c t i v e s t a t e i n cages w i t h a measured amount o f p l a n t m a t e r i a l f o r 24 o r 48 h r . O nly the p l a n t found t o be t h e major d i e t a r y component was used i n o r d e r t o s i m p l i f y t h e c o n v e r s i o n o f f e e d i n g d a t a t o h e a t energy e q u i v a l e n t ( c a l o r i e s ) o f f o o d a b s o r b e d (based on c a l o r i c c o n t e n t s and a b s o r p t i o n e f f i c i e n c i e s d e t e r m i n e d i n th e l a b o r a t o r y ) . The amount o f f o o d e a t e n was d e t e r m i n e d by w e i g h i n g i t (damp l i v e w e i g h t ±0.1 mg on a M e t t l e r H20 b a l a n c e ) b e f o r e and a f t e r t h e f e e d -i n g p e r i o d . The change i n w e i g h t was d i v i d e d by t h e number o f days o f f e e d i n g t o y i e l d a f e e d i n g r a t e p e r i s o p o d p e r day. A b s o r p t i o n E f f i c i e n c y I n o r d e r t o determine the p r o p o r t i o n o f absorbed energy a l l o c a t e d t o r e -p r o d u c t i o n , t h e a b s o r p t i o n e f f i c i e n c y o f the female i s o p o d s was measured. Groups o f 6-12 female i s o p o d s o f u n i f o r m l e n g t h (15 mm f o r I_. m o n t e r e y e n s i s , 22 mm f o r I_. w o s n e s e n s k i i ) were s t a r v e d f o r 24 h r i n o r d e r t o c l e a r t h e i r g u t s . These i s o p o d s were t h e n p l a c e d i n l a r g e f i n g e r bowls w i t h f o o d o f known weight ( P h y l l o s p a d i x f o r I_. m o n t e r e y e n s i s and Fucus f o r I_. w o s n e s e n s k i i ) t o f e e d f o r p e r i o d s o f 24 o r 48 h r . A t t h e end o f t h i s f e e d i n g p e r i o d t h e r e m a i n i n g f o o d was reweighed, and th e t o t a l f o o d consumed was c a l c u l a t e d from the d i f f e r e n c e . The water i n the f i n g e r bowl was t h e n f i l t e r e d t h r o u g h a p i e c e o f Whatman #1 f i l t e r p a p e r o f known i n i t i a l weight t o c o l l e c t t h e f e c e s . ,The a n i m a l s were h e l d f o r a n o t h e r 24 h r w i t h o u t f o o d , and the water was r e -f i l t e r e d t o c o l l e c t t h e r e m a i n i n g f e c e s . A f t e r c o n v e r t i n g t h e s e v a l u e s t o e q u i v a l e n t c a l o r i e s ( d e s c r i b e d below), the a b s o r p t i o n e f f i c i e n c y was t h e n c a l -c u l a t e d as (energy o f f o o d consumed - energy o f f e c e s ) / e n e r g y o f f o o d consumed ( C a r e f o o t , 1973). 21 Calorimetry In order to estimate the c a l o r i c content of the plant foods as well as the c a l o r i c content contained i n both the somatic and reproductive t i s s u e s of the isopods, samples of the somatic tissues of females of both species (entire isopod minus brood), the eggs, the feces, and plant food used i n determination of absorption e f f i c i e n c y were combusted i n a P h i l l i p s o n Microbomb Calorimeter. Following the procedure o u t l i n e d by Gentry and Weigert Instruments, Inc. (1970), samples dri e d at 75°C for 24 hr were homogenized with a mortar and pe s t l e , pressed i n t o p e l l e t s weighing 4-20 mg, and combusted. This procedure y i e l d e d heat energy values measured as calories/mg. 22 Results Population Samples Population s i z e c l a s s structures of Idotea montereyensis and Idotea wos-nesenskii f or each month are i l l u s t r a t e d by the s i z e frequency histograms shown i n Figures 2 and 3, r e s p e c t i v e l y . From these figures, i t i s apparent that the populations of both species of Idotea contain gravid females throughout the year. Figure 4 shows the percentage of reproductive-sized females that were found to be carrying broods each month. This i s based on the minimum size of 12 mm f o r gravid females of I_. montereyensis and a minimum s i z e of 16 mm for gravid females of I_. wosnesenskii. Examination of Figures 2 and 4 reveals the basic changes i n composition of the population throughout the year for Idotea montereyensis. Although o v i -gerous females were present throughout the year, the percentage was highest during the spring and summer, and lowest during the winter months. From May to August of 1975, greater than 50% of the females were gravid, while from September 1975 to March 1976, le s s than 40% of the females c a r r i e d broods. This apparent seasonality i n breeding i s also r e f l e c t e d i n the siz e frequency histograms, which show the greatest proportion of young isopods (<10 mm i n length) from June to September 1975, during and following the breeding peak of the females. A decrease i n s i z e range of the females from a maximum of 23 mm i n March 1975 to a maximum of 18 mm i n July, suggests that the l a r g e r females experience heavy mortality during t h i s period. This mortality may be associated with brooding the young, since l a r g e r females are most often o v i -gerous, and few post-brooding females with empty marsupiums larger than 16 mm can be found i n the f i e l d throughout the year. The fa c t that only a small 23 F i g u r e 2. S i z e f r e q u e n c y h i s t o g r a m s f o r monthly samples o f I d o t e a monterey- e n s i s . The p o r t i o n s o f the h i s t o g r a m s below the dashed l i n e r e p r e s e n t young i s o p o d s o f i n d e t e r m i n a t e sex. E n c l o s e d w i t h i n the female d i s t r i b u t i o n , t h e s o l i d b l a c k h i s t o g r a m r e p r e s e n t s the p o r t i o n o f t h e female p o p u l a t i o n which were g r a v i d . The t o t a l sample s i z e f o r each month i s shown i n t h e upper l e f t hand c o r n e r o f each b l o c k ; t h e sa m p l i n g date i s i n t h e upper r i g h t c o r n e r . - | — . 1—i—|—T 1—| ' 1 — r — T— r 15 10 5 0 5 10 15 15 10 5 0 5 10 15 % F R E Q U E N C Y 25 S i z e f r e q u e n c y h i s t o g r a m s f o r montly samples o f I d o t e a w o s n e s e n s k i i . The p o r t i o n s o f the h i s t o g r a m s below the dashed l i n e r e p r e s e n t ,young i s o p o d s o f i n d e t e r m i n a t e sex. E c l o s e d w i t h i n t h e female d i s t r i b u -t i o n s , the s o l i d b l a c k h i s t o g r a m s r e p r e s e n t t h e p o r t i o n o f the female p o p u l a t i o n which was g r a v i d . The t o t a l sample s i z e f o r each month i s shown i n t h e upper l e f t hand c o r n e r o f each b l o c k ; the samp l i n g date i s i n t h e upper r i g h t c o r n e r . 26 % F R E Q U E N C Y 27 F i g u r e 4. The p e r c e n t a g e o r r e p r o d u c t i v e - s i z e d females t h a t were found t o be c a r r y i n g broods each month. T h i s i n c l u d e s a l l females l o n g e r t h a n 12 mm f o r I d o t e a m o n t e r e y e n s i s and a l l females l o n g e r than 16 mm f o r I d o t e a w o s n e s e n s k i i . 29 proportion of isopods are less than 10 mm i n length from December 1975 to March 1976, even though gravid females are s t i l l present i n the population, suggests that mortality of young isopods may be high during the winter. Thus i t seems that the majority of young are r e c r u i t e d i n t o the population during the l a t e spring and summer months, and grow to reach reproductive s i z e the following year. The change i n composition of the Idotea wosnesenskii population i s i l l u s -t r a t e d i n Figures 3 and 4. No major changes i n monthly population structure are evident i n Figure 3. Young I_. wosnesenskii appear to be continually re-c r u i t e d i n t o the population, and, s i m i l a r l y , reproductive-sized females c o n t i n u a l l y enter the breeding pool. The proportion of ovigerous females present throughout the year varies l e s s with time for Idotea wosnesenskii than for Idotea montereyensis, although a somewhat smaller proportion of females was ovigerous during the summer months (May-August) than during the winter months (Figure 4). However, a corresponding seasonal change i n population si z e c l a s s structure i s not apparent from Figure 3. The only evidence from the s i z e frequency d i s t r i b u -tions which might i n d i c a t e seasonality i n breeding i s the reduction i n the proportion of i n d i v i d u a l s i n the 3-4 mm size class from June to August. How-ever, t h i s i s more l i k e l y a d i r e c t consequency of the decrease i n number of gravid females present i n the samples, than independent evidence i n support of seasonality, since newly released young are usually found near gravid females. Furthermore, since there are never fewer than one-third of the reproductive-sized females gravid at any time (Figure 4) , a d e s c r i p t i o n of I_. wosnesenskii as a c o n t i n u a l l y reproducing species seems accurate. Idotea wosnesenskii appears not to be d i s t r i b u t e d at random with respect 30 t o sex and age c l a s s i n t h e i r two h a b i t a t s on Haines I s l a n d (under r o c k s and i n t h e Fucus) . F o u r s e a s o n a l samples o f I_. w o s n e s e n s k i i from u n d e r - r o c k h a b i t a t s , compared t o t h o s e t a k e n from among the Fucus and o t h e r a l g a e , show t h a t t h e u n d e r - r o c k p o p u l a t i o n s a r e s t r o n g l y b i a s e d toward g r a v i d females and newly r e l e a s e d young (3 mm) (Table I ) . S i n c e a l l f o u r samples show a s i g n i f i -c a n t l y h i g h e r p r o p o r t i o n o f g r a v i d females and newly r e l e a s e d young under r o c k s t h a n i n t h e Fucus, i t s u g g e s t s t h a t g r a v i d females might seek out t h e u n d e r - r o c k h a b i t a t f o r p r o t e c t i o n w h i l e b r o o d i n g t h e i r young. A l t e r n a t i v e l y , t h i s b e h a v i o u r might improve the s u r v i v a l o f t h e young i s o p o d s by r e d u c i n g t h e chances t h a t t h e y w i l l be swept away by waves and by r e d u c i n g t h e s t r e s s imposed by changes i n temperature and m o i s t u r e when t h e t i d e i s o u t . E m i g r a t i o n and I m m i g r a t i o n The r e s u l t s o f t h e t e s t s f o r e m i g r a t i o n and i m m i g r a t i o n suggest t h a t t h e r e was no exchange o f a d u l t i s o p o d s between a d j a c e n t p o p u l a t i o n s , s i n c e no marked i s o p o d s were found o u t s i d e o f the i s o l a t e d h a b i t a t s i n which t h e y were r e l e a s e d . The r e c o v e r y o f 17-41% o f the marked i s o p o d s w i t h i n t h e h a b i t a t s i n which t h e y were r e l e a s e d , however, c o n f i r m e d t h a t the i s o p o d s which d i d n o t m o l t r e t a i n e d the marking dye i n t h e i r e x o s k e l e t o n s f o r a t l e a s t a week. S i n c e t h e s e i s o p o d s m o l t once e v e r y 30 days on th e average, a maximum o f 25% o f t h e marked i s o p o d s may have shed t h e i r dyed e x o s k e l e t o n s a f t e r 7 days. The absence o f e v i d e n c e f o r e m i g r a t i o n from o r i m m i g r a t i o n t o , s i t e s I and m I s u g g e s t e d t h a t t h e monthly samples o f p o p u l a t i o n s i z e c l a s s s t r u c t u r e t aken a t t h e s e s i t e s would r e f l e c t demographic changes w i t h i n e s s e n t i a l l y c l o s e d p o p u l a t i o n s . Growth Rates and B r o o d i n g P e r i o d The s i m u l a t e d growth c u r v e s o f b o t h s p e c i e s o f I d o t e a were dete r m i n e d by o v e r l a p p i n g t h e d a t a c o l l e c t e d from growth measurements o f t h r e e d i f f e r e n t Table I. Percentage of females (F), gravid females (GF), and newly released young (NRY) i n samples of Idotea wosnesenskii c o l l e c t e d from under-rock and Fucus habitats at s i t e I . w Date % F under rocks % F i n Fucus % GF under rocks % GF i n Fucus % NRY under rocks % NRY i n Fucus June 10, 1975 Sept. 4, 1975 Jan. 15, 1976 Mar. 13, 1976 58% 75%* 66%* 64% 49% 53%* 46%* 5U 72%* 64%* 78%* 77%* 34%* 45%* 63%* 56%* 2%* 14%J 13%* 21%* 0%* 1%* 1%* 1%* *These p a i r s of observations were s i g n i f i c a n t l y d i f f e r e n t , based on an independent sample chi-squared t e s t at a=0.05. s i z e classes of isopods kept i n separate f i e l d enclosures from June through December of 1975. The r e s u l t i n g growth curves are presented f o r I_. monterey- ensis i n Figure 5, and for I_. wosnesenskii i n Figure 6. Since the data simu-l a t e a year's growth, but a l l growth measurements were made within a seven-month i n t e r v a l , not a l l seasonal e f f e c t s on growth may be accounted for. For example, the normal sequence of seasonal temperatures that would have been experienced by a si n g l e cohort of newly released isopods growing to adults was not experienced;,the curves would represent the growth of a cohort r a i s e d through r e p e t i t i o n s of the summer and autumn seasonal temperatures. However, since I am mainly i n t e r e s t e d i n q u a l i t a t i v e comparison of the l i f e h i s t o r y c h a r a c t e r i s t i c s of the two species, and not quantitative measures of growth rates I assumed that seasonal e f f e c t s on growth would a f f e c t both species i n the same d i r e c t i o n , making Figures 5 and 6 v a l i d f o r q u a l i t a t i v e comparisons. The mean brooding period (±95% confidence l i m i t s ) f o r Idotea montereyen- s i s and Idotea wosnesenskii under laboratory conditions (water temperature of 11 ±1°C) are shown below. Mean Brooding Period Species N (±95% confidence l i m i t s ) I_. montereyensis 51 26 ± 1 days _I. wosnesenskii 46 , 56 ± 3 days Since there i s no overlap of the 95% confidence l i m i t s , the-brooding period of Idotea wosnesenskii i s s i g n i f i c a n t l y longer than that of Idotea monterey- ensis under laboratory conditions. In the f i e l d enclosures, 79% of the female I_. montereyensis had brooding periods of le s s than 30 days, with the remaining females re l e a s i n g t h e i r broods within 44 days, while 67% of the fe-male I_. wosnesenskii had brooding periods of l e s s than 60 days, with the re-maining females r e l e a s i n g t h e i r broods within 73 days. 33 F i g u r e 5. The s i m u l a t e d growth r a t e o f I d o t e a m o n t e r e y e n s i s b a s e d on t h e growth r a t e s o f t h r e e d i f f e r e n t s i z e c l a s s e s o f i s o p o d s kept i n f i e l d e n c l o s u r e s . I n i t i a l sample s i z e s were: 36 i s o p o d s f o r Curve I , and 24 i s o p o d s f o r each o f Curves I I and I I I . R e d u c t i o n s i n sample s i z e r e s u l t e d from the l o s s o f i s o p o d s t h r o u g h m o r t a l i t y , escape from t h e e n c l o s u r e s , and t h e d e s t r u c t i o n o f one e n c l o s u r e . F i n a l sample s i z e s were: 17 i s o p o d s f o r Curve I , 16 i s o p o d s f o r Curve I I , and 23 i s o p o d s f o r Curve I I I . 18 0 50 100 150 200 250 300 350 400 TIME IN DAYS 35 F i g u r e 6. The s i m u l a t e d growth r a t e o f I d o t e a w o s n e s e n s k i i based on the growth r a t e s o f t h r e e d i f f e r e n t s i z e c l a s s e s o f i s o p o d s k e p t i n f i e l d e n c l o s u r e s . I n i t i a l sample s i z e s were: 36 i s o p o d s f o r Curve I, and 24 i s o p o d s f o r each o f Curves I I and I I I . Reduc-t i o n s i n sample s i z e r e s u l t e d from t h e l o s s o f i n d i v i d u a l s t h r o u g h m o r t a l i t y and escape from t h e e n c l o s u r e . The f i n a l sample s i z e s were: 14 i s o p o d s f o r Curve I , 20 i s o p o d s f o r Curve I I , and 22 i s o p o d s f o r Curve I I I . TIME IN DAYS 37 Brood Size The r e l a t i o n s h i p between brood s i z e and female body length for Idotea  montereyensis and Idotea wosnesenskii i s presented i n Figure 7. A l l data are for females containing g(l) embryos (refer to table on p. 16 ), which were c o l l e c t e d i n June, 1975. M o r t a l i t y The mortality rate of newly released isopods during the f i r s t two months of t h e i r l i v e s was estimated. The mean value of the estimates over the period of March 1975-May 1976 was 95% mortality during the f i r s t two months for newly released I_. montereyensis, and 98% mortality during the f i r s t two months for newly released I_. wosnesenskii. However, as some of these young isopods may have emigrated to other areas, these estimates of mortality might be more properly termed disappearance rates. Dry Weight-Length Relationship The d i s t r i b u t i o n of dry weights with length f o r I_. montereyensis i s pre-sented i n Figure 8. The body of I_. montereyensis i s long and narrow, a form well adapted to existence on the f l a t , narrow blades of Phyllospadix. Repro-ductive s i z e d females are, however, t y p i c a l l y wider and heavier than the males. Figure 8 shows that the l a r g e r sized females (15-16 mm) are s i g n i f i c a n t l y more massive than males of equal length. This sexual dimorphism might serve to increase the volume of the brood pouch. The d i s t r i b u t i o n of dry weights with length for I_. wosnesenskii i s pre-sented i n Figure 9. The body of Idotea wosnesenskii i s wider and heavier than that of I_. montereyensis, and no d i f f e r e n c e i n dry weight between males and females of I_. wosnesenskii of the same length i s apparent. 38 F i g u r e 7. The r e l a t i o n s h i p o f t h e mean number o f g ( l ) embryos p e r brood t o female body l e n g t h f o r I d o t e a m o n t e r e y e n s i s and I d o t e a w o s n e s e n s k i i . The b a r s show 95% c o n f i d e n c e l i m i t s , and t h e number above o r below the b a r s i n d i c a t e s sample s i z e . 40 F i g u r e 8. The r e l a t i o n s h i p o f dry weight o f I d o t e a m o n t e r e y e n s i s t o body l e n g t h . 95% c o n f i d e n c e l i m i t s are shown f o r i s o p o d s l a r g e r than 10 mm. Female d r y weights a r e d i s p l a c e d s l i g h t l y t o the r i g h t o f males o f t h e same l e n g t h , f o r c l a r i t y . Sample s i z e s range from 4-9 i s o p o d s . 100 80-E \-i o UJ >-Q 60-^  40 H 20 H 10 15 20 25 30 B O D Y L E N G T H (mm) 42 F i g u r e 9. The r e l a t i o n s h i p o f dry weight o f I d o t e a w o s n e s e n s k i i t o body l e n g t h . 95% c o n f i d e n c e l i m i t s a r e shown f o r i s o p o d s l a r g e r t h a n 10 mm. Female d r y wei g h t s a r e d i s p l a c e d s l i g h t l y t o the r i g h t o f male wei g h t s o f t h e same l e n g t h f o r c l a r i t y . Sample s i z e s range from 4-9 i s o p o d s . 44 Food and F e e d i n g The gut c o n t e n t s o f I_. m o n t e r e y e n s i s c o l l e c t e d from t h e f i e l d appeared t o c o n s i s t m a i n l y o f p i e c e s o f P h y l l o s p a d i x , as dete r m i n e d by comparison w i t h the gut c o n t e n t s o f i s o p o d s f e d o n l y P h y l l o s p a d i x i n t h e l a b o r a t o r y . I n a d d i t i o n , s m a l l amounts o f t h e e p i p h y t i c a l g a , S m i t h o r a , and s m a l l numbers o f s e s s i l e diatoms were found. Thus, I_. m o n t e r e y e n s i s appears t o be p r i m a r i l y an h e r b i v o r e , f e e d i n g m a i n l y on t h e s u p e r f i c i a l l a y e r s o f P h y l l o s p a d i x . The stomach c o n t e n t s o f I_. w o s n e s e n s k i i c o l l e c t e d from Fucus f r o n d s con-s i s t e d o f b i t s o f Fucus and i t s a s s o c i a t e d e p i p h y t e s . However, those animals c o l l e c t e d from under r o c k s c o n t a i n e d a brown ooze from which l i t t l e e x c e p t p l a n t f i b e r s and diatoms c o u l d be i d e n t i f i e d . These a n i m a l s may f e e d on the d e t r i t u s and di a t o m scum t r a p p e d beneath t h e r o c k s . The f e e d i n g r a t e o f I d o t e a m o n t e r e y e n s i s f e d o n l y P h y 1 l o s p a d i x , and I d o t e a w o s n e s e n s k i i f e d o n l y Fucus, were measured. The mean o f 12 measure-ments o f wet weight o f P h y l l o s p a d i x consumed (±95% c o n f i d e n c e l i m i t s ) by female I_. m o n t e r e y e n s i s 15 mm i n l e n g t h was 43.8 ±11.1 mg p e r day, and the mean o f 12 measurements o f Fucus consumed (±95%. c o n f i d e n c e l i m i t s ) by female I_. w o s n e s e n s k i i 22 mm i n l e n g t h was 72.5 ±16.1 mg p e r day. C a l o r i m e t r y The c a l o r i c v a l u e s o f the new embryos, female body, major p l a n t foods ( P h y l l o s p a d i x f o r I_. m o n t e r e y e n s i s , and Fucus f o r I_. w o s n e s e n s k i i ) , and t h e f e c e s f o r each s p e c i e s a r e ' p r e s e n t e d i n T a b l e I I . A b s o r p t i o n E f f i c i e n c y The a b s o r p t i o n e f f i c i e n c y o f f o o d energy was dete r m i n e d f o r I d o t e a monter-e y e n s i s f e e d i n g on P h y l l o s p a d i x , and I d o t e a w o s n e s e n s k i i f e e d i n g on Fucus. 45 T a b l e I I . C a l o r i f i c v a l u e s o f i s o p o d t i s s u e s , f o o d and f e c e s (±95% c o n f i d e n c e l i m i t s ) used t o e s t i m a t e r e p r o d u c t i v e e f f o r t . Twelve samples o f each t i s s u e were combusted. C a l o r i f i c V a l u e s Sample D e s c r i p t i o n ( c a l / d r y mg) I d o t e a m o n t e r e y e n s i s female body t i s s u e s 2.92 ± 0.06 embryos 6.50 ± 0.09 f o o d , ( P h y l l o s p a d i x , s c o u l e r i ) 2.84 ± 0.11 f e c e s 3.06 ± 0.19 I d o t e a w o s n e s e n s k i i female body t i s s u e s 3.08 ± 0.15 embryos 6.31 ± 0.12 f o o d (Fucus d i s t i c h u s ) 3. 38 ± 0.14 f e c e s 2.67 ± 0.09 46 The mean a b s o r p t i o n e f f i c i e n c y f o r 20 female I_. m o n t e r e y e n s i s 15 mm i n l e n g t h (±95% c o n f i d e n c e l i m i t s ) was 71.3 ±9.5%, and t h e mean f o r 24 female I_. wosnes- e n s k i i 22 mm i n l e n g t h (±95% c o n f i d e n c e l i m i t s ) was 78.8 ±4.8%. 47 D i s c u s s i o n P i a n k a (1970) m a i n t a i n s t h a t a l l s p e c i e s can be p o s i t i o n e d on an r-K continuum, and he p r e s e n t s a l i s t o f c o r r e l a t e s which r e p r e s e n t the c h a r a c t e r -i s t i c s o f s p e c i e s a t t h e e n d p o i n t s o f the continuum (Table I I I ) . The f i r s t two c o r r e l a t e s o u t l i n e the assumptions upon which the p r e s e n t s t u d y i s based. The " c l i m a t e " o r environment o f I_. w o s n e s e n s k i i (upper i n t e r t i d a l , s u b j e c t t o changes i n temperature and h u m i d i t y when the t i d e i s out) i s thought t o be more v a r i a b l e and u n p r e d i c t a b l e t h a n t h a t o f I_. m o n t e r e y e n s i s (lower i n t e r -t i d a l , l e s s o f t e n exposed t o a i r ) , and t h i s i s thought t o l e a d t o DI m o r t a l i t y i n t h e former, and DD m o r t a l i t y i n the l a t t e r . P i a n k a a l s o c o r r e l a t e s v a r i - . a b l e p o p u l a t i o n s i z e , , u s u a l l y below the c a r r y i n g c a p a c i t y o f t h e environment and l a x i n t r a s p e c i f i c c o m p e t i t i o n , w i t h r - s e l e c t e d s p e c i e s . However, d e t e r -m i n a t i o n o f th e c a r r y i n g c a p a c i t y o f an environment, o r the degree o f i n t r a -s p e c i f i c c o m p e t i t i o n would be s t u d i e s i n t h e mselves, and no i n f o r m a t i o n i s a v a i l a b l e f o r the i s o p o d s . However, i f i t i s assumed t h a t th e environment o f _I. w o s n e s e n s k i i i s more v a r i a b l e and u n p r e d i c t a b l e t h a n t h a t o f I_. montereyen-s i s , under t h e s e c o n d i t i o n s P i a n k a p r e d i c t s t h a t s e l e c t i o n s h o u l d f a v o u r r a p i d development, s e m e l p a r i t y , e a r l y r e p r o d u c t i o n , s m a l l body s i z e , and i n c r e a s e d b i r t h r a t e f o r t h e r - s t r a t e g i s t (I_. w o s n e s e n s k i i ) and slow development, i n t e r o p a r i t y , d e l a y e d r e p r o d u c t i o n , l a r g e body s i z e , and d e c r e a s e d d e a t h r a t e f o r t h e K- s t r a t e g i s t m o n t e r e y e n s i s ) . The p r e d i c t i o n s o f t h e s e c o r r e l a t e s w i l l be i n v e s t i g a t e d i n the f o l l o w i n g s e c t i o n s . P i a n k a a l s o c o r r e l a t e s l e n g t h o f l i f e w i t h l i f e h i s t o r y t a c t i c , c l a i m i n g t h a t r - s t r a t e g i s t s g e n e r a l l y have s h o r t e r l i v e s ( l e s s t h a n one year) than K-s t r a t e g i s t s ( g r e a t e r than one y e a r ) , and p r o v i d e s e v i d e n c e showing a s t r o n g i n v e r s e c o r r e l a t i o n between the i n t r i n s i c r a t e o f n a t u r a l i n c r e a s e and t h e T a b l e I I I . Some o f the c o r r e l a t e s o f r - and K - s e l e c t i o n ( m o d i f i e d a f t e r P i a n k a , 1970, and S t e a r n s , 1975). C l i m a t e V a r i a b l e and/or u n p r e d i c t a b l e C o n s t a n t and/or p r e d i c t a b l e M o r t a l i t y D e n s i t y independent D e n s i t y dependent U n c e r t a i n a d u l t s u r v i v a l U n c e r t a i n j u v e n i l e s u r v i v a l P o p u l a t i o n s i z e V a r i a b l e i n time; C o n s t a n t i n t i m e ; u s u a l l y below c a r r y i n g a t o r n e a r c a r r y i n g c a p a c i t y c a p a c i t y 0 0 C o m p e t i t i o n O f t e n l a x U s u a l l y keen S e l e c t i o n f a v o u r s 1. Rapid development 1. Slow development 2. S e m e l p a r i t y 2. I t e r o p a r i t y 3. E a r l y r e p r o d u c t i o n 3. D e l a y e d r e p r o d u c t i o n 4. S m a l l body s i z e 4. Large body s i z e 5. I n c r e a s e d b i r t h r a t e 5. Decreased d e a t h r a t e L e n g t h o f l i f e S h o r t , u s u a l l y l e s s t h a n Longer, u s u a l l y more one y e a r • t h a n one y e a r Leads t o P r o d u c t i v i t y E f f i c i e n c y 49 l e n g t h o f l i f e . T h i s i s n o t s u r p r i s i n g s i n c e C o l e (1954) has shown t h a t r i s most r e s p o n s i v e t o changes i n th e age o f f i r s t r e p r o d u c t i o n , a, and animals w i t h s h o r t l i v e s would be e x p e c t e d t o have low v a l u e s o f a. R a p i d Development The f i r s t c h a r a c t e r i s t i c t h a t P i a n k a s u g g e s t s s h o u l d be c o r r e l a t e d w i t h r - s e l e c t i o n i s " r a p i d development", a l t h o u g h he does n o t d e f i n e what he means by t h i s r a t h e r vague term. - I n t h i s s t u d y the time from the f o r m a t i o n o f the zygote t o the r e l e a s e o f the young from the marsupium, o r the b r o o d i n g p e r i o d , i s used as t h e developmental time. Hence i t might be advantageous f o r an r -s t r a t e g i s t t o have i t s young d e v e l o p as r a p i d l y as p o s s i b l e , s i n c e f i t n e s s i s measured as the a b i l i t y t o dominate the environment n u m e r i c a l l y , and the soon-e r t h e o r g a n i s m i s o u t i n the environment, the sooner i t can b e g i n t o grow toward r e p r o d u c t i v e s i z e . R a p i d development may a l s o be c o r r e l a t e d w i t h o t h e r r - c h a r a c t e r i s t i c s such as s m a l l body s i z e and e a r l y age o f f i r s t r e p r o d u c t i o n . Based on the measure o f b r o o d i n g time, I d o t e a m o n t e r e y e n s i s , which has a mean b r o o d i n g time o f 26 days, has a s h o r t e r d e v e l o p m e n t a l time than I d o t e a  w o s n e s e n s k i i , which has a mean b r o o d i n g time o f 58 days. However, i t must be k e p t i n mind t h a t a l l b r o o d i n g time measurements were made i n the l a b o r a t o r y under c o n s t a n t temperature c o n d i t i o n s (11 ± 1 ° C ) , and t h e b r o o d p e r i o d o f t h e two s p e c i e s may r e s p o n d d i f f e r e n t l y t o changes i n temperature, r e s u l t i n g i n s e a s o n a l changes i n b r o o d i n g time i n the f i e l d . However s i n c e no such tempera-t u r e e f f e c t has been measured f o r t h e s e s p e c i e s , i t i s assumed t h a t any tem-p e r a t u r e e f f e c t w i l l a c t i n the same d i r e c t i o n f o r b o t h s p e c i e s , and n o t a f f e c t q u a l i t a t i v e comparison o f b r o o d i n g time. I_. m o n t e r e y e n s i s i s t h e r e f o r e assumed t o have the more r a p i d d e v e l o p m e n t a l time o f young t h r o u g h o u t the y e a r , and i s thus r - s e l e c t e d r e l a t i v e t o I_. w o s n e s e n s k i i on the b a s i s o f d e v e l o p -mental time. 50 Number o f Broods P i a n k a has s u g g e s t e d t h a t s e l e c t i o n f a v o u r s s e m e l p a r i t y f o r r - s e l e c t e d s p e c i e s , and i t e r o p a r i t y f o r K- s e l e c t e d s p e c i e s . Both I d o t e a m o n t e r e y e n s i s and I d o t e a w o s n e s e n s k i i are i t e r o p a r o u s . Females o f b o t h s p e c i e s have been k e p t i n th e l a b o r a t o r y t h r o u g h two c o n s e c u t i v e b r o o d i n g p e r i o d s , and a d d i t i o n -a l broods may be p o s s i b l e . Even though t h e r e i s some s e a s o n a l i t y t o t h e r e -p r o d u c t i v e c y c l e o f I_. m o n t e r e y e n s i s , the b r o o d i n g p e r i o d i s l e s s than one month, and o n l y r e p e a t e d r e p r o d u c t i o n would a l l o w g r e a t e r than 50% o f the r e -p r o d u c t i v e s i z e d females t o be c a r r y i n g broods d u r i n g t h e f o u r months o f the b r e e d i n g peak (May-August) o f 1975 ( F i g u r e 5 ) . Thus, s i n c e b o t h s p e c i e s a r e i t e r o p a r o u s , t h e y a r e b o t h K- s e l e c t e d i n t h i s r e s p e c t , based on Pianka's c o r r e l a t e s . Age a t F i r s t R e p r o d u c t i o n P i a n k a (1970) i l l u s t r a t e s t h e s t r o n g c o r r e l a t i o n between the age a t f i r s t r e p r o d u c t i o n and the i n t r i n s i c r a t e o f i n c r e a s e o f a s p e c i e s , s u g g e s t i n g t h a t e a r l y age o f f i r s t r e p r o d u c t i o n s h o u l d be a c h a r a c t e r i s t i c o f r - s e l e c t e d s p e c i e s . I n o r d e r t o e s t i m a t e age a t f i r s t r e p r o d u c t i o n f o r the two s p e c i e s o f i s o p o d s b e i n g s t u d i e d , the e s t i m a t e d s i z e a t f i r s t r e p r o d u c t i o n was con-v e r t e d t o an age e q u i v a l e n t , u s i n g t h e growth d a t a from F i g u r e s 5 and 6. The minimum s i z e o f g r a v i d females found i n the f i e l d t h r o u g h o u t the y e a r (12 mm f o r I_. m o n t e r e y e n s i s , 16 mm f o r I_. w o s n e s e n s k i i ) was used as s i z e o f f i r s t r e p r o d u c t i o n s i n c e females r e p r o d u c e more t h a n once and sometimes do n o t grow between subsequent b r o o d s , making i t i m p o s s i b l e t o a s s e s s the age o r b r o o d number o f the l a r g e r r e p r o d u c t i v e females. Based on the s i m u l a t e d growth r a t e s , I_. m o n t e r e y e n s i s would r e a c h 12 mm i n 205 days, and I_. w o s n e s e n s k i i would r e a c h 16 mm i n 175 days, a d i f f e r e n c e o f t h i r t y days, s u g g e s t i n g t h a t I_. w o s n e s e n s k i i i s r - s e l e c t e d r e l a t i v e t o I_. m o n t e r e y e n s i s on the b a s i s o f 51 age o f f i r s t r e p r o d u c t i o n . Body S i z e P i a n k a a s s e r t s t h a t i n s e c t s a r e r - s e l e c t e d as a group compared t o t e r r e s -t r i a l v e r t e b r a t e s , and thus c o r r e l a t e s the g e n e r a l l y s m a l l body s i z e o f the former group w i t h h i g h i n t r i n s i c r a t e o f p o p u l a t i o n i n c r e a s e . I_. montereyen- s i s i s s m a l l e r than I_. w o s n e s e n s k i i b o t h i n o v e r a l l body l e n g t h , and i n terms o f t o t a l d r y weight. Female I_. mo n t e r e y e n s i s r e a c h a maximum l e n g t h o f 23 mm, w i t h g r a v i d females h a v i n g a mean l e n g t h o f 15 mm, w h i l e female I_. w o s n e s e n s k i i r e a c h a maximum l e n g t h o f 30 mm, w i t h g r a v i d females h a v i n g a mean l e n g t h o f 22 mm. Based on the dry w e i g h t - l e n g t h r e l a t i o n s h i p s f o r each s p e c i e s ( F i g u r e s 8 and 9) , the mean l e n g t h s o f g r a v i d I_. m o n t e r e y e n s i s and I_. w o s n e s e n s k i i c o r r e s p o n d t o d r y weig h t s o f 23 mg and 100 mg, r e s p e c t i v e l y . Thus, I_. monter- e y e n s i s i s r - s e l e c t e d r e l a t i v e t o 1^ . w o s n e s e n s k i i on t h e b a s i s o f body s i z e . Brood S i z e The t h e o r y o f r-K s e l e c t i o n p r e d i c t s t h a t r - s e l e c t e d s p e c i e s s h o u l d have l a r g e r b r o o d s i z e s t h a n s p e c i e s which a r e K- s e l e c t e d . The r e l a t i o n s h i p be-tween b r o o d s i z e and female body l e n g t h f o r I d o t e a m o n t e r e y e n s i s and I d o t e a  w o s n e s e n s k i i a r e shown i n F i g u r e 7. From t h e s e c u r v e s i t i s e v i d e n t t h a t a l t h o u g h t h e 95% c o n f i d e n c e l i m i t s f o r 18 mm and 19 mm i s o p o d s o v e r l a p , I_. w o s n e s e n s k i i c a r r i e s more young p e r b r o o d on the average than does 1^ . monter- e y e n s i s . T r e a t i n g t h e b r o o d s i z e samples as s i n g l e d i s t r i b u t i o n s , n o r m a l l y d i s t r i b u t e d , f o r each s p e c i e s , a S t u d e n t ' s t - t e s t showed them t o be s i g n i f i -c a n t l y d i f f e r e n t a t the 5% l e v e l o f s i g n i f i c a n c e . Thus I_. w o s n e s e n s k i i i s r - s e l e c t e d r e l a t i v e t o I_. m o n t e r e y e n s i s on t h e b a s i s o f b r o o d s i z e . P o s i t i o n on t h e r-K Continuum Comparison o f the l i f e h i s t o r y c h a r a c t e r i s t i c s o f I d o t e a m o n t e r e y e n s i s 52 and I d o t e a w o s n e s e n s k i i t o the p r e d i c t i o n s o f P i a n k a ' s r-K c o r r e l a t e s shows t h a t two o f the c o r r e l a t e s ( e a r l y r e p r o d u c t i o n , i n c r e a s e d b i r t h r a t e ) f a v o u r I_. w o s n e s e n s k i i as t h e r e l a t i v e r - s t r a t e g i s t , w h i l e two o f t h e o t h e r c o r r e l -a t e s ( r a p i d development, s m a l l body s i z e ) f a v o u r I_. m o n t e r e y e n s i s as th e r e l a t i v e r - s t r a t e g i s t , and a f i f t h ( s e m e l p a r i t y ) , f a v o u r s n e i t h e r s p e c i e s . Thus the l i m i t a t i o n o f t h e p r e d i c t i v e c a p a b i l i t y o f P i a n k a ' s r-K c o r r e l a t e s i s a p p a r e n t . One main problem i s t h a t t h e c o r r e l a t e s a r e not ranked w i t h r e s p e c t t o t h e i r importance toward d e f i n i n g an r - o r K- t a c t i c ; s u r e l y age o f f i r s t r e p r o d u c t i o n i s more i m p o r t a n t t o an r - s t r a t e g i s t than s m a l l body s i z e , f o r example. Fu r t h e r m o r e , the use o f a s e t o f c o r r e l a t e s f o r r a n k i n g a s p e c i e s on an r-K continuum t r e a t s r e p r o d u c t i v e parameters such as b r o o d s i z e and age o f f i r s t r e p r o d u c t i o n i n i s o l a t i o n , y e t they a r e coadapted t r a i t s . The f a c t t h a t t h e s e two s p e c i e s o f i s o p o d s d i d not f o l l o w t h e p r e d i c t i o n t h a t t h e r e l a t i v e r - s t r a t e g i s t s h o u l d be semelparous, and the r e l a t i v e K-s t r a t e g i s t i t e r o p a r o u s , i s not d i s c o n c e r t i n g . P i a n k a i n t e n d s the c o r r e l a t e s t o r e p r e s e n t the e n d p o i n t s o f t h e continuum. Thus c e r t a i n p a r t s o f t h e con-tinuum w i l l be o c c u p i e d by o n l y i t e r o p a r o u s s p e c i e s , which can s t i l l be ranked on an r-K continuum on the b a s i s o f o t h e r c h a r a c t e r i s t i c s . F u rthermore, Charnov and S c h a f f e r (1973) p r e d i c t t h a t a semelparous s p e c i e s would have t o add P/C o f f s p r i n g t o i t s average bro o d s i z e t o a c h i e v e the same r a t e o f popu-l a t i o n i n c r e a s e as an i t e r o p a r o u s s p e c i e s (where P i s p a r e n t a l s u r v i v a l , and C i s j u v e n i l e s u r v i v a l ) . S i n c e C i s low f o r b o t h s p e c i e s o f I d o t e a (5% f o r I_. m o n t e r e y e n s i s and 2% f o r I_. w o s n e s e n s k i i a f t e r two months) , i t seems l i k e l y t h a t P/C i s l a r g e , and t h a t s e l e c t i o n s h o u l d f a v o u r i t e r o p a r i t y f o r b o t h s p e c i e s . I n o r d e r t o rank the r e m a i n i n g c o r r e l a t e s as t o t h e i r importance t o r-K t h e o r y , the i n f l u e n c e o f each parameter on the i n t r i n s i c r a t e o f n a t u r a l i n -c r e a s e must be determined. C o l e (1954) has shown t h a t r i s most r e s p o n s i v e t o change i n age o f f i r s t r e p r o d u c t i o n and b r o o d s i z e . S m a l l body s i z e and r a p i d development have no d i r e c t i n f l u e n c e on r ; P i a n k a (1970) i s o n l y t r y i n g t o suggest t h a t body s i z e and developmental time a r e i n v e r s e l y c o r r e l a t e d t o r . He a s s e r t s t h a t i n s e c t s , w i t h t h e i r g e n e r a l l y s m a l l body s i z e and r a p i d d e v e l -opment, a r e r - s e l e c t e d as a group r e l a t i v e t o t e r r e s t r i a l v e r t e b r a t e s , w i t h t h e i r l a r g e r body s i z e and s l o w e r development. T h i s a s s e r t i o n may g e n e r a l l y h o l d , b u t i t s p r e d i c t i v e power f o r comparison o f the l i f e h i s t o r y t a c t i c s o f c o n g e n e r i c s p e c i e s i s l i m i t e d . F a c t o r s o t h e r than the a b i l i t y t o maximize r may have an e f f e c t on s e l e c -t i o n o f such t r a i t s as body s i z e and d e v e l o p m e n t a l t i m e . F o r example, i s o p o d s the s i z e and shape o f I_. w o s n e s e n s k i i might not be as w e l l adapted t o c l i n g i n g t o t h e wave-swept b l a d e s o f P h y l l o s p a d i x as i s m o n t e r e y e n s i s because o f t h e i n c r e a s e d r e s i s t a n c e o f t h e i r b r o a d e r and t h i c k e r b o d i e s t o water f l o w . L i k e w i s e , the s m a l l , narrow shape o f I_. m o n t e r e y e n s i s might be l e s s r e s i s t a n t t o t h e temperature o r h u m i d i t y changes o f the upper i n t e r t i d a l environment, s i n c e t h e r a t i o o f s u r f a c e a r e a t o volume would be h i g h e r f o r an a n i m a l t h e s i z e and shape o f I . monterey ens i s t h a n f o r I_. w o s n e s e n s k i i . T h i s type o f s e l e c t i v e p r e s s u r e might account f o r t h e l a r g e r body s i z e o f I_. wosnes- e n s k i i . S i m i l a r l y , i f the h i g h m o r t a l i t y o f young i s o p o d s i n t h e i r f i r s t two months was a t l e a s t i n p a r t due t o s t r e s s imposed by temperature and h u m i d i t y change, s e l e c t i o n might f a v o u r l a r g e r young which have a l o n g e r d e v e l o p m e n t a l time. T h i s presumes t h a t l a r g e r young are more r e s i s t a n t t o p h y s i c a l s t r e s s e s imposed by the environment. I n d i r e c t e v i d e n c e f o r t h e importance o f s t r e s s imposed by temperature and h u m i d i t y on t h e s u r v i v a l o f i s o p o d s i s p r o v i d e d by t h e p r e f e r e n c e o f newly r e l e a s e d I_. w o s n e s e n s k i i f o r the presumably l e s s 54 v a r i a b l e u n d e r - r o c k h a b i t a t ( T a b l e I ) . Thus, t h e s e t y p e s o f s e l e c t i v e p r e s -s u r e s , which a f f e c t the s u r v i v a l o f young o r a d u l t s , would seem t o be more i m p o r t a n t than s e l e c t i o n f o r t h e m a x i m i z a t i o n o f r , c o n s i d e r i n g t h e l a c k o f d i r e c t impact o f body s i z e and d e v e l o p m e n t a l time on r . I t seems most r e a s o n a b l e , t h e n , t o rank t h e s e two s p e c i e s on an r-K con-tinuum on the b a s i s o f age o f f i r s t r e p r o d u c t i o n and b r o o d s i z e . Thus, I d o t e a  w o s n e s e n s k i i , w i t h i t s somewhat e a r l i e r age o f f i r s t r e p r o d u c t i o n , and i t s s i g n i f i c a n t l y l a r g e r b r o o d s i z e , i s c o n s i d e r e d an r - s t r a t e g i s t r e l a t i v e t o I_. m o n t e r e y e n s i s . The i n d i c a t i o n t h a t the s p e c i e s from the more u n p r e d i c t a b l e environment was the r - s t r a g e t i s t , s u p p o r t s the t h e o r y o f r-K s e l e c t i o n . I t w i l l be r e c a l l e d , however, t h a t S c h a f f e r (1974) p r e d i c t e d t h a t when a d u l t m o r t a l i t y was h i g h , and j u v e n i l e m o r t a l i t y was low, s e l e c t i o n would f a v o u r r - s t r a t e g i s t s 0 i n u n p r e d i c t a b l e environments, whereas when j u v e n i l e m o r t a l i t y was h i g h , and a d u l t m o r t a l i t y low, s e l e c t i o n would f a v o u r K- s t r a t e g i s t s i n u n p r e d i c t a b l e environments. My r e s u l t s do n o t s u p p o r t the p r e d i c t i o n s o f S c h a f f e r ' s model s i n c e j u v e n i l e m o r t a l i t y i s h i g h f o r b o t h I_. m o n t e r e y e n s i s and I_. w o s n e s e n s k i i (>90%), and s i n c e I_. w o s n e s e n s k i i , which l i v e s i n t h e more v a r i a b l e upper i n t e r t i d a l environment, was found t o be an r - s t r a t e g i s t r e l a t i v e t o I_. monter- e y e n s i s , which l i v e s i n a l e s s v a r i a b l e environment. One o b v i o u s d i f f i c u l t y w i t h S c h a f f e r ' s model i s t h a t a d u l t and j u v e n i l e m o r t a l i t y a r e c o n s i d e r e d as independent phenomena. I n h i s model, i n the f i r s t case he assumes t h a t the p r e - r e p r o d u c t i v e i n d i v i d u a l s a r e s u b j e c t t o a l l the m o r t a l i t y , w h i l e the p o s t - r e p r o d u c t i v e i n d i v i d u a l s a r e s u b j e c t t o none, and i n t h e second case he assumes t h a t the r e v e r s e h o l d s t r u e . I t would seem more r e a l i s t i c t o c o n s i d e r t h e e f f e c t s o f d i f f e r i n g degrees o f m o r t a l i t y t h r o u g h o u t 55 t h e l i f e h i s t o r y . Any model t h a t assumes no m o r t a l i t y i n any phase o f the l i f e c y c l e i s l i k e l y t o make p r e d i c t i o n s t h a t w i l l be d i f f i c u l t t o s u p p o r t w i t h f i e l d d a t a , s i n c e the assumption i s so u n r e a l i s t i c . Even a s m a l l amount o f a d u l t m o r t a l i t y where none, i s assumed i n S c h a f f e r ' s model, c o u l d be impor-t a n t i n d e t e r m i n i n g the d i r e c t i o n o f n a t u r a l s e l e c t i o n on l i f e h i s t o r y t a c t i c s . P u b l i s h e d T e s t s o f r-K Theory One o f the o n l y t e s t s o f r-K t h e o r y d e a l i n g w i t h l i f e h i s t o r y parameters o t h e r t h a n r e p r o d u c t i v e e f f o r t i s t h a t o f S t e a r n s (1975) who i n v e s t i g a t e d the e f f e c t s o f s t a b l e and f l u c t u a t i n g environments on the l i f e h i s t o r y t r a i t s o f Gambusia a f f i n i s i n H a w a i i . S t e a r n s found t h a t e i g h t o f n i n e i n t r a s p e c i f i c d i f f e r e n c e s i n r e p r o d u c t i v e t r a i t s between f i s h from f l u c t u a t i n g and s t a b l e r e s e r v o i r s (based on water l e v e l ) were i n the d i r e c t i o n p r e d i c t e d by r-K t h e o r y , b u t t h a t seven o f t h e s e d i f f e r e n c e s were n o t s t a t i s t i c a l l y s i g n i f i c a n t . F u r t h e r m o r e , by v a r y i n g f o o d and temperature i n t h e l a b o r a t o r y , d i f f e r e n c e s i n r e p r o d u c t i v e t r a i t s as g r e a t as any o b s e r v e d i n the f i e l d c o u l d be pro d u c e d from a s i n g l e l a b o r a t o r y s t o c k , a l t h o u g h f i s h from d i f f e r e n t r e s e r v o i r s r a i s e d under s i m i l a r c o n d i t i o n s d i f f e r e d s i g n i f i c a n t l y i n l i f e h i s t o r y t r a i t s . T h i s s u g g e s t s t h a t a l t h o u g h much o f the o b s e r v e d v a r i a b i l i t y i n r e p r o d u c t i v e p a r a -meters r e s u l t s from d e v e l o p m e n t a l p l a s t i c i t y , a d e t e c t a b l e p o r t i o n o f t h e v a r i a b i l i t y among s t o c k s from d i f f e r e n t r e s e r v o i r s has a g e n e t i c base. S t e a r n s c o n c l u d e s t h a t Gambusia i n t h e f l u c t u a t i n g r e s e r v o i r s a r e under-g o i n g o s c i l l a t i n g s e l e c t i v e p r e s s u r e s . They cope w i t h t h e s e p r e s s u r e s by p r o d u c i n g young w i t h a l a r g e range o f v a r i a b i l i t y i n growth r a t e s and ages a t f i r s t r e p r o d u c t i o n . He p o i n t s o u t t h a t t h e models l e a d i n g t o the p r e d i c t i o n s c o n c e r n i n g r - and K- s t r a t e g i e s have f a i l e d t o c o n s i d e r d e v e l o p m e n t a l p l a s t i c -i t y i n l i f e h i s t o r y t r a i t s and the importance o f t h e degree o f v a r i a b i l i t y o f t r a i t s . 56 The problems involved with f i t t i n g Steam's r e s u l t s to the predictions of the various theories of l i f e h i s t o r y t a c t i c s emphasizes an important point: these theories assume that s e l e c t i o n w i l l drive a population towards a com-bina t i o n of t r a i t s that represents an optimal t a c t i c . However, t h i s optimal t a c t i c depends on the assumptions of the p a r t i c u l a r model, which are often u n r e a l i s t i c a l l y s i m p l i f i e d . Furthermore, we must bear i n mind that natural populations may not achieve "optimal" t a c t i c s because of the changing nature of how the organism perceives i t s environment as i t evolves, and because general models cannot take i n t o account the natural v a r i a b i l i t y of l i f e h i s -tory t r a i t s a v a i l a b l e to be selected i n a species. The problem of a temporally s h i f t i n g optimal t a c t i c might well apply to the present study of isopod reproductive t a c t i c s , since the importance of DI mortality as a s e l e c t i v e force may vary unpredictably from year to year. Since only a sing l e population of isopods of each species was studied i n d e t a i l for a period of 14 months, i t i s impossible to assess the degree to which l o c a l i z e d populations may s h i f t t h e i r p o s i t i o n s along an r-K continuum with temporal changes i n the environment. The large amount of v a r i a b i l i t y i n the measures of the reproductive parameters of these isopods might well be, as Stearns suggests, a t a c t i c for dealing with o s c i l l a t i n g s e l e c t i v e pressures. Long term study of several isopod populations would be required to add i n s i g h t to t h i s question. Another t e s t of r-K s e l e c t i o n i s that of Menge (1974), who considers both the i n t e r s p e c i f i c comparison of l i f e h i s t o r y c h a r a c t e r i s t i c s between the sea stars P i s a s t e r ochraceus and Leptasterias hexactis, and the i n t r a s p e c i f i c com-parison of populations of Leptasterias subject to density-independent mortality (wave shock) and density-dependent mortality (competition). In comparing the l i f e h i s t o r i e s o f P i s a s t e r and L e p t a s t e r i a s , Menge su g g e s t s t h a t t h e d i f f e r e n c e between t h e i r modes o r r e p r o d u c t i o n reduces the v a l u e o f f e c u n d i t y as a measure o f t h e i r r e l a t i v e p o s i t i o n s on an r-K continuum. P i s a s t e r produces numerous s m a l l eggs which must pa s s t h r o u g h a p l a n k t o n i c l a r v a l s t a g e b e f o r e metamorphosis, w h i l e L e p t a s t e r i a s p r o d u c e s fewer, l a r g e r eggs which a r e b r o o d -ed u n t i l they a r e r e l e a s e d as s m a l l s t a r f i s h . S i n c e the eggs v a r y b o t h i n s i z e and number p e r b r o o d , a more s u i t a b l e measure o f p o s i t i o n on an r-K con-tinuum might be r e p r o d u c t i v e e f f o r t . In o r d e r t o determine the energy a l l o c a t -ed t o r e p r o d u c t i o n by L e p t a s t e r i a s , the energy expended d u r i n g t h e b r o o d i n g p e r i o d , when t h e a d u l t cannot f e e d , must be c a l c u l a t e d . Menge does t h i s by m easuring the w e i g h t change o f the p y l o r i c caecae o v e r the b r o o d i n g p e r i o d , and c o n v e r t i n g t h i s t o an energy e q u i v a l e n t based on the c a l o r i f i c v a l u e o f t h e caecae. U s i n g t h i s v a l u e , and t h e c a l o r i c c o n t e n t s o f t h e gonads, Menge determines t h a t L e p t a s t e r i a s d e v o t e s a g r e a t e r p r o p o r t i o n o f i t s energy t o r e p r o d u c t i o n t h a n does P i s a s t e r . Based on t h i s , the s m a l l e r body s i z e and the a p p a r e n t e a r l i e r age o f f i r s t r e p r o d u c t i o n o f L e p t a s t e r i a s (based on s i z e ) ' , Menge c o n c l u d e s t h a t L e p t a s t e r i a s i s an r - s t r a t e g i s t r e l a t i v e t o P i s a s t e r . Menge a l s o makes an i n t r a s p e c i f i c comparison o f the r e p r o d u c t i v e e f f o r t o f d i f f e r e n t p o p u l a t i o n s o f L e p t a s t e r i a s b a s e d on the DI e f f e c t s o f wave shock and the DD e f f e c t s o f c o m p e t i t i o n w i t h P i s a s t e r . C o m p e t i t i o n w i t h P i s a s t e r a p p a r e n t l y reduces the mean body s i z e o f L e p t a s t e r i a s , w i t h o u t d e c r e a s i n g i t s abundance, and thus the p r e s e n c e o f P i s a s t e r a f f e c t s t h e f e c u n d i t y o f L e p t a s - t e r i a s o n l y i n an i n d i r e c t manner s i n c e t h e f e c u n d i t y o f L e p t a s t e r i a s i n c r e a s e s w i t h body s i z e . Menge c o n s i d e r s t h e e f f e c t s o f wave a c t i o n t o be a DI m o r t a l -i t y f a c t o r , and i f L e p t a s t e r i a s were s u b j e c t e d t o i n c r e a s e d amounts o f m o r t a l -i t y as a r e s u l t o f wave a c t i o n , r e p r o d u c t i v e e f f o r t s h o u l d be g r e a t e s t where 58 wave a c t i o n i s most i n t e n s e . T h i s i s n o t s u p p o r t e d by Menge's r e s u l t s , how-eve r . A n a l y s i s o f s i z e s p e c i f i c f e c u n d i t y shows t h a t females a t p r o t e c t e d a r e a s produce more eggs than do e q u a l s i z e d females a t exposed a r e a s . Menge's r e s u l t s s u p p o r t S c h a f f e r ' s model, s i n c e wave a c t i o n presumably i n c r e a s e s the chances o f m o r t a l i t y o f p r e - r e p r o d u c t i v e s more than f o r a d u l t s . S c h a f f e r ' s r e s u l t s p r e d i c t r e d u c e d r e p r o d u c t i v e e f f o r t under t h e s e c i r c u m s t a n c -e s , as Menge o b s e r v e d . However, Menge's stu d y can be c r i t i c i z e d on s e v e r a l grounds. F i r s t l y , he has n o t demonstrated t h a t c o m p e t i t i o n w i t h P i s a s t e r i s a DD m o r t a l i t y s o u r c e , nor has he s u c c e s s f u l l y demonstrated t h a t wave a c t i o n a f f e c t s t h e s u r v i v a l o f L e p t a s t e r i a s i n a DI manner. Thus the f a i l u r e o f Menge's d a t a t o s u p p o r t r-K t h e o r y may n o t r e f u t e i t ; i t s i m p l y may n o t t e s t i t . F u r thermore, h i s c o n c l u s i o n t h a t r e p r o d u c t i v e e f f o r t i s h i g h e r f o r L e p t a s t e r i a s than f o r P i s a s t e r depends upon the i n c l u s i o n o f the energy change i n the p y l o r i c caecae i n d e t e r m i n a t i o n o f r e p r o d u c t i v e e f f o r t , because L e p t a s t e r i a s devotes o n l y 6.7% o f i t s w e i g h t t o r e p r o d u c t i o n , compared w i t h 9.5% f o r P i s a s t e r . However, e v i d e n c e from Menge's (1972) s t u d y o f th e f o r a g -i n g s t r a t e g y o f L e p t a s t e r i a s s u g g e s t s t h a t most a n i m a l s reduce o r cease f e e d -i n g d u r i n g t h e w i n t e r months (when the females b r o o d ) . Thus i t may be i n c o r -r e c t t o c o u n t t h i s l a c k o f f o r a g i n g d u r i n g b r o o d i n g as a c o s t o f r e p r o d u c t i o n , and t h e r e f o r e I would rank P i s a s t e r as an r - s t r a t e g i s t r e l a t i v e t o L e p t a s t e r -i a s , on t h e b a s i s o f r e p r o d u c t i v e e f f o r t . I n summary, the r e s u l t s o f t h e t h r e e t e s t s o f r-K t h e o r y c o n s i d e r e d thus f a r have p r o v i d e d s u p p o r t f o r the t h e o r y , but n o t w i t h o u t q u a l i f i c a t i o n . S t e a m ' s work b a s i c a l l y s u p p o r t s the t h e o r y o f r-K s e l e c t i o n w i t h r e s p e c t t o i n t r a s p e c i f i c comparisons, a l t h o u g h he found the m a j o r i t y o f r e p r o d u c t i v e p a r a -meters not t o d i f f e r by s t a t i s t i c a l l y s i g n i f i c a n t amounts f o r p o p u l a t i o n s from 59 s t a b l e and u n s t a b l e e n vironments. The r e s u l t s o f Menge (1974) and o f t h e p r e s e n t s t u d y l e n d l i m i t e d s u p p o r t t o t h e c o n c e p t o f an r-K continuum f o r com-p a r i s o n o f l i f e h i s t o r y t a c t i c s , b u t Menge found one major c o r r e l a t e , b r o o d s i z e , w hich d i d n o t meet t h e p r e d i c t i o n s o f t h e t h e o r y , and t h i s s t u d y found two l e s s i m p o r t a n t c o r r e l a t e s , body s i z e and d e v e l o p m e n t a l t i m e , which d i d n o t meet t h e p r e d i c t i o n s o f P i a n k a . The d i f f i c u l t y may be t h a t l i f e h i s t o r y t a c t i c s a r e t o o complex f o r q u a l i t a t i v e a n a l y s i s b a s e d on t h e b r o a d c o r r e l a -t i o n o f r e p r o d u c t i v e p a r a m e t e r s t o i n t r i n s i c r a t e o f i n c r e a s e , as s u g g e s t e d by P i a n k a . S i n c e t h e c o r r e l a t e s a r e d e f i n e d i n terms o f t h e c h a r a c t e r i s t i c s ex-p e c t e d f o r s p e c i e s a t t h e e n d p o i n t s o f the r-K continuum, t h e r a n k i n g o f spec-i e s l o c a t e d i n t h e m i d d l e p o r t i o n o f t h e continuum by t h e s e c o r r e l a t e s can l e a d t o u n c l e a r r e s u l t s . Even r a n k i n g t h e c o r r e l a t e s w i t h r e s p e c t t o t h e i r i n f l u e n c e on t h e i n t r i n s i c r a t e o f i n c r e a s e has i t s drawbacks s i n c e each c o r -r e l a t e i s s t i l l compared on a q u a l i t a t i v e b a s i s , and s m a l l d i f f e r e n c e s i n one parameter have t h e same i n f l u e n c e as l a r g e d i f f e r e n c e s i n a n o t h e r parameter f o r p o s i t i o n i n g a s p e c i e s on t h e r-K continuum. The s o l u t i o n may be t o con-s i d e r a system o f a n a l y s i s o f l i f e h i s t o r y t a c t i c s w hich does n o t r e l y on t h e measurement o f demographic p a r a m e t e r s , such as the' measure o f r e p r o d u c t i v e ' e f f o r t employed by Menge. G a d g i l and S o l b r i g (1972) a s s e r t t h a t r e p r o d u c t i v e e f f o r t i s t h e most a p p r o p r i a t e measure o f a s p e c i e s ' p o s i t i o n a l o n g an r-K continuum. The e f f e c t s o f s e v e r a l r e p r o d u c t i v e p a r a m e t e r s on t h e l i f e h i s t o r y t a c t i c o f a s p e c i e s can be s i m u l t a n e o u s l y measured on a q u a n t i t a t i v e b a s i s by c o n s i d e r i n g the p a r t i -t i o n i n g o f f o o d energy r e s o u r c e s t o somat i c and r e p r o d u c t i v e growth. F o r i n s t a n c e , s p e c i e s w i t h an e a r l i e r age o f f i r s t r e p r o d u c t i o n w i l l have a s h o r t -e r p e r i o d o f s o m a t i c growth and w i l l r e q u i r e l e s s e n ergy f o r body maintenance b e f o r e r e p r o d u c i n g t h a n a s p e c i e s w i t h a l a t e r age o f f i r s t r e p r o d u c t i o n . 60 Thus the species with an e a r l y age of f i r s t reproduction w i l l devote a smaller proportion of i t s food energy resources to reproduction and have a lower re-productive e f f o r t . S i m i l a r l y , species with large brood s i z e s w i l l have to devote a l a r g e r proportion of i t s food energy resources to reproductive growth than species with smaller brood s i z e s and s i m i l a r somatic growth and body maintenance requirements, and thus w i l l have a higher reproductive e f f o r t . Although Gadgil and S o l b r i g define reproductive e f f o r t as "the product of the t o t a l resources obtained by the organism and the f r a c t i o n devoted to re-production" , they measure reproductive e f f o r t as the r a t i o of dry weight of reproductive t i s s u e to the t o t a l dry weight of somatic t i s s u e . S i m i l a r l y , T i n k l e (1969) estimates reproductive e f f o r t as the r a t i o of egg weight to t o t a l female weight (both l i v e weight) for l i z a r d s . This does not seem an appropriate measure of the a l l o c a t i o n of food energy resources, however, since the somatic and reproductive t i s s u e s may d i f f e r g r eatly i n c a l o r i c content, which suggests that d i f f e r e n t amounts of food energy resources may be required to b u i l d s i m i l a r weights of the d i f f e r e n t t i s s u e s . Abrahamson and Gadgil (1973) improve upon the measure of reproductive e f f o r t by considering not only the r a t i o of the dry weights of somatic and re-productive t i s s u e s , but also the r a t i o of the c a l o r i c contents of these t i s s u e s . This y i e l d s a more accurate p i c t u r e of the p a r t i t i o n i n g of energy resources, but i t s t i l l does not measure the f r a c t i o n of t o t a l resources a l -located to reproduction; i t only measures the f r a c t i o n of food energy resources which were t i e d up i n the body ti s s u e s as reproductive products. A more accurate measure of reproductive e f f o r t would be the r a t i o of the c a l o r i c con-tent of the reproductive t i s s u e s to the amount of food energy absorbed by the animal. Gadgil and S o l b r i g .(1972) . p r e d i c t that both the reproductive e f f o r t per female per brood and the reproductive e f f o r t per 61 female p e r l i f e t i m e s h o u l d be h i g h e r f o r r - s t r a t e g i s t s t h a n f o r K- s t r a t e g i s t s . Because o f t h e f a c t t h a t female i s o p o d s o f t e n do not grow between the r e l e a s e o f b r o o d s , i t i s i m p o s s i b l e t o r e l a t e s i z e t o age i n r e p r o d u c t i v e females, and thus t h e i r s u r v i v o r s h i p cannot be det e r m i n e d from f i e l d c o l l e c t i o n d a t a . T h e r e f o r e no e s t i m a t e o f t h e mean number o f broods p e r female p e r l i f e t i m e a r e a v a i l a b l e f o r f i e l d p o p u l a t i o n s , and I can o n l y compare r e p r o d u c t i v e e f f o r t p e r female p e r b r o o d as a measure o f p o s i t i o n on t h e r-K continuum. The r e p r o d u c t i v e e f f o r t s p e r brood o f I_. m o n t e r e y e n s i s and L w o s n e s e n s k i i were det e r m i n e d f o r females o f mean r e p r o d u c t i v e s i z e as t h e r a t i o o f the dry weight o f the r e p r o d u c t i v e p r o d u c t s t o the dry weight o f t h e female body (termed Method I ) , as t h e r a t i o o f t h e c a l o r i c c o n t e n t o f t h e r e p r o d u c t i v e p r o d u c t s t o the c a l o r i c c o n t e n t o f the female (termed Method I I ) , and as t h e r a t i o o f t h e c a l o r i c c o n t e n t o f the r e p r o d u c t i v e p r o d u c t s t o t h e t o t a l f o o d energy a b s o r b e d by t h e a n i m a l p r i o r t o t h e f o r m a t i o n o f t h e b r o o d (termed Method I I I ) . ( T a b l e I V ) . In Method I I I , t h e p r o d u c t o f t h e f r a c t i o n o f f o o d energy a b s o r b e d and the f e e d i n g r a t e i n t h e f i e l d ( c o n v e r t e d t o i t s c a l o r i c e q u i v a l e n t ) was used t o e s t i m a t e the food energy a b s o r b e d p e r day on c e r t a i n t e s t d i e t s . The mean time between r e p r o d u c t i v e m o l t s was d e t e r m i n e d i n t h e l a b o r a t o r y , a n d the energy t a k e n up by the female o v e r t h i s time i n t e r v a l was assumed t o be t h a t from which t h e subsequent b r o o d was formed (Table I V ) . A l l t h r e e methods o f c a l c u l a t i n g r e p r o d u c t i v e e f f o r t s u p p o r t the p r e d i c -t i o n s o f r-K t h e o r y ; I_. w o s n e s e n s k i i always has the h i g h e r r e p r o d u c t i v e e f f o r t . However, s i n c e t h e measure o f r e p r o d u c t i v e e f f o r t i s meant t o o r d e r s p e c i e s on an r-K continuum, i t i s i m p o r t a n t t o note t h a t the t h r e e methods show v a r y i n g degrees o f d i f f e r e n c e between the r e p r o d u c t i v e e f f o r t s o f t h e two s p e c i e s . Comparison o f Methods I and I I show o n l y s l i g h t d i f f e r e n c e s i n t h e measure o f r e p r o d u c t i v e e f f o r t . Method I y i e l d s a r e p r o d u c t i v e e f f o r t 1.8 times as g r e a t 62 T a b l e IV. V a l u e s used i n t h e c a l c u l a t i o n o f r e p r o d u c t i v e e f f o r t (±95% c o n f i d e n c e l i m i t s where a p p r o p r i a t e ) , and t h e t h r e e measures o f r e p r o d u c t i v e e f f o r t ( e x p r e s s e d as r a t i o s ) . V a l u e I d o t e a m o n t e r e y e n s i s I d o t e a w o s n e s e n s k i i Mean d r y weight g ( l ) embryo Mean b r o o d s i z e Dry weight o f b r o o d Dry weight o f female C a l o r i c c o n t e n t of. b r o o d C a l o r i c c o n t e n t o f female I n t e r - b r o o d p e r i o d Dry w e i g h t o f f o o d consumed p e r day i n the f i e l d Food consumed d u r i n g i n t e r - b r o o d p e r i o d % f o o d energy a b s o r b e d 0.077 ± 0.003 mg 47 3.6 mg 23.5 mg 23.6 c a l 68.6 c a l 41 days 9.1 ± 2.3 mg 378 mg 71 ± 9% 0.219 ± 0.008 mg 132 28.9 mg 100.4 mg 182.5 c a l 309.2 c a l 37 days 15.6 ± 3.4 581 mg 78 ± 4% R e p r o d u c t i v e e f f o r t (Method I) 0.15 0.29 R e p r o d u c t i v e e f f o r t (Method I I ) 0.34 0.59 R e p r o d u c t i v e e f f o r t (Method I I I ) - 0.09 0.40 63 f o r JL. w o s n e s e n s k i i t h a n 1^ m o n t e r e y e n s i s , w h i l e Method I I shows the r e p r o d u c -t i v e e f f o r t o f I_. w o s n e s e n s k i i t o be 1.7 times as l a r g e as f o r I_. montereyen- s i s . R e p r o d u c t i v e e f f o r t c a l c u l a t e d by Method I I I , however, shows I_. wosnes- e n s k i i t o have a r e p r o d u c t i v e e f f o r t 4.6 times as l a r g e as t h a t o f I_. monter- e y e n s i s . O b v i o u s l y , i f one were t r y i n g t o rank numerous s p e c i e s on an r-K continuum based on r e p r o d u c t i v e e f f o r t , t h e d i f f e r i n g r e s u l t s o b t a i n e d t h r o u g h t h e v a r i o u s methods c o u l d p u t s p e c i e s i n d i f f e r e n t o r d e r s , depending on which method was used f o r c a l c u l a t i o n . One r e a s o n f o r t h e g r e a t d i f f e r e n c e i n t h e measurement o f r e p r o d u c t i v e e f f o r t w i t h Method I I I i n comparison t o t h e f i r s t two methods i s the f a c t t h a t I_. m o n t e r e y e n s i s has a much h i g h e r f e e d i n g r a t e t h a n does I_. w o s n e s e n s k i i . The former consumes an average o f 0.39 grams dry weight o f food p e r gram dry w e i g h t o f i s o p o d p e r day, compared w i t h o n l y 0.15 grams o f f o o d p e r gram o f i s o p o d p e r day f o r the l a t t e r . S i n c e I_. m o n t e r e y e n s i s has a h i g h e r a b s o l u t e i n t a k e o f f o o d energy than I_. w o s n e s e n s k i i , i t may be e i t h e r u s i n g more o f t h i s f o o d energy f o r body maintenance ( r e s p i r a t i o n ) , o r i t may be l o s i n g l a r g e r amounts o f a b s o r b e d food energy t h r o u g h o t h e r c h a n n e l s (e.g. d i s s o l v e d o r g a n i c m a t t e r , u r i n e ) . The f a c t t h a t b o t h s p e c i e s remain i n a c t i v e ( u n l e s s d i s t u r b e d ) and do n o t f e e d when exposed t o the a i r s u g g e s t s one r e a s o n why I_. m o n t e r e y e n s i s , which spends more time submerged, has a h i g h e r f e e d i n g r a t e t h a n I_. w o s n e s e n s k i i . I f I . m o n t e r e y e n s i s were a c t i v e f o r a l a r g e r p a r t o f the day, the amount o f energy f o r maintenance ( r e s p i r a t i o n ) would a l s o be h i g h e r f o r a g i v e n weight o f I_. m o n t e r e y e n s i s t h a n f o r I_. w o s n e s e n s k i i . T h i s seems l i k e l y , a l t h o u g h no q u a n t i t a t i v e e v i d e n c e f o r a d i f f e r e n c e i n r e s p i r a t o r y r a t e s between exposed and submerged i s o p o d s i s a v a i l a b l e . 64 The p r e s e n t s t u d y p r o v i d e s s u p p o r t f o r t h e p r e d i c t i o n s o f r-K t h e o r y w i t h r e g a r d t o r e p r o d u c t i v e e f f o r t . Of t h e two p u b l i s h e d s t u d i e s which have i n v e s -t i g a t e d t h e r e sponse o f r e p r o d u c t i v e e f f o r t t o changes i n e n v i r o n m e n t a l s t a b -i l i t y , one s u p p o r t s the p r e d i c t i o n s o f the t h e o r y ( G a d g i l and S o l b r i g , 1972), w h i l e the o t h e r does n o t (Menge, 1974). The use o f r e p r o d u c t i v e e f f o r t p e r b r o o d as a measure o f a s p e c i e s ' p o s i t i o n on an r-K continuum a v o i d s t h e p roblem o f r a n k i n g a s e r i e s o f q u a l i -t a t i v e comparisons as e n c o u n t e r e d w i t h the use o f P i a n k a ' s c o r r e l a t e s , but i t i g n o r e s t h e e f f e c t s o f s e v e r a l i m p o r t a n t f a c t o r s which i n f l u e n c e a p o p u l a t i o n ' s i n t r i n s i c r a t e o f i n c r e a s e , such as age o f f i r s t r e p r o d u c t i o n , and the number o f broods p e r l i f e t i m e . Thus r e p r o d u c t i v e e f f o r t p e r b r o o d can o n l y s e r v e as a n o t h e r c o r r e l a t e o f r-K s e l e c t i o n ; i t cannot s e r v e as an independent measure o f l i f e h i s t o r y t a c t i c . I n o r d e r t o account f o r the d i f f e r e n t v a l u e o f o f f -s p r i n g produced a t d i f f e r e n t times i n the l i f e h i s t o r y t o t h e r a t e o f i n c r e a s e o f a p o p u l a t i o n , r e p r o d u c t i v e e f f o r t would have t o be c a l c u l a t e d by Method I I I t h r o u g h o u t t h e l i f e t i m e o f an organism. Thus s p e c i e s w i t h l a t e r ages o f f i r s t r e p r o d u c t i o n would be s h i f t e d toward the K- s e l e c t e d end o f t h e continuum because o f t h e i n c r e a s e d amount o f energy d e v o t e d t o body maintenance u n t i l the f i r s t r e p r o d u c t i o n . S i m i l a r l y , i t e r o p a r o u s s p e c i e s would have added ener-gy r e q u i r e m e n t s f o r body maintenance between p r o d u c t i o n o f broods which would s h i f t them towards the K- s e l e c t e d end o f t h e continuum. However, the d i f f i -c u l t i e s i n v o l v e d i n measuring the energy budget o f an o r g a n i s m t h r o u g h o u t i t s l i f e h i s t o r y a r e f o r m i d a b l e , and thus t h i s approach s h o u l d be employed o n l y when l e s s complex t e c h n i q u e s , s uch as th e use o f P i a n k a ' s c o r r e l a t e s , f a i l t o p r o v i d e s t r a i g h t f o r w a r d r e s u l t s . 65 Summary In the present study I have investigated the evolution of l i f e h i s t o r y t a c t i c s f o r two congeneric species of i n t e r t i d a l isopod with respect to the theory of r - and K- s e l e c t i o n , by t e s t i n g the predictions of Pianka's c o r r e l -ates of r- and K- s e l e c t i o n , and by measuring reproductive e f f o r t . The r e s u l t s of both t e s t s are thought to support the basic premise of r-K theory; that species subjected to more variable environments should have evolved l i f e h i s t o r y patterns which maximize r, the i n t r i n s i c rate of natural increase, when compared to species l i v i n g i n more stable environments. Thus Idotea  wosnesenskii, an isopod l i v i n g i n the mid- and u p p e r - i n t e r t i d a l zones,and subject to a more var i a b l e p h y s i c a l environment than Idotea montereyensis from the lower i n t e r t i d a l , was found to have an e a r l i e r age of f i r s t repro-duction, l a r g e r brood s i z e , and higher reproductive e f f o r t . Several major problems with the methods for determining the r e l a t i v e p o s i t i o n of a species on an r-K continuum were found i n t h i s study. The use of Pianka's set of r-K correlates was found to lack p r e d i c t i v e power, since each c o r r e l a t e makes only q u a l i t a t i v e p r e dictions concerning the r e l a t i v e p o s i t i o n of a species on the continuum. Reproductive parameters had to be i s o l a t e d from a set of coadapted t a c t i c s i n order to be compared, with no p r o v i s i o n f o r ranking the importance of i s o l a t e d correlates i n a system i n which evolutionary trade-offs seem to abound. The use of reproductive e f f o r t for determining the p o s i t i o n of a species on an r-K continuum seems to be the most reasonable approach, although i t must be measured throughout the e n t i r e l i f e of the organism i n order to account for the e f f e c t s of early age of f i r s t reproduction and repeated reproduction on the i n t r i n s i c rate of popula-t i o n increase. Furthermore, reproductive e f f o r t should be measured as the 66 ratio of the caloric value of the reproductive products to the energy absorb-ed throughout the lifetime, since reproductive effort may be highly influenced by the respiration component of the energy budget, the precise theoretical significance of which to l i f e history tactics i s unclear. 67 L i t e r a t u r e C i t e d Abrahamson, W. G., and M. D. Gadgil. 1973. Growth form and reproductive e f f o r t i n goldenrods (Solidago, Compositae). Amer. Nat. 107: 651-661. Carefoot, T. H. 1973. Feeding, food preference, and the uptake of food energy by the s u p r a l i t t o r a l isopod L i g i a p a l l a s i i . Mar. b i o l . 18: 228-236. Charnov, E. I., and W. M. Sch a f f e r . 1973. L i f e h i s t o r y consequences of na t u r a l s e l e c t i o n : Cole's r e s u l t r e v i s i t e d . Amer. Nat. 107:_. 791-793. Cole, I. C. 1954. The population consequences o f l i f e h i s t o r y phenomena. Q. Rev. B i o l . 29: 103-137. Dobzhansky, T. 1950. E v o l u t i o n i n t h e ' t r o p i c s . Amer. S c i . 38: 209-221. Gadgil, M. D.,and 0. T. S o l b r i g . 1972. The concept of r - and K- s e l e c t i o n : evidence from some w i l d flowers and some t h e o r e t i c a l considera-t i o n s . Amer. Nat..106: 14-31. Gentry and Wiegert Instruments, Inc. 1970. I n s t r u c t i o n s f o r the P h i l l i p s o n microbomb calorimeter. Aiken, South C a r o l i n a . Lee, W. L. 1966. C o l o r change and the ecology of the marine isopod Idotea (Pentidotea) montereyensis Maloney, 1933. Ecology 47: 930-941. MacArthur, R. H. 1962. Some generalized theorems o f n a t u r a l s e l e c t i o n . Proc. Nat. Acad. S c i . 48: 1893-1897. MacArthur, R. H. , and E. 0. Wilson. 1967. The Theory o f Is l a n d Biogeography. Princeton U n i v e r s i t y Press, Princeton. Menge, B. A. 1972. . Competition f o r food between two i n t e r t i d a l s t a r f i s h species and i t s e f f e c t on body s i z e and feeding. Ecology 53: 635-644. . " Menge, B. A. 1974. E f f e c t of wave a c t i o n on brooding and reproductive e f f o r t i n a seastar, L e p t a s t e r i a s hexactis. Ecology 55: 84-93. Menzies, R. J . 1950. The taxonomy, ecology, and d i s t r i b u t i o n of northern C a l i f o r n i a isopods of the genus Idothea with the d e s c r i p t i o n of a new species. Wassman Jour. B i o l . 8: 155-195. Murphy, G. I. 1968. Pa t t e r n i n l i f e h i s t o r y and the environment. Amer. Nat. 102: 391-403. 68 Pearl, R., and I. J. Reed. 1920. On the rate of growth of the population of the United States since 1790 and i t s mathematical representa-t i o n . Proc. Nat. Acad. S c i . U. S. 6: 275-288. Pianka, E. 1970. On r- and K- s e l e c t i o n . Amer. Nat. 104: 592-597. Schaffer, W. M. 1974. Optimal reproductive e f f o r t i n f l u c t u a t i n g environ-ments. Amer. Nat. 108: 783-790. Stearns, S. C. 1975. A comparison of the evolution and expression of l i f e h i s t o r y t r a i t s i n stable and f l u c t u a t i n g environments: Gambusia  a f f i n i s i n Hawaii. Ph.D. Thesis. University of B r i t i s h Columbia. Tinkle, D. W. 1969. The concept of reproductive e f f o r t and i t r e l a t i o n to the evolution of l i f e h i s t o r i e s of l i z a r d s . Amer. Nat. 103: 501-516. 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0093822/manifest

Comment

Related Items