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UBC Theses and Dissertations

The maintenance of shore-level size gradients in an intertidal snail (Littorina sitkana) McCormack, Susan Mary Denise 1981

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THE MAINTENANCE OF SHORE-LEVEL SIZE GRADIENTS IN AN INTERTIDAL SNAIL (LITTORINA SITKANA) by SUSAN MARY DENISE MCCORMACK .S c . ( h o n s ) , D a l h o u s i e U n i v e r s i t y , H a l i f a x , 1978 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA 24 A p r i l 1981 c) Susan Mary D e n i s e McCormack, 1981 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a gree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f ZOOLOGY  The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e V ancouver, Canada V6T 1W5 Date /fact' 29, /Wf r\p c. i o /na \ i i ABSTRACT The p o p u l a t i o n s i z e s t r u c t u r e of many i n t e r t i d a l a n i m a l s v a r i e s w i t h t i d a l h e i g h t so t h a t the mean a n i m a l s i z e i s i n c r e a s i n g or d e c r e a s i n g w i t h t i d a l h e i g h t . These s i z e g r a d i e n t s c o u l d be produced by growth or s u r v i v a l v a r y i n g w i t h t i d a l h e i g h t , or by a n i m a l s moving t o a p r e f e r r e d t i d a l l e v e l . The body s i z e of the s n a i l , L i t t o r i n a s i t k a n a , i n c r e a s e s s t e a d i l y w i t h t i d a l h e i g h t i n r o c k y h i g h i n t e r t i d a l h a b i t a t s of B r i t i s h C olumbia. To det e r m i n e how s i z e g r a d i e n t s were m a i n t a i n e d i n L . s i t k a n a , I q u a n t i f i e d how growth, s u r v i v a l , and s n a i l movement v a r i e d w i t h t i d a l h e i g h t . I s t u d i e d p o p u l a t i o n s of L . s i t k a n a found on s h e l t e r e d p e b b l e beach and exposed b a s a l t i c s h e l f h a b i t a t s . M a r k - r e c a p t u r e s t u d i e s and e x p e r i m e n t a l t r a n s p l a n t s showed t h a t growth c o u l d not have produced the s i z e g r a d i e n t s because s n a i l growth r a t e s i n both h a b i t a t s were as f a s t or f a s t e r a t low t i d a l l e v e l s (where the s n a i l s were the s m a l l e s t ) than a t h i g h t i d a l l e v e l s . However, s u r v i v a l r a t e s were l o w e s t a t low t i d a l l e v e l s . On pe b b l e beaches, t h i s was due t o s i z e s e l e c t i v e p r e d a t i o n on l a r g e s n a i l s by the p i l e p e r c h , R h a c o c h i l u s v a c c a . On b a s a l t i c s h e l v e s , heavy wave a c t i o n a t low t i d a l l e v e l s may have caused the poor s u r v i v a l r a t e s . T r a n s p l a n t e d s n a i l s moved homeward on p e b b l e beaches, but not on b a s a l t i c s h e l v e s . Reduced s u r v i v a l r a t e s a t low t i d a l l e v e l s cause s i z e g r a d i e n t s on b a s a l t i c s h e l v e s . S i z e s e l e c t i v e p r e d a t i o n by f i s h t o g e t h e r w i t h snail.movement m a i n t a i n s s i z e g r a d i e n t s on pe b b l e beaches. i i i TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES i v LIST OF FIGURES v ACKNOWLEDGEMENTS v i INTRODUCTION 1 STUDY AREAS 6 METHODS 12 S i z e D i s t r i b u t i o n 13 Growth "14 S u r v i v a l 18 S u r v i v a l on Pebble Beaches 18 S u r v i v a l on B a s a l t i c S h e l v e s 20 Movement 21 Movement on Pebble Beaches .; 21 Movement on B a s a l t i c S h e l v e s 22 RESULTS 23 S n a i l S i z e D i s t r i b u t i o n s 23 Growth 27 S u r v i v a l . 30 S u r v i v a l on Pebble Beaches 30 1. P r e d a t i o n E x p e r i m e n t s 30 i v F i s h D i e t and F e e d i n g Experiments 33 F i s h P r e d a t i o n Model 36 2. S u r v i v a l i n Growth Bags 40 S u r v i v a l on B a s a l t i c S h e l v e s 42 Movement 53 Pebble Beach E x p e r i m e n t s 53 Movement on B a s a l t i c S h e l v e s 57 L i t t o r i n a s c u t u l a t a 59 DISCUSSION 63 Pebble Beaches . . . • 64 B a s a l t i c S h e l v e s 69 G e n e r a l D i s c u s s i o n 71 REFERENCES 74 APPENDIX 79 Appendix I I I . S i m u l a t i o n Models 83 F i s h P r e d a t i o n Model 83 V a r i a b l e Range 85 B a s a l t i c S h e l f Model 87 V a r i a b l e Range 88 V LIST OF TABLES Tab l e I . Summary of hypotheses and t e s t s 13 Ta b l e I I . Egg mass d i s t r i b u t i o n and abundance 26 T a b l e I I I . S u r v i v a l of t r a n s p l a n t s on Pebble Beach A 31 Ta b l e IV. S u r v i v a l of P a i n t e d and U n p a i n t e d S n a i l s 33 Ta b l e V. Stomach c o n t e n t s of 2 p i l e p e r c h 34 Ta b l e V I . Responses of p i l e p e r c h t o o f f e r e d prey 36 Tab l e V I I . M o r t a l i t y of s n a i l s i n growth bags 41 Ta b l e V I I I . C h i - s q u a r e d v a l u e s f o r s u r v i v a l c u r v e s 48 Tab l e IX. F a t e s of l e a s h e d s n a i l s 52 T a b l e X. Summary of p e b b l e beach movement e x p e r i m e n t s .... 54 Tab l e X I . Summary of s n a i l movements on B a s a l t i c S h e l f A . 58 Ta b l e X I I . S u r v i v a l of both l i t t o r i n e s on Pebble Beach A . 62 Ta b l e X I I I . Summary of r e s u l t s 64 Ta b l e XIV. Summary of mean s n a i l s i z e s i n r e p l i c a t e h a b i t a t s 80 Ta b l e XV. O u t l i n e of f i s h p r e d a t i o n model 85 Ta b l e XVI. Summary of f i s h p r e d a t i o n model s i m u l a t i o n s ... 86 Ta b l e X V I I . O u t l i n e of b a s a l t i c s h e l f model 88 Ta b l e X V I I I . Summary of b a s a l t i c s h e l f model s i m u l a t i o n s . 90 v i LIST OF FIGURES F i g u r e 1. L o c a t i o n of st u d y s i t e s 7 F i g u r e 2. Diagram of s a m p l i n g s t a t i o n s on main study s i t e s 10 F i g u r e 3. Diagram of s h e l l l i p - l e n g t h r e l a t i o n s h i p 15 F i g u r e 4. S i z e d i s t r i b u t i o n s of L . s i t k a n a 24 F i g u r e 5. Growth r a t e s of L . s i t k a n a a t d i f f e r e n t t i d a l l e v e l s 28 F i g u r e 6. F i s h p r e d a t i o n model s i m u l a t i o n s 38 F i g u r e 7. S u r v i v a l r a t e s of s n a i l s on B a s a l t i c S h e l f A ... 43 F i g u r e 8. S u r v i v a l r a t e s of d i f f e r e n t s i z e d s n a i l s 45 F i g u r e 9. S i m u l a t i o n s from b a s a l t i c s h e l f model 49 F i g u r e 10. Sample diagrams of s n a i l movements 55 F i g u r e 11. S i z e d i s t r i b u t i o n s of L . s c u t u l a t a 60 F i g u r e 12. R e g r e s s i o n of l i p v e r s u s l e n g t h increment 81 v i i ACKNOWLEDGEMENTS T h i s study has been e n j o y a b l e thanks t o the k i n d n e s s of many p e o p l e a t UBC and B a m f i e l d . My s u p e r v i s o r , Dr. J.N.M. Sm i t h , p r o v i d e d m o r a l , i n t e l l e c t u a l , and f i n a n c i a l s u p p o r t . D r s . P.A. Breen, W.E. N e i l l , and J.N.M. Smith c r i t i c a l l y r e a d e a r l i e r d r a f t s of t h i s t h e s i s and t o g e t h e r w i t h Dr. J.D. M c P h a i l d i s c u s s e d the p r o j e c t w i t h me throughout i t s e v o l u t i o n . My study b e n e f i t t e d g r e a t l y from the i n t e l l e c t u a l and t e c h n i c a l i n p u t of J e f f Lynskey who a s s i s t e d me i n the f i e l d i n 1980. The s t a f f a t B a m f i e l d Marine S t a t i o n were always h e l p f u l , e s p e c i a l l y Bob Baden, as were many B a m f i e l d e r s who c h e e r f u l l y h e l p e d me i n the f i e l d on s e v e r a l o c c a s i o n s . Many thanks t o the l i t t o r i n a ; may f u t u r e summers be l e s s d i s r u p t i v e . R i c h a r d Palmer and S y l v i a Behrens Yamada sh a r e d t h e i r knowledge of s n a i l e c o l o g y w i t h me. The f a c u l t y , s t u d e n t s , and s t a f f of the I n s t i t u t e of Anim a l Resource E c o l o g y p r o v i d e d an e n t e r t a i n i n g and s t i m u l a t i n g atmosphere and many of them p r o v i d e d t i m e l y s m i l e s or encouragement. I would l i k e t o a l s o thank Mark Dale f o r s t a t i s t i c a l a d v i c e , D a v i d Marmorek and Tim Webb f o r commenting on s e c t i o n s of the t h e s i s , and Dave Z i t t e n f o r computer a s s i s t a n c e . I was s u p p o r t e d by a N a t i o n a l Research C o u n c i l of Canada S c h o l a r s h i p . 1 INTRODUCTION The i n t e r t i d a l zone i s a s t e e p e n v i r o n m e n t a l g r a d i e n t b r i d g i n g the gap between f u l l y marine and f u l l y t e r r e s t r i a l c o n d i t i o n s . T h e r e f o r e , s e v e r a l p h y s i c a l and b i o t i c c o n d i t i o n s may change w i t h i n an i n t e r t i d a l s p e c i e s ' v e r t i c a l d i s t r i b u t i o n . T h i s i n t e r t i d a l g r a d i e n t s h o u l d a f f e c t the a g e / s i z e s t r u c t u r e of p o p u l a t i o n s by c a u s i n g the dynamics of p o p u l a t i o n s t o v a r y w i t h i n t h e i r v e r t i c a l range. In f a c t , body s i z e v a r i e s w i t h t i d a l h e i g h t i n many i n t e r t i d a l a n i m a l s ( V e r m e i j 1972). V e r m e i j (1972) s y n t h e s i z e d the l i t e r a t u r e on s i z e g r a d i e n t s • i n i n t e r t i d a l g a s t r o p o d s . He noted t h a t i n a n i m a l s found i n the upper i n t e r t i d a l , body s i z e i n c r e a s e d w i t h t i d a l h e i g h t . In a n i m a l s found i n the lower i n t e r t i d a l , body s i z e d e c r e a s e d w i t h t i d a l h e i g h t . He proposed t h a t the s i z e g r a d i e n t s were a response t o g r a d i e n t s i n the i n t e n s i t y of j u v e n i l e m o r t a l i t y . J u v e n i l e m o r t a l i t y i s not the o n l y f a c t o r t h a t may v a r y w i t h t i d a l h e i g h t t o produce s i z e g r a d i e n t s . For s e s s i l e a n i m a l s , o n l y d i f f e r e n c e s i n growth or j u v e n i l e and/or a d u l t m o r t a l i t y can m a i n t a i n s i z e g r a d i e n t s . But, m o t i l e a n i m a l s can a l s o move t o t h e i r p r e f e r r e d t i d a l h e i g h t s . Growth c o u l d a f f e c t s i z e g r a d i e n t s i f a n i m a l s grow f a s t e s t where they a r e the l a r g e s t . Growth r a t e s a re known t o v a r y w i t h t i d a l h e i g h t f o r many reasons ( V e r m e i j 1978). For example, food q u a l i t y and q u a n t i t y , f e e d i n g t i m e , and m e t a b o l i c r a t e may v a r y w i t h t i d a l h e i g h t . M o r t a l i t y c o u l d c o n t r i b u t e t o s i z e g r a d i e n t s i f i t i s g r e a t e s t where i n d i v i d u a l s a r e the s m a l l e s t or i f t h e r e a r e 2 s i z e s e l e c t i v e m o r t a l i t y agents p r e s e n t where those s i z e c l a s s e s a r e absent. M o r t a l i t y c o u l d v a r y w i t h t i d a l h e i g h t because of d i f f e r e n c e s i n exposure (wave a c t i o n , d e s i c c a t i o n ) or p r e d a t i o n and c o m p e t i t i o n ( C o n n e l l 1972, Dayton 1971). Movement c o u l d produce s i z e g r a d i e n t s i f l a r g e a n i m a l s choose t o l i v e a t d i f f e r e n t t i d a l l e v e l s from s m a l l a n i m a l s . Food and s h e l t e r a r e two of many h a b i t a t f e a t u r e s t h a t may i n f l u e n c e where an a n i m a l d e c i d e s t o l i v e . S t u d y i n g how and why s i z e g r a d i e n t s a r e m a i n t a i n e d s h o u l d l e a d t o a g r e a t e r u n d e r s t a n d i n g of how the environment a f f e c t s the dynamics of i n t e r t i d a l p o p u l a t i o n s . What i s the e v i d e n c e f o r s i z e g r a d i e n t s b e i n g m a i n t a i n e d by growth, s u r v i v a l , or movement? A l t h o u g h many s t u d i e s have compared growth r a t e s of a n i m a l s from d i f f e r e n t t i d a l l e v e l s , few have r e l a t e d growth t o the maintenance of s i z e g r a d i e n t s . S u t h e r l a n d (1970), however, o b s e r v e d t h a t the l i m p e t C o l l i s e l l a (=Acmaea) d i g i t a l i s grew f a s t e r and reached l a r g e r s i z e s a t h i g h t i d a l l e v e l s than a t low l e v e l s . The f a s t e r growth r a t e s were due t o reduced d e n s i t i e s a t h i g h t i d a l l e v e l s . In c o n t r a s t , o t h e r s t u d i e s on b a r n a c l e s and mussels have shown growth r a t e s t o be f a s t e s t a t low t i d a l l e v e l s , where the a n i m a l s were the s m a l l e s t ( C o n n e l l 1970, K i t c h i n g e t a l . 1959). T h e r e f o r e , growth c o u l d not be c o n t r i b u t i n g t o these s i z e g r a d i e n t s . C o n n e l l (1970) and K i t c h i n g e t a l . (1959) showed t h a t d i f f e r e n c e s i n p r e d a t i o n r a t e s produced the ob s e r v e d s i z e g r a d i e n t s i n b a r n a c l e s and m u s s e l s . P r e d a t i o n r a t e s were 3 g r e a t e s t a t low t i d a l l e v e l s . H i g h t i d a l l e v e l s were a r e f u g e from p r e d a t i o n where the a n i m a l s c o u l d l i v e l o n g e r and so grow l a r g e r . Another m o r t a l i t y f a c t o r t h a t has been w i d e l y s t u d i e d i s exposure t o d e s i c c a t i o n . S m a l l i n d i v i d u a l s of a n i m a l s found i n the upper i n t e r t i d a l c o u l d have poor s u r v i v a l a t h i g h t i d a l l e v e l s because they a r e more s u s c e p t i b l e t o d e s i c c a t i o n . T h i s has been proposed by r e s e a r c h e r s who, f i n d i n g t h a t s m a l l i n d i v i d u a l s were s c a r c e a t h i g h t i d a l l e v e l s , d i d l a b o r a t o r y s t u d i e s which showed t h a t s m a l l i n d i v i d u a l s were more s u s c e p t i b l e t o d e s i c c a t i o n than l a r g e ones ( B o y l e 1970, D a v i e s 1969, Coombs 1973, Edwards 1969). W o l c o t t (1973) demonstrated i n the f i e l d t h a t d e s i c c a t i o n m o r t a l i t y o c c u r r e d and a f f e c t e d s m a l l l i m p e t s more than l a r g e ones. M o r t a l i t y due t o wave a c t i o n has a l s o been d i f f i c u l t t o measure d i r e c t l y i n the f i e l d . Emson and F a l l e r - F r i t s c h (1976) and R a f f a e l l i and Hughes (1978) showed t h a t i n wave exposed a r e a s the s i z e d i s t r i b u t i o n and abundance of l i t t o r i n e s was a f f e c t e d by the s i z e d i s t r i b u t i o n and abundance of c r e v i c e s . However, i t was not known i f the c r e v i c e s were p r o v i d i n g s h e l t e r from wave a c t i o n or p r e d a t i o n . More f i e l d work needs t o be done on i d e n t i f y i n g and q u a n t i f y i n g m o r t a l i t y g r a d i e n t s . A l t h o u g h t h e r e i s good e v i d e n c e t h a t s i z e g r a d i e n t s can be m a i n t a i n e d by movement, not much i s known about how and why a n i m a l s s e l e c t c e r t a i n t i d a l h e i g h t s . Body s i z e i n c r e a s e s w i t h t i d a l h e i g h t i n many l i m p e t s of the h i g h i n t e r t i d a l zone because the young s e t t l e out and s u r v i v e a t low t i d a l h e i g h t s and move up as they get l a r g e r (Branch 1975, Breen 1972, Frank 4 1965). The proposed advantages of the s e m i g r a t i o n s a re t o widen the s p e c i e s ' ranges and move i n t o a r e a s w i t h b e t t e r growth and s u r v i v a l c o n d i t i o n s . Breen (1972) showed t h a t m i g r a n t s grew f a s t e r than n o n - m i g r a n t s . B e r t n e s s (1977) and B u t l e r (1979) suggest t h a t s p e c i e s of T h a i s m a i n t a i n s i z e g r a d i e n t s i n response t o g r a d i e n t s i n fo o d , s h e l t e r , and d i f f e r e n t i a l d e s i c c a t i o n t o l e r a n c e s . These hypotheses need f u r t h e r t e s t i n g . Smith and N e w e l l (1956) showed t h a t movement c o n t r i b u t e d t o s i z e g r a d i e n t s i n l i t t o r i n e s ; Gendron (1977) a l s o found s i z e g r a d i e n t s i n l i t t o r i n e s and showed t h a t e x p e r i m e n t a l l y d i s p l a c e d a n i m a l s c o u l d r e g a i n t h e i r former p o s i t i o n on the sh o r e . Many of the above s t u d i e s on movement su g g e s t e d , w i t h l i t t l e d a t a , t h a t the a n i m a l s were moving i n response t o growth' and s u r v i v a l c o n d i t i o n s . B e f o r e we can propose a r o b u s t t h e o r y f o r why s i z e g r a d i e n t s a r e m a i n t a i n e d , we need t o know how they a r e m a i n t a i n e d . A l l t h r e e f a c t o r s may c o n t r i b u t e t o s i z e g r a d i e n t s , and i f they do we need t o a s s e s s t h e i r r e l a t i v e c o n t r i b u t i o n s . I s t u d i e d how s i z e g r a d i e n t s were m a i n t a i n e d i n p o p u l a t i o n s of L i t t o r i n a s i t k a n a near B a m f i e l d , B.C. I compared p o p u l a t i o n s i n two d i s t i n c t h a b i t a t s , p e b b l e beaches and b a s a l t i c s h e l v e s , t o see what were the common and perhaps most i m p o r t a n t mechanisms a f f e c t i n g the s i z e d i s t r i b u t i o n p a t t e r n s . To determine how the s n a i l s i z e g r a d i e n t s were m a i n t a i n e d on pebble beaches and b a s a l t i c h e a d l a n d s , I asked the f o l l o w i n g q u e s t i o n s : 5 1) Does the s i z e g r a d i e n t p a t t e r n v a r y s e a s o n a l l y ? 2) Does s n a i l growth, s u r v i v a l , or movement v a r y w i t h t i d a l h e i g h t ? 3) I f any or a l l of the above f a c t o r s v a r y w i t h t i d a l h e i g h t , a r e the obser v e d d i f f e r e n c e s s u f f i c i e n t t o produce the s i z e g r a d i e n t s ? I found L i t t o r i n a s c u t u l a t a w i t h L . s i t k a n a i n t h e s e upper i n t e r t i d a l h a b i t a t s , but d e c i d e d t o c o n c e n t r a t e on L . s i t k a n a because i t s grooved s h e l l made i t e a s i e r t o mark and a l l l i f e h i s t o r y s t a g e s a r e p r e s e n t on the sh o r e . L . s c u t u l a t a has p l a n k t o n i c l a r v a e , but L . s i t k a n a l a y s b e n t h i c egg masses t h a t h a t c h d i r e c t l y i n t o young s n a i l s . T h e r e f o r e , I examined the above t h r e e q u e s t i o n s f o r L . s i t k a n a , but o n l y sampled L . s c u t u l a t a i n the two h a b i t a t s and q u a n t i f i e d i t s s u r v i v a l w i t h t i d a l h e i g h t on pebble beaches. The r e s u l t s f o r L . s c u t u l a t a a r e r e p o r t e d i n a s e p a r a t e s e c t i o n . Chow (1975) found no c o n c l u s i v e e v i d e n c e f o r how s i z e g r a d i e n t s were m a i n t a i n e d i n C a l i f o r n i a n p o p u l a t i o n s of L . s c u t u l a t a ; body s i z e i n c r e a s e d w i t h t i d a l h e i g h t as V e r m e i j (1972) p r e d i c t e d f o r h i g h i n t e r t i d a l a n i m a l s . The o n l y e c o l o g i c a l s t u d i e s done on L . s i t k a n a d i d not i n v e s t i g a t e s i z e g r a d i e n t s (Behrens 1971, 1972). 6 STUDY AREAS A l l f i e l d w o r k was done i n B a r k l e y Sound,B.C.,near the B a m f i e l d M a r i n e S t a t i o n ( F i g u r e 1 ) . Pebble beaches c o n t a i n i n g L . s i t k a n a a r e found i n s h e l t e r e d i n l e t s . The v e r t i c a l d i s t r i b u t i o n of the s n a i l e xtends from about 1.3- 3.0m above c h a r t datum. Datum i s a p l a n e below which t i d e s seldom f a l l . P ebble beaches a r e g e n t l y f l o o d e d by the t i d e s . B a s a l t i c headlands j u t out between sandy beaches found on more exposed c o a s t s . L . s i t k a n a extended from 3.3- 3.8m above datum on the headlands s t u d i e d . The lower end of the s n a i l s ' v e r t i c a l d i s t r i b u t i o n on b a s a l t i c headlands i s s u b j e c t t o heavy wave a c t i o n . Incoming waves break and f l o o d the h i g h e r h o r i z o n t a l s h e l v e s more g e n t l y except d u r i n g s e v e r e w i n t e r storms. These s h e l v e s a r e d o t t e d w i t h t i d e p o o l s c o n t a i n i n g many l i t t o r i n e s . B a s a l t i c s h e l v e s a r e more heterogeneous s n a i l h a b i t a t than pebble beaches because the t i d e p o o l s a r e not always c o n n e c t e d by streams. Dry a r e a s t h a t have no s n a i l s o f t e n s e p a r a t e the t i d e p o o l s . A l s o , the d i r e c t i o n i n which the s h e l v e s were f i r s t f l o o d e d depended upon sea s u r g e , t i d a l h e i g h t , and wind d i r e c t i o n . The t i d e p o o l s on most of the h i g h h o r i z o n t a l s h e l v e s a r e i r r e g u l a r l y f l o o d e d by the summer t i d e s . The t i d e s may not f l o o d them f o r up t o two weeks. I f t h i s c o i n c i d e s w i t h warm, dry weather many t i d e p o o l s d r y up and the s n a i l s form l a r g e a g g r e g a t i o n s on the s h e l f . In c o n t r a s t , most of the s n a i l h a b i t a t on pebble beaches i s f l o o d e d d a i l y by summer t i d e s . In a d d i t i o n , the u n d e r s u r f a c e s of p e b b l e s p r o v i d e a moist m i c r o h a b i t a t f o r the s n a i l s . The t i d e s on t h i s p a r t of the 7 F i g u r e 1. Maps showing l o c a t i o n of B a m f i e l d and study s i t e s i n t he B a m f i e l d a r e a . Legend: — • st u d y s i t e s P p e b b l e beaches BH b a s a l t i c h e a d l a n d s BMS B a m f i e l d M a r i n e S t a t i o n *•:.-/.;.• sandy beaches 8 9 P a c i f i c c o a s t a r e mixed, m a i n l y s e m i - d i u r n a l (Canadian H y d r o g r a p h i c S e r v i c e ) . In s p r i n g and summer the lower low t i d e o c c u r s d u r i n g the day and the h i g h e r h i g h t i d e o c c u r s a t n i g h t . In f a l l and w i n t e r t h i s p a t t e r n i s r e v e r s e d . The w i n t e r h i g h t i d e s a re a l s o h i g h e r than the summer h i g h t i d e s . T h e r e f o r e , b a s a l t i c s h e l v e s a r e f l o o d e d q u i t e r e g u l a r l y i n the w i n t e r . In both h a b i t a t s most s n a i l s g r a z e m i c r o a l g a e o f f the rock s u r f a c e . Most e x p e r i m e n t s on growth, s u r v i v a l , and movement were done, on peb b l e beach A i n B a m f i e l d I n l e t and b a s a l t i c s h e l f A near Second Beach ( F i g u r e 1 ) . However, I d i d some e x p e r i m e n t s and s a m p l i n g on b a s a l t i c s h e l f B a t A g u i l a r P o i n t and pebble beach B i n G r a p p l e r I n l e t . E x p e r i m e n t s were conducted a t t h r e e t i d a l h e i g h t s on pebble beaches, and a t two t i d a l h e i g h t s on b a s a l t i c s h e l v e s ( F i g u r e 2 ) . These s t a t i o n s spanned the v e r t i c a l d i s t r i b u t i o n of L . s i t k a n a i n t h e s e two h a b i t a t s . U n l e s s o t h e r w i s e s t a t e d , ' s n a i l s ' or ' l i t t o r i n e s ' , always r e f e r s t o L . s i t k a n a . gure 2. Diagram of the main study s i t e s showing t i d a l h e i g h t s and h o r i z o n t a l d i s t a n c e s between s a m p l i n g s t a t i o n s ( O ) . The darkened a r e a s of the t o p views show approximate a r e a s sampled a t each s t a t i o n . Sea a c c e s s i s i n d i c a t e d by the b i g a r r o w s . 12 METHODS Ta b l e I summarizes how growth, s u r v i v a l , and movement must v a r y i f they produce or c o n t r i b u t e t o an upshore i n c r e a s e i n s n a i l s i z e . I used m a r k - r e c a p t u r e t e c h n i q u e s and c o n t r o l l e d t r a n s p l a n t s t o measure the t h r e e f a c t o r s on p ebble beaches and b a s a l t i c s h e l v e s . I f any of t h e f a c t o r s v a r i e d w i t h t i d a l h e i g h t , I c a l c u l a t e d i f the observed d i f f e r e n c e s were s u f f i c i e n t t o produce a s i z e g r a d i e n t . In some c a s e s , I used s i m u l a t i o n models t o e x p l o r e the range of d i f f e r e n c e s t h a t were s u f f i c i e n t t o a f f e c t the s i z e d i s t r i b u t i o n of the p o p u l a t i o n . S i z e D i s t r i b u t i o n To see i f s n a i l s i z e d i s t r i b u t i o n v a r i e d w i t h t i d a l h e i g h t and season, I sampled the f o u r s t u d y s i t e s ( F i g u r e 1) every t h r e e months. Ten cm 2 q u a d r a t s were randomly chosen a t each t i d a l h e i g h t . The number sampled depended upon- the s n a i l d e n s i t y and s i z e d i s t r i b u t i o n a t each t i d a l h e i g h t . On pebble beach A and b a s a l t i c s h e l f A, as many q u a d r a t s were taken as needed t o get a r e p r e s e n t a t i v e sample. The r e p l i c a t e h a b i t a t s , p e b b l e beach B and b a s a l t i c s h e l f B, were not sampled so t h o r o u g h l y . My measure of s n a i l s i z e was the l o n g e s t a x i s of the s n a i l s h e l l . T h i s was measured w i t h v e r n i e r c a l i p e r s . To see i f egg mass d i s t r i b u t i o n v a r i e d w i t h t i d a l h e i g h t , I a l s o r e c o r d e d egg mass d i s t r i b u t i o n and abundance on pebble beach A and b a s a l t i c s h e l f A. 13 T a b l e I . P r e d i c t i o n s and t e s t s f o r the t h r e e f a c t o r s t h a t c o u l d cause or c o n t r i b u t e t o l i t t o r i n e s i z e g r a d i e n t s . P r e d i c t i o n s i f FACTOR c o n t r i b u t e s t o g r a d i e n t s : TEST Pebble Beaches B a s a l t i c S h e l v e s F a c t o r s 1. Growth r a t e s s h o u l d be f a s t e s t a t h i g h t i d a l l e v e l s measure growth r a t e s of s n a i l s e n c l o s e d a t 3 t i d a l l e v e l s measure growth r a t e s of s n a i l s from h i g h and low t i d a l l e v e l s 2. S u r v i v a l r a t e s s h o u l d be lo w e s t a t low t i d a l l e v e l s t r a n s p l a n t l a r g e s n a i l s t o low l e v e l s and mo n i t o r s u r v i v a l r e c o r d s u r v i v a l of s n a i l s i n growth bags compare s u r v i v a l o f : a) s n a i l s marked f o r growth b) l a r g e and medium s n a i l s t r a n s p l a n t e d t o low l e v e l s Movement l a r g e s n a i l s s h o u l d move t o h i g h t i d a l l e v e l s r e c o r d movements of l a r g e s n a i l s t r a n s p l a n t e d t o low t i d a l l e v e l s r e c o r d movements of t r a n s p l a n t e d and l o c a l s n a i l s a t low l e v e l s . 14 'Growth To measure the v a r i a t i o n i n growth r a t e s w i t h t i d a l h e i g h t , I marked t h r e e s i z e c l a s s e s of s n a i l s a t the t i d a l h e i g h t s on p e b b l e beach A and b a s a l t i c s h e l f A shown i n F i g u r e 2. I d e f i n e d t h r e e s i z e c l a s s e s f o r a l l e x p e r i m e n t s : s m a l l ( 3 -4mm), medium (6-7mm), and l a r g e (8-llmm). The s n a i l s were s i e v e d w i t h s o i l s i e v e s f o r s i z e matching and the s i z e groups were p a i n t e d d i f f e r e n t l y w i t h 'Humbrol enamel base' model p a i n t . The l i p of the s h e l l was p a i n t e d and l i p increment was measured w i t h v e r n i e r c a l i p e r s a t weekly t o monthly time i n t e r v a l s from May- October 1980. I used l i p increment as an index of growth r a t e . The r e l a t i o n s h i p between l i p increment and l e n g t h increment was c a l c u l a t e d by measuring i n i t i a l s h e l l l e n g t h , f i n a l s h e l l l e n g t h , and l i p increment ( F i g u r e 3 ) . T h i s method was used by Behrens (1971). For a l l s n a i l s i z e s and a r e a s , the mean l i p increment was c a l c u l a t e d f o r a two month p e r i o d on pebble beaches (June 15-August 15) and on b a s a l t i c s h e l v e s (May 15- J u l y 1 5 ) . Means were t e s t e d f o r d i f f e r e n c e s w i t h p a i r e d t - t e s t s f o r e q u a l but unknown p o p u l a t i o n v a r i a n c e s . On b a s a l t i c s h e l f A , s n a i l s were c o l l e c t e d , p a i n t e d , and r e t u r n e d t o t h e i r o r i g i n a l t i d e p o o . l s w i t h i n 24 h o u r s . The s u r r o u n d i n g a r e a was mapped and s y s t e m a t i c a l l y s e a r c h e d a t each v i s i t . On May 13, 80 s m a l l and 50 medium s n a i l s were p a i n t e d and r e t u r n e d t o a low l e v e l t i d e p o o l a t 3.3m above datum. F i f t y s m a l l , 50 medium and 50 l a r g e were a l s o r e t u r n e d t o two h i g h l e v e l t i d e p o o l s (3.8m(a), 3.8m(b)). On May 27, a n o t h e r b a t c h 15 F i g u r e 3. Diagram of a g e n e r a l i z e d l i t t o r i n e showing r e l a t i o n s h i p between l i p increment (growth i n d e x ) and s h e l l l e n g t h i n crement. Paint Mark New Growth '4—M lip increment 17 was c o l l e c t e d from the same t h r e e t i d e p o o l s . F i f t y medium s n a i l s were p a i n t e d a t the low p o o l , and 50 l a r g e and 50 medium s n a i l s were p a i n t e d and r e t u r n e d t o the two h i g h p o o l s . On pe b b l e beaches, s m a l l s n a i l s were absent a t h i g h l e v e l s and so were t r a n s p l a n t e d up from low l e v e l s . L a r g e s n a i l s were a l s o t r a n s p l a n t e d down t o low l e v e l s . I t was d i f f i c u l t t o f i n d marked s n a i l s a t l a r g e i n the pe b b l e m a t r i x and so s n a i l s were e n c l o s e d i n 1 m2 bags of 'permascreen', a p l a s t i c meshing used f o r window s c r e e n i n g ( L i n k Hardware L t d . , V a n c o u v e r ) . Two bags were s e t up a t each of the t h r e e t i d a l h e i g h t s (1.5, 2.2, 2.8m) on June 11 on pebble beach A. The bags c o n t a i n e d 30 s m a l l , 30 medium, and 15 l a r g e marked s n a i l s . They a l s o c o n t a i n e d n a t u r a l d e n s i t i e s of unmarked s n a i l s , r o c k s , and a s h e l l / m u d m a t r i x from the s u r r o u n d i n g a r e a . T h e r e f o r e , bags a t low t i d a l l e v e l s c o n t a i n e d h i g h d e n s i t i e s of s n a i l s and h i g h l e v e l bags had low d e n s i t i e s of s n a i l s . I thought my r e c o v e r y r a t e s of marked s m a l l s n a i l s might s t i l l be poor i n the b i g bags. T h e r e f o r e , I' a l s o s e t up two s m a l l bags (45 by 35 cm) on June 11 a t the t h r e e t i d a l h e i g h t s . The s m a l l bags c o n t a i n e d o n l y 15 p a i n t e d s m a l l s n a i l s each and n a t u r a l d e n s i t i e s of unmarked s n a i l s and r o c k s . Growth r a t e s were a l s o measured on p e b b l e beach B i n G r a p p l e r I n l e t . Ten medium s n a i l s were put i n the s m a l l bags .at the t h r e e t i d a l h e i g h t s . Three s m a l l bags c o n t a i n i n g t en marked s n a i l s , r o c k s , and unmarked s n a i l s were put out a t each t i d a l h e i g h t on June 17. 18 S u r v i v a l The s u r v i v a l r a t e s of L . s i t k a n a c o u l d v a r y w i t h t i d a l h e i g h t because of changes i n p r e d a t i o n or exposure (wave a c t i o n , d e s i c c a t i o n ) . I measured the s u r v i v a l r a t e s of two groups of marked s n a i l s . The f i r s t group were s n a i l s t h a t were p a i n t e d and r e t u r n e d t o t h e i r o r i g i n a l t i d a l l e v e l so t h a t t h e i r growth r a t e s c o u l d be measured. The second group were s n a i l s t h a t were t r a n s p l a n t e d t o t i d a l l e v e l s where t h e i r s i z e c l a s s was r a r e . I f I found d i f f e r e n c e s i n s u r v i v a l r a t e s , I then t r i e d t o q u a n t i f y how m o r t a l i t y from p r e d a t i o n or exposure v a r i e d w i t h t i d a l h e i g h t on pebble beaches and b a s a l t i c s h e l v e s . ; L S u r v i v a l on Pebble Beaches To measure how s u r v i v a l r a t e s v a r i e d w i t h t i d a l h e i g h t and s n a i l s i z e , I put out known s i z e c o m p o s i t i o n s of marked s n a i l s a t the t h r e e t i d a l h e i g h t s on p e b b l e beach A. I checked the a r e a s a f t e r one h i g h t i d e . To s t o p the s n a i l s from d i s p e r s i n g w i d e l y , the t r a n s p l a n t s were put i n t o a 0.8 m2 aluminum e n c l o s u r e which was 5 cm h i g h w i t h an a t t a c h e d 10 cm h i g h permascreen f e n c e . The e n c l o s u r e reduced the s n a i l s ' movements w i t h o u t h i n d e r i n g any p o s s i b l e p r e d a t o r s . I put out two f e n c e s 37 m a p a r t a t each t i d a l h e i g h t . E x p e r i m e n t s were c a r r i e d on from June t o August 1980. The d e s i g n i s d e t a i l e d i n the r e s u l t s . S u r v i v o r s h i p of s n a i l s i n growth bags was a l s o measured. 19 These measurements gave e s t i m a t e s of s u r v i v a l of a l l s i z e c l a s s e s a t a l l t i d a l l e v e l s i n t h e absence of p r e d a t i o n . In p r e l i m i n a r y t r a n s p l a n t s of p a i n t e d l a r g e s n a i l s t o low l e v e l s of pebble beach A, I found many p a i n t e d s h e l l fragments a f t e r o n l y one h i g h t i d e . To d i s c o v e r what was e a t i n g the s n a i l s , I c l o s e l y watched the e n c l o s u r e s a t low t i d a l l e v e l s t h r oughout t h e i r e n t i r e p e r i o d of submersion. Four p i l e p e r c h , R h a c o c h i l u s v a c c a , were obser v e d e n t e r i n g the a r e a a t h i g h t i d e . They sucked the l i t t o r i n e s i n t o t h e i r mouths and spat out the fragments moments l a t e r . To f i n d out why the p i l e p e r c h were r e g u l a r l y v i s i t i n g t h i s a r e a , I examined the stomach c o n t e n t s of two f i s h caught i n the study a r e a . I a l s o o f f e r e d the p i l e p e r c h o t h e r p o t e n t i a l prey t h a t were common a t low t i d a l l e v e l s t o see how i n t e r e s t e d the f i s h were i n p u r s u i n g them. I dropped the prey i n t o the water from an anchored boat at h i g h t i d e and o b s e r v e d the f i s h e s ' responses through a v i e w i n g box. To see i f the p i l e p e r c h were o n l y a t t r a c t e d t o p a i n t e d s n a i l s , I o f f e r e d the f i s h both p a i n t e d and u n p a i n t e d s n a i l s . I r e c o r d e d the number of l i v e s n a i l s and a l l s h e l l fragments found a f t e r one h i g h t i d e . I used a t e s t f o r comparing b i n o m i a l p r o p o r t i o n s (Walpole and Myers 1972) t o see i f t h e r e was any d i f f e r e n c e i n the p r o p o r t i o n found a l i v e i n p a i n t e d and u n p a i n t e d groups. T h i s t e s t i s used i n a l l c a s e s where I am comparing p r o p o r t i o n s . 20 S u r v i v a l on B a s a l t i c S h e l v e s S u r v i v o r s h i p a t d i f f e r e n t t i d a l l e v e l s of b a s a l t i c s h e l f A was measured by the s u r v i v a l r a t e s of the same s n a i l s marked and put out a t the t h r e e t i d e p o o l s t o measure growth r a t e s . The r e s u l t i n g d i s t r i b u t i o n s were compared w i t h a Kolmogorov-Smirnov two-sample t e s t ( S i e g e l 1956). To see i f d i s a p p e a r a n c e r a t e s i n c r e a s e d w i t h s n a i l s i z e a t the low t i d a l l e v e l , I t r a n s p l a n t e d l a r g e , medium, and s m a l l -s i z e d s n a i l s from the h i g h s h e l v e s t o the low p o o l . Oh June 26, I t r a n s p l a n t e d 25 l a r g e and 25 s m a l l (4-5mm) s n a i l s . On August 8, I t r a n s p l a n t e d an a d d i t i o n a l 50 l a r g e and 50 medium-sized s n a i l s t o the low p o o l . The s u r v i v a l r a t e s were r e c o r d e d a t weekly i n t e r v a l s from May t o August. To s o r t out the e f f e c t s , i f any, of wave a c t i o n and p r e d a t i o n on the d i s p p e a r a n c e r a t e s of marked s n a i l s , I a t t a c h e d l a r g e s n a i l s t o l e a s h e s a t two low l e v e l a r e a s . The l e a s h e s s h o u l d p r o t e c t the s n a i l s from wave a c t i o n m o r t a l i t y w i t h o u t p r o t e c t i n g them from p r e d a t i o n . H o l e s were d r i l l e d t h r o u g h the l i p of the s h e l l and the s n a i l s were a t t a t c h e d t o w i r e b a r s w i t h one k i l o g r a m t e s t .nylon m o nofilament. The b a r s were cemented t o the rock i n August 1980 w i t h 'Burke P l u g Quick S e t t i n g H y d r a u l i c Cement' (Burke C o n c r e t e Acces. I n c . , 863 M i t t e n Road, B u r l i n g a m e , C a l i f o r n i a 94010). The l e a s h e d s n a i l s were checked two weeks and two months l a t e r . 21 Movement Movement on Pebble Beaches To t e s t the h y p o t h e s i s t h a t s n a i l movement m a i n t a i n s s i z e g r a d i e n t s , I d i d t r a n s p l a n t e x p e r i m e n t s from J u l y 21 t o August 7, 1980 on pebble beach A. I f t r a n s p l a n t e d s n a i l s move toward t h e i r o r i g i n a l t i d a l l e v e l s (homeward), i t s u g g e s t s t h a t the s n a i l s a r e c a p a b l e of m a i n t a i n i n g t h e i r p r e f e r r e d p o s i t i o n on the s h o r e . To t e s t the h y p o t h e s i s t h a t s n a i l movement produces s i z e g r a d i e n t s , one would have t o f o l l o w the movements of s m a l l s n a i l s and see i f they moved t o h i g h e r t i d a l l e v e l s as they get l a r g e r . T r a n s p l a n t e x p e r i m e n t s a r e l o g i s t i c a l l y e a s i e r and a r e a good p r e l i m i n a r y t e s t because n e g a t i v e r e s u l t s would r u l e out the p o s s i b i l i t y t h a t s n a i l s can a c t i v e l y s e l e c t c e r t a i n t i d a l h e i g h t s . Large s n a i l s were t r a n s p l a n t e d down from t h e i r o r i g i n a l t i d a l l e v e l s and s m a l l s n a i l s were t r a n s p l a n t e d up. The up/down a x i s was p e r p e n d i c u l a r t o the s h o r e l i n e . 0° was d e f i n e d as s t r a i g h t up the beach; 180° was s t r a i g h t down. Large s n a i l s were t r a n s p l a n t e d down the beach 4m (a change i n t i d a l h e i g h t of 0.7m). S m a l l s n a i l s were t r a n s p l a n t e d up the beach 6m (a change i n t i d a l h e i g h t of 0.8m). Large s n a i l s c o u l d not be t r a n s p l a n t e d lower because of heavy l o s s t o p r e d a t o r s . To c o n t r o l f o r the e f f e c t of d i s p l a c e m e n t , s n a i l s were t r a n s p l a n t e d l a t e r a l l y (l-5m) w i t h i n t h e i r o r i g i n a l t i d a l h e i g h t . Each t r e a t m e n t p l a c e d 30 l a r g e or 40 s m a l l marked s n a i l s 22 w i t h i n a 20 cm d i a m e t e r c i r c l e . P o s i t i o n s of the s n a i l s ( d i s t a n c e and d i r e c t i o n from the p o i n t of r e l e a s e ) were r e c o r d e d 24 hours and 2 h i g h t i d e s l a t e r . The a r e a s e a r c h e d was w i t h i n a lm r a d i u s c e n t e r e d about the r e l e a s e p o i n t . The d i s t r i b u t i o n s of the p o s i t i o n s of the s n a i l s were a n a l y z e d w i t h a m o d i f i e d R a y l e i g h t e s t (Moore 1980). T h i s t e s t w e i g h t s d i s t a n c e and d i r e c t i o n and l o o k s f o r s i g n i f i c a n t d e v i a t i o n s from a random or u n i f o r m d i s t r i b u t i o n . A mean v e c t o r was c a l c u l a t e d f o r each t r e a t m e n t . Movement on B a s a l t i c S h e l v e s To measure any movement up from the low l e v e l of b a s a l t i c s h e l f A, the p o s i t i o n s of the marked s n a i l s i n the low l e v e l t i d e p o o l were r e c o r d e d on f o u r o c c a s i o n s . These marked s n a i l s were the l a r g e and medium s i z e d s n a i l s t r a n s p l a n t e d t h e r e t o measure s u r v i v a l and the l o c a l s n a i l s t h a t were p a i n t e d t o measure growth. A minimum ar e a of 21 m2 was s e a r c h e d about t h i s low l e v e l t i d e p o o l . Any s n a i l s t h a t had moved 1-2 m down the b a r n a c l e s l o p e below the t i d e p o o l (a 0.1-0.2 m change i n t i d e h e i g h t ) were r e c o r d e d as moving down. S n a i l s r e c o r d e d as moving up, had moved i n t o t i d e p o o l s t h a t were 2-3 m above the o r i g i n a l t i d e p o o l (a change i n t i d e h e i g h t of 0.1-0.2 m). A l l o t h e r p o s i t i o n s r e c o r d e d were a t the same t i d a l l e v e l as the o r i g i n a l t i d e p o o l . 23 RESULTS S n a i l S i z e D i s t r i b u t i o n s W i t h i n b oth h a b i t a t s , the s i z e s t r u c t u r e of the p o p u l a t i o n v a r i e d w i t h t i d a l h e i g h t so mean s n a i l s i z e was i n c r e a s i n g w i t h t i d a l h e i g h t ( F i g u r e 4 ) . Sample s i z e s were s m a l l a t the h i g h s t a t i o n of pebble beaches, d e s p i t e s a m p l i n g t w i c e the a r e a sampled a t lower s t a t i o n s . S m a l l s n a i l s were always s c a r c e a t h i g h t i d a l l e v e l s of p e b b l e beaches, but were s e a s o n a l l y abundant on b a s a l t i c s h e l v e s . S m a l l s i z e c l a s s e s were abundant and l a r g e r s i z e c l a s s e s were r a r e a t low t i d a l l e v e l s of both h a b i t a t s . T a b l e I I shows t h a t the d i s t r i b u t i o n and abundance of egg masses i s c o r r e l a t e d w i t h the d e n s i t y of r e p r o d u c t i v e (>6mm) s n a i l s (Spearman's rank c o r r e l a t i o n =.75, p<.05). Egg masses were l a i d a t a l l l e v e l s of b a s a l t i c s h e l f A i n the s p r i n g . But i n O c t o b e r , f a l l r e p r o d u c t i o n had o n l y s t a r t e d a t 3.8m(b). On p e b b l e beach A, two e x t r a t i d e h e i g h t s were sampled t o ensure f i n d i n g the a r e a where most egg masses were l a i d . Egg masses were never ob s e r v e d a t the h i g h e s t s t a t i o n d e s p i t e the c o n t i n u a l p r e s ence of l a r g e s n a i l s . Nor were they found a t the two l o w e s t s t a t i o n s where l a r g e s n a i l s were v e r y s c a r c e . 24 F i g u r e 4. S i z e f r e q u e n c y d i s t r i b u t i o n s of L . s i t k a n a sampled i n 1980 on main study s i t e s . Sample s i z e i s g i v e n on each s i z e f r e q u e n c y h i s t o g r a m . Note t h a t l a r g e r s i z e c l a s s e s a r e absent from low t i d a l l e v e l s of both h a b i t a t s . T a b l e XIV i n Appendix I I I shows s i m i l a r t r e n d s f o r p o p u l a t i o n s on P ebble Beach B and B a s a l t i c S h e l f B; mean s n a i l s i z e i n c r e a s e d w i t h t i d a l h e i g h t . 1.5m PEBBLE BEACH A 2.2 m 2.8m 26 Ta b l e I I . Egg mass abundance of L . s i t k a n a a t d i f f e r e n t t i d a l l e v e l s . D e n s i t i e s of s n a i l s and egg masses are f o r a 10 cm squared a r e a . s a m p l i n g time (1980) Feb. A p r i l J u l y Oct. A p r i l d e n s i t y of s n a i l s > 6mm mean (s.d.) i n mm, 1.5 0 0 0 0 PEBBLE BEACH A t i d a l h e i g h t (m) 1.7 2.2 2.5 Egg Mass d e n s i t y 0 0 0 0 4 1 0 0 1 0 0 0 2.8 0 0 0 0 K . 5 ) 2.1(1.6) 9.7(9.5) 3(2.2) .7(1.2) s a m p l i n g time (1980) Feb. A p r i l J u l y Oct. A p r i l d e n s i t y of s n a i l s > 6mm mean (s.d.) i n mm. BASALTIC SHELF A t i d a l h e i g h t (m) 3.3 3.8a 3.8b Egg Mass d e n s i t y 0.5 0 0 0 7.5 7.6 0 0 1( .36) 8.2(7.6) 4.4 5.3 0 2.0 6.2(5.9) 27 Growth I measured growth r a t e s of L . s i t k a n a a t d i f f e r e n t t i d a l l e v e l s on p e b b l e beaches and b a s a l t i c s h e l v e s . I f v a r i a t i o n i n growth r a t e s c o n t r i b u t e s t o s i z e g r a d i e n t s , growth r a t e s s h o u l d be h i g h e s t a t h i g h t i d a l l e v e l s where the s n a i l s a r e l a r g e s t . F i g u r e 5 summarizes how growth r a t e s v a r y w i t h t i d a l h e i g h t on pebble beaches and b a s a l t i c s h e l v e s . The p l o t of l i p v e r s u s l e n g t h increment i n Appendix I I shows how t h i s growth r a t e index c o r r e s p o n d s t o changes i n s h e l l l e n g t h . On p e b b l e beaches, a l l s i z e c l a s s e s grew f a s t e s t a t the low t i d a l h e i g h t s . Means from low t i d a l l e v e l s were s i g n i f i c a n t l y g r e a t e r than means from h i g h l e v e l s ( t - t e s t , p<.005). T h e r e f o r e , v a r i a t i o n i n growth r a t e s cannot e x p l a i n why l a r g e s n a i l s a re absent from low l e v e l s . On b a s a l t i c s h e l f A, the l i t t o r i n e s a t the low s t a t i o n (3.3m) grew s l i g h t l y f a s t e r than the s n a i l s a t 3.8m(a), but the d i f f e r e n c e was not s i g n i f i c a n t (p=.07). T h i s s u g g e s t s t h a t v a r i a t i o n i n growth r a t e s cannot e x p l a i n why l a r g e s n a i l s a r e absent from 3.3m but p r e s e n t a t 3.8m(a). Only the growth r a t e s of medium s n a i l s a re shown i n F i g u r e 5 f o r b a s a l t i c s h e l f A, because l a r g e s n a i l s were not t r a n s p l a n t e d t o low l e v e l s i n May t o measure growth. A l s o , my r e c o v e r y r a t e s of marked s m a l l s n a i l s were poor a t a l l t i d a l l e v e l s . S n a i l s from some h i g h l e v e l s of b a s a l t i c s h e l v e s grew v e r y f a s t . For example, the s n a i l s a t 3.8b grew f a s t e r than the s n a i l s a t 3.8a (p<.001). I c o n c l u d e t h a t v a r i a t i o n i n growth r a t e s cannot account f o r the s i z e g r a d i e n t s found on pebble beaches and b a s a l t i c 28 F i g u r e 5. Growth r a t e s of L . s i t k a n a a t d i f f e r e n t t i d a l l e v e l s of Pebble Beach A,B and B a s a l t i c S h e l f A. The mean l i p i n c r e m e n t s f o r two month p e r i o d s and t h e i r s t a n d a r d e r r o r s a re p l o t t e d . Legend: s n a i l s i z e A s m a l l A medium • l a r g e P E B B L E B E A C H A P E B B L E B E A C H B B A S A L T I C S H E L F A 1.5 2.2 2.8 1.5 2.2 2.8 3.2 3.8a 3.8b TIDAL HEIGHT ( m ) 30 s h e l v e s . In both h a b i t a t s s n a i l s a t low l e v e l s (where o n l y s m a l l s n a i l s a r e found) grow as f a s t or f a s t e r than s n a i l s a t h i g h l e v e l s (where the l a r g e s t s n a i l s are f o u n d ) . S u r v i v a l S u r v i v a l on P e b b l e Beaches On p e b b l e beaches, l a r g e s n a i l s were absent from low t i d a l l e v e l s and s m a l l s n a i l s were absent from h i g h l e v e l s . I t e s t e d the h y p o t h e s i s t h a t s i z e s e l e c t i v e m o r t a l i t y was c a u s i n g t h i s g r a d i e n t by: (1) p u t t i n g out p a i n t e d l a r g e and medium-sized s n a i l s i n f e n c e s a t t h r e e t i d a l h e i g h t s and r e c o r d i n g t h e i r s u r v i v a l ; (2) e n c l o s i n g s m a l l , medium, and l a r g e s n a i l s i n permascreen bags at t h r e e t i d a l l e v e l s and measuring t h e i r growth and s u r v i v a l . 1. P r e d a t i o n , E x p e r i m e n t s To q u a n t i f y s u r v i v a l r a t e s of d i f f e r e n t s i z e d s n a i l s , 20 l a r g e and 20 medium-sized s n a i l s were put i n s i d e the s n a i l f e n c e s a t the t h r e e t i d a l h e i g h t s on p ebble beach A on 6 o c c a s i o n s . At the h i g h s t a t i o n , 414 out of 430 s n a i l s were found a l i v e and no s n a i l s h e l l fragments were ever found. T a b l e I I I shows t h a t the s u r v i v a l r a t e was much lower a t the low s t a t i o n s than at the m i d d l e s t a t i o n s . There i s a n e g a t i v e c o r r e l a t i o n between the number of fragments found and the mean number of l a r g e s n a i l s found a l i v e (Spearman's rank c o r r e l a t i o n 31 T a b l e I I I . Mean s u r v i v a l of 2 0 ' l a r g e and 20 medium-size s n a i l s a f t e r one h i g h t i d e . The s n a i l s were p l a c e d i n e n c l o s u r e s a t 3 t i d a l h e i g h t s on 6 o c c a s i o n s . See t e x t f o r r e s u l t s of h i g h t i d a l l e v e l (2.8m) e n c l o s u r e s . t i d a l h e i g h t (m) mean # a l i v e S.E. T o t a l # fragments found low (1.5) s n a i l s i z e L arge Medium 9.2 15.2 1.1 0.6 105 31 m i d d l e (2.2) s n a i l s i z e Large Medium 18.9 19 0.3 0.3 8 0 32 =-.90, p<.05). A two way ANOVA was done on the number of s n a i l s found a l i v e w i t h s n a i l s i z e and t i d a l h e i g h t as f a c t o r s . Both f a c t o r s were s i g n i f i c a n t (p<.001). T h e r e f o r e , s u r v i v a l r a t e i s s i g n i f i c a n t l y lower f o r l a r g e s n a i l s a t the low t i d a l h e i g h t s . To see i f , i n the absence of l a r g e s n a i l s , more medium-s i z e d (=M) s n a i l s would be e a t e n , I put out 40 M s n a i l s i n s i d e the m i d d l e and low f e n c e s on t h r e e o c c a s i o n s . In f a c t , 89% of the medium-sized s n a i l s s u r v i v e d t h i s t r e a t m e n t whereas o n l y 76% s u r v i v e d i n the f e n c e s c o n t a i n i n g 20 M and 20 l a r g e s n a i l s . These p r o p o r t i o n s were s i g n i f i c a n t l y d i f f e r e n t ( t e s t f o r b i n o m i a l p r o p o r t i o n s , p<.001). These two e x p e r i m e n t s show t h a t p r e d a t o r s p r e f e r r e d l a r g e s n a i l s and suggest t h a t medium-sized s n a i l s were o n l y eaten when the p r e d a t o r s were a t t r a c t e d i n t o an a r e a by the presence of l a r g e s n a i l s . In a l l e x p e r i m e n t s , the h i g h c o r r e l a t i o n between the number of s n a i l s m i s s i n g and s h e l l fragments found s t r o n g l y s u g g e s t s t h a t p r e d a t i o n was r e s p o n s i b l e f o r the m i s s i n g s n a i l s . I c o n c l u d e from t h e s e e x p e r i m e n t s t h a t the p r e d a t i o n r a t e i s h i g h e s t on l a r g e s n a i l s found a t low l e v e l s of p e b b l e beaches. Because p i l e p e r c h were c o n s i s t e n t l y found i n pebble beach A a t h i g h t i d e and were obse r v e d e a t i n g the s n a i l s , I b e l i e v e they were the major p r e d a t o r . Another p o s s i b l e p r e d a t o r i s the r e d rock c r a b , Cancer p r o d u c t u s . I t i s n o r m a l l y found a t lower t i d a l l e v e l s than the l i t t o r i n e s . C.productus feeds on o t h e r l a r g e r g a s t r o p o d s such as T h a i s spp. ( B e r t n e s s 1977, Z i p s e r and 33 V e r m e i j 1978) t h a t are more abundant below the l i t t o r i n e s ' d i s t r i b u t i o n . To see i f the p r e d a t o r s were e a t i n g such a h i g h p r o p o r t i o n of the t r a n s p l a n t e d l a r g e l i t t o r i n e s o n l y because they were p a i n t e d , I put out groups of p a i n t e d and u n p a i n t e d l i t t o r i n e s a t low t i d a l l e v e l s on two o c c a s i o n s . T a b l e IV below shows t h a t groups of 25 l a r g e u n p a i n t e d s n a i l s d i s a p p e a r e d as f a s t as groups of 25 l a r g e p a i n t e d s n a i l s d i d . There was no d i f f e r e n c e i n the p r o p o r t i o n s found a l i v e ( t e s t f o r b i n o m i a l p r o p o r t i o n s , p=.36). Large numbers of fragments were found i n a l l t h r e e low t i d a l l e v e l a r e a s . T a b l e IV. Numbers of l i v e s n a i l s found a f t e r one h i g h t i d e . p a i n t e d u n p a i n t e d u n p a i n t e d J u l y 29 4 10 10 J u l y 31 15 5 10 F i s h D i e t and F e e d i n g E x p e r i m e n t s I examined the d i e t of the p i l e p e r c h t o see what e l s e they a t e on pebble beaches. The stomach c o n t e n t s of two p i l e p e r c h caught a t a low s t a t i o n are shown i n T a b l e V. B a r n a c l e s and s m a l l c r a b s ( m o s t l y 'Hemigrapsus spp.) formed a major p a r t of t h e s e f i s h e s ' d i e t s . These prey items were commonly found on the pebble beaches between the t i d a l h e i g h t s of 1.5 and 2.2 m. M u s s e l s v a r i e d i n abundance on the pebble beaches s t u d i e d , but a l s o o c c u r r e d between the above t i d a l h e i g h t s . The f i s h were 34 T a b l e V. % Wet w e i g h t s of stomach c o n t e n t s of two p i l e p e r c h caught on Pebble Beach A. . F i s h A %wet weight rank PREY mussels c r a b s b a r n a c l e s and s m a l l b i v a l v e s l i t t o r i n e s t o t a l wet weight (g) f i s h l e n g t h (cm) 42 35 18 5 13.5 20 1 2 F i s h B %wet weight rank 15 23 44 18 8.5 30 4 2 35 o b s e r v e d e a t i n g b a r n a c l e s between v i s i t s t o the s n a i l e n c l o s u r e s . T h e r e f o r e , i t i s l i k e l y t h a t the f i s h r e g u l a r l y v i s i t low l e v e l s of p e b b l e beaches because t h e i r major prey items a r e abundant t h e r e . To see what o t h e r p o t e n t i a l prey o c c u r r e d i n t h i s zone, I o f f e r e d the p i l e p e r c h g a s t r o p o d s whose upper l i m i t s b e g i n i n t h i s zone. I a l s o o f f e r e d the f i s h s m a l l c r a b s and l i t t o r i n e s d u r i n g the same experiment t o compare the p i l e p e r c h e s ' r e s p o n s e s . The r e s u l t s a r e shown i n T a b l e V I . W h i l e l i t t o r i n e s were r e a d i l y e a t e n , o t h e r l a r g e r g a s t r o p o d s were e j e c t e d whole almost i m m e d i a t e l y or were too l a r g e t o even e n t e r some f i s h e s ' mouths. The s m a l l c r a b s dropped i n t o the water a t t r a c t e d the p i l e p e r c h e s ' a t t e n t i o n , but the f i s h were l e s s s u c c e s s f u l a t c a t c h i n g them (Table V I ) . I put out l a r g e l i t t o r i n e s a t low t i d a l l e v e l s of pebble beach B t o see i f p i l e p e r c h p r e d a t i o n . o c c u r r e d on o t h e r pebble beaches. A f t e r one h i g h t i d e , s e v e r a l s h e l l fragments were found. The a r e a was watched f o r one h a l f hour of the next h i g h t i d e and p i l e p e r c h were obser v e d i n the a r e a , but none were ob s e r v e d t o eat l i t t o r i n e s . F i s h P r e d a t i o n Model S i z e s e l e c t i v e p r e d a t i o n by p i l e p e r c h may be the reason t h a t l a r g e l i t t o r i n e s a r e not found a t low t i d a l l e v e l s on pebble beaches i n B a r k l e y Sound. I made a s i m u l a t i o n model t o see i f the observed p r e d a t i o n ' r a t e s c o u l d be s u f f i c i e n t t o e x c l u d e l a r g e s n a i l s from low t i d a l l e v e l s . The model a l l o w e d 36 Tab l e V I . , Responses of p i l e p e r c h p r e s e n t on Pebble Beach A t o o f f e r e d l i v e p r e y . PRPV J . RESPONSES . ° f f e r e d ^ " O " * Covered a t e e j e c t e d f i s h d i s -a) g a s t r o p o d s ° V e r whole appeared size(mm)* sm t e g u l a l l . (9-10) 1? 5 5 l g t e g u l a 3 l (14-19) 2 sm t h a i s 6 (15-19) 5 i l g t h a i s 2 l (20-27) 1 S e a r l e s i a d i r a (18-20) 6 l i t t o r i n e s 15 u 4 2 (8-11) 1 5 b) c r a b s l g H.nudus 3 r> (30,30) J sm H.nudus 3 i ' o (15,15) 1 2 sm H.oregon- 3 2 i e n s i s (15,15) P e t r o l i s h h o g i erioniern . q (10,10) * f o r g a s t r o p o d s , s i z e measured was l o n g e s t a x i s f o r c r a b s , i t was dime n s i o n s of c a r a p a c e t e g u l a = T e g u l a f u n e b r a l i s t h a i s = T h a i s l a m e l l o s a 37 me t o examine the e f f e c t s of a few s i m p l e assumptions on the s u r v i v a l of l a r g e s n a i l s . The model was w r i t t e n i n A p p l e s o f t BASIC and run on an Apple microcomputer. The dynamics of the l a r g e s i z e c l a s s of the s n a i l p o p u l a t i o n were s i m u l a t e d f o r a 40m by 4m a r e a . T h i s a r e a i s e q u i v a l e n t t o the a r e a on pebble beach A where the f i s h were most a c t i v e . The model makes the f o l l o w i n g a s s u m p t i o n s : (1) no l i t t o r i n e movement up t o h i g h l e v e l s ; (2) the d e n s i t y of l a r g e and s m a l l s n a i l s i s s i m i l a r t o t h a t o b s e r v e d a t 2.2m above datum on pebble beach A; (3) the number of young p r e s e n t i s d i r e c t l y r e l a t e d t o the number of a d u l t s . E v e r y y e a r , the number of young i s s e t e q u a l t o the number of a d u l t s a l i v e ; (4) the f i s h show a type two f u n c t i o n a l response t o s n a i l d e n s i t y ( H o l l i n g 1959). Growth, n a t u r a l m o r t a l i t y and p r e d a t i o n r a t e s were d e t e r m i n e d from f i e l d d a t a and then v a r i e d f o r a range of i n i t i a l l i t t o r i n e d e n s i t i e s . An o u t l i n e of the model and r e s u l t s of s e v e r a l s i m u l a t i o n s a r e g i v e n i n Appendix I I I . F i s h p r e d a t i o n was s u f f i c i e n t , under a wide range of i n i t i a l d e n s i t i e s and l i t t o r i n e r e c r u i t m e n t r a t e s , t o e l i m i n a t e the l a r g e s n a i l s . F i g u r e 6 p l o t s the r e s u l t s of f o u r s i m u l a t i o n s which each show the e f f e c t of i n i t i a l l i t t o r i n e d e n s i t y and growth r a t e s on the e x t i n c t i o n r a t e s . The r a t e t h a t s m a l l s n a i l s grew i n t o l a r g e s n a i l s had a g r e a t e f f e c t on when and i f e x t i n c t i o n o c c u r e d . The l a r g e s n a i l p o p u l a t i o n i n c r e a s e d when s n a i l d e n s i t y and growth r a t e s were 38 F i g u r e 6. S i m u l a t i o n s from f i s h p r e d a t i o n model ( o u t l i n e d i n Appendix I I I ) t h a t show e f f e c t of s n a i l r e c r u i t m e n t and f i s h p r e d a t i o n r a t e s on the s u r v i v a l of l a r g e s n a i l s . In t h e s e examples the i n i t i a l d e n s i t y of s m a l l (D2) and l a r g e (D3) s n a i l s were 3.0, except i n 6d where they were both o n l y 2.0. F i s h p r e d a t i o n had a much s t r o n g e r e f f e c t on the p o p u l a t i o n a t D2=D3=2.0. Legend: o s m a l l (5-7mm) s n a i l s • l a r g e (7-llmm) s n a i l s G 2 = p r o p o r t i o n of s m a l l s n a i l s t h a t grow i n t o l a r g e s n a i l s K=maximum number of s n a i l s e a t e n / f i s h / v i s i t NO. ALIVE (xlOOO) NO. AL IVE (x1000) 40 h i g h and the f i s h s t i l l a t e o n l y 10 s n a i l s each per v i s i t ( F i g u r e 6 a ) . Growth r a t e s i n L . s i t k a n a d e c r e a s e w i t h d e n s i t y (Behrens 1971, a u t h o r ' s u n p u b l i s h e d d a t a ) . T h e r e f o r e , the growth r a t e - d e n s i t y s c e n a r i o i n F i g u r e 6a i s u n l i k e l y . A l s o , my f e e d i n g e x p e r i m e n t s suggest t h a t t en l i t t o r i n e s / f i s h / v i s i t i s a c o n s e r v a t i v e e s t i m a t e i n the presence of many l a r g e l i t t o r i n e s . When the maximum number of s n a i l s e a t e n / f i s h / v i s i t was i n c r e a s e d t o 12, the l a r g e s n a i l p o p u l a t i o n d e c r e a s e d ( F i g u r e 6 c ) . These r e s u l t s s t r o n g l y suggest t h a t p i l e p e r c h p r e d a t i o n i s s u f f i c i e n t t o e l i m i n a t e l a r g e s n a i l s from low t i d a l l e v e l s of pebble beaches. 2. S u r v i v a l i n Growth Bags Can poor s u r v i v a l of s m a l l s n a i l s a t h i g h l e v e l s , owing t o d e s i c c a t i o n , be r e s p o n s i b l e f o r t h e i r absence t h e r e ? My d a t a do not show d e c r e a s e d s u r v i v a l of young a t h i g h t i d a l l e v e l s . For the f i v e month p e r i o d t h a t I measured growth and s u r v i v a l i n t he bags, the c u m u l a t i v e p e r c e n t a g e of s n a i l s found dead a t h i g h l e v e l s was 6% f o r s m a l l s n a i l s and 20% f o r l a r g e and medium-sized s n a i l s . However, I t h i n k the m o r t a l i t y r a t e s f o r s m a l l s n a i l s were u n d e r e s t i m a t e d because dead s m a l l s n a i l s h e l l s were v e r y hard t o f i n d . They q u i c k l y became c o v e r e d by the o r g a n i c and mud m a t r i x . The c u m u l a t i v e p e r c e n t a g e s of dead l a r g e and medium-sized s n a i l s were l o w e s t a t the low t i d a l l e v e l s ( T able V I I ) . T h i s s u g g e s t s t h a t i n the absence of p r e d a t i o n , the p h y s i c a l and b i o t i c c o n d i t i o n s f o r growth and s u r v i v a l a r e b e s t at low t i d a i l e v e l s . 41 Ta b l e V I I . M o r t a l i t y of s n a i l s i n growth bags put out on Pebble Beach A. The number of dead s n a i l s (and t o t a l number found) i s shown; note the t r e n d s i n the c u m u l a t i v e p e r c e n t a g e of dead s n a i l s a t d i f f e r e n t t i d a l l e v e l s . s m a l l s n a i l s B i g Bags S m a l l Bags t i d a l h e i g h t t i d a l h e i g h t h i g h m i d d l e low h i g h middle' low June 26 1(17) 1(39) 0(28) 0(30) 1(25)- 3(21) J u l y 16 2 ( 3 0 ) * 0(35) 0 ( 5 0 ) * 3(26) 1(28) 1(18) Aug 12 1(15) 0(30) 0(20) 3(22) 0(23) 2(20) Oct 27 0(5) 0(15) 0(5) 0(10) 1(20) 2(17) Cum. f r e q . 4(64) 1(119) 0(98) 6(88) 3(96) 8(77) Cum. % 6 0.8 0.0 7 3 10 B i g Bags -medium s n a i l s l a r g e s n a i l s h i g h m i d d l e low h i g h m i d d l e low June 26 0(30) 1(54) 0(28) 0(13) 0(28) 0(30) J u l y 16 1 0 ( 3 3 ) * 0(47) 0 ( 5 7 ) * 5(21) 0(28) 0(43) Aug 12 2(19) 4(44) 1(43) 4(12) 1(27) 0(28) Oct 27 8(12) 2(30) 2(20) 1(5) 0(21) 2(46) Cum. f r e q . 20(94) 7(175) 3(158) 10(51) 1(104) 2(146) Cum. % 21 4 2 20 1 2 * the t o t a l number found i n c r e a s e d because o n l y one r e p l i c a t e had been sear c h e d on June 26. 42 S u r v i v a l on B a s a l t i c S h e l v e s Are l a r g e s n a i l s absent from low t i d a l l e v e l s of b a s a l t i c s h e l v e s because of i n c r e a s e d m o r t a l i t y r a t e s a t low l e v e l s ? F i g u r e 7 shows t h a t t h e r e was a lower s u r v i v a l r a t e of marked s n a i l s a t the low t i d e p o o l of b a s a l t i c s h e l f A than at the two h i g h t i d e p o o l s . T h i s o c c u r r e d f o r the groups of s n a i l s r e l e a s e d on May 13 ( b a t c h A) and on May 27 ( b a t c h B ) . For both b a t c h e s , the s u r v i v a l c u r v e s a t the low t i d e p o o l a r e s i g n i f i c a n t l y d i f f e r e n t from the o t h e r two ( Kolmogorov-Smirnov t e s t , p<.005). When l a r g e and s m a l l e r - s i z e d s n a i l s were t r a n s p l a n t e d t o the low s t a t i o n , t h e r e was no d i f f e r e n c e (p=.5) i n t h e i r s u r v i v a l r a t e s ( F i g u r e 8 ) . T h e r e f o r e , t h e r e i s no e v i d e n c e f o r s i z e - d e p e n d e n t s u r v i v a l r a t e s a t the low t i d e p o o l . D i s a p p e a r a n c e of marked s n a i l s from b a s a l t i c s h e l v e s c o u l d be due t o p r e d a t i o n , wave a c t i o n , movement, or my f a i l u r e t o f i n d the s n a i l s . A l l t i d e p o o l s were s y s t e m a t i c a l l y s e a r c h e d and I see no reason f o r t h e p r o b a b i l i t y of my f i n d i n g marked s n a i l s t o be lower at the low t i d e p o o l . To q u a n t i f y and compare the s u r v i v a l r a t e s of the s n a i l s from the t h r e e t i d e p o o l s of b a s a l t i c s h e l f A, I f i t t e d a n e g a t i v e e x p o n e n t i a l f u n c t i o n t o a l l the s u r v i v a l c u r v e s i n F i g u r e 7. T h i s f u n c t i o n assumes t h a t t h e r e i s a c o n s t a n t s u r v i v a l r a t e which i s independent of the p o p u l a t i o n s i z e . The p r o p o r t i o n s u r v i v i n g i s e x p ( - r * t ) , where t i s time and r i s the m o r t a l i t y parameter. I used a c h i - s q u a r e goodness of f i t t e s t t o see i f t h i s model was a good f i t t o t h e s e d a t a . For the h i g h t i d e p o o l s , a l l the s u r v i v a l c u r v e s were good f i t s (p>.5) except 43 F i g u r e 7. S u r v i v a l r a t e s of b a t c h A and b a t c h B medium-sized s n a i l s a t t h e t h r e e s t a t i o n s of B a s a l t i c S h e l f A. Note t h a t i n b o t h c a s e s the s u r v i v a l r a t e i s l o w e s t a t the low s t a t i o n . Legend: b a t c h A date r e l e a s e d • 3.3m 3 1 . 5 * ( e x p ( - . 0 2 9 * t ) ) O 3.8a A 3.8b 4 6 . 5 * ( e x p ( - . 0 1 6 * t ) ) b a t c h B V^" date r e l e a s e d • 3.3m 2 5 . 7 * ( e x p ( - . 0 1 3 7 * t ) ) O 3.8a 4 7 . 0 * ( e x p ( - . 0 0 9 * t ) ) A 3.8b 4 9 . 0 * ( e x p ( - . 0 1 * t ) ) The f i t t e d n e g a t i v e e x p o n e n t i a l e q u a t i o n s a r e g i v e n f o r the lower s t a t i o n (see T a b l e V I I I ) and f o r the o t h e r s t a t i o n s where they were a good f i t t o the d a t a (p>.5). 45 F i g u r e 8. S u r v i v a l r a t e s of l a r g e , medium, and s m a l l - s i z e d s n a i l s t r a n s p l a n t e d t o the low s t a t i o n (3.3m) of B a s a l t i c S h e l f A. Note t h a t the s u r v i v a l r a t e s were u n a f f e c t e d by s n a i l s i z e . Legend: ^ date r e l e a s e d • l a r g e (9-llmm) A medium (6-7mm) A s m a l l (4-5mm) - i n a l l o t h e r e x p e r i m e n t s , small= 3-4mm 46 47 f o r the b a t c h A 3.8m(a) c u r v e (p<.05). In bo t h r e p l i c a t e s from the low t i d e p o o l , the c u r v e s were not a good f i t because the p r e d i c t e d i n i t i a l v a l u e u n d e r e s t i m a t e d the a c t u a l v a l u e (Table V I I I ) . T h i s d i s c r e p a n c y o c c u r r e d because the s u r v i v a l r a t e was p o o r e s t a t the low t i d e p o o l f o r the f i r s t s a m p l i n g p e r i o d . T h i s s u g g e s t s t h a t the s n a i l s a t the low t i d e p o o l were more a f f e c t e d by b e i n g p a i n t e d and d i s p l a c e d than were s n a i l s from the h i g h t i d e p o o l s . The d i s t u r b a n c e may have made the s n a i l s from low ar e a s more v u l n e r a b l e t o wave a c t i o n . T h i s e f f e c t would be s t r o n g e s t d u r i n g the f i r s t h i g h t i d e s a f t e r they had been r e t u r n e d t o t h e i r home t i d e p o o l . The r v a l u e s of the e q u a t i o n s shown i n F i g u r e 7 p r e d i c t t h a t f o r the b a t c h A s n a i l s , the pe r c e n t a g e of s n a i l s r e c a p t u r e d 60 days a f t e r t a g g i n g would be 17% a t the low a r e a and 38% a t the h i g h a r e a s . For the b a t c h B s n a i l s , 43% would s u r v i v e a t the low a r e a and 58% would s u r v i v e a t the h i g h a r e a . I s t h i s d i f f e r e n c e i n s u r v i v a l r a t e s s u f f i c i e n t t o e x p l a i n why t h e r e a r e no l a r g e s n a i l s found a t low l e v e l s of b a s a l t i c s h e l v e s ? To answer t h i s q u e s t i o n , I d i d an o t h e r s i m u l a t i o n model which i s o u t l i n e d i n Appendix I I I . The dynamics of a l a r g e s i z e c l a s s (6-8mm) was s i m u l a t e d f o r a 6m2 a r e a . Every two months., a c e r t a i n p r o p o r t i o n of the s m a l l s i z e c l a s s (4-6mm) grew i n t o the l a r g e s i z e c l a s s . T h i s r a t e was c a l c u l a t e d from the observ e d p r o p o r t i o n s of these s i z e c l a s s e s i n the p o p u l a t i o n and t h e i r observed growth r a t e s . A l s o , a p r o p o r t i o n ( l - e x p ( - r * t ) ) was l o s t every two months. The r e s u l t s of s e v e r a l s i m u l a t i o n s a r e g i v e n i n Appendix I I I , and F i g u r e 9 shows the 48 T a b l e V I I I . Observed numbers of s n a i l s r e c o v e r e d a t low t i d a l l e v e l s of B a s a l t i c S h e l f A and numbers of s n a i l s p r e d i c t e d from f i t t e d e q u a t i o n s . B a t c h A s n a i l s 31.5*exp(-.029*t) Time obs exp (obs-exp)/exp 0 50 31.5 10.9 4 30 . 28 .14 10 21 23.5 .26 17 13 19.2 2.0 24 18 15.6 .37 31 15 12.7 .42 45 4 8.5 2.3 63 7 5 .8 86 3 2.5 .1 99 1 1 0 Batc h B s n a i l s 25.7*exp(-.0137*t) Time obs exp (obs-exp)/exp 0 50 25.7 . 22.9 3 20 24.6 .86 10 22 22.4 .01 17 25 20.4 1.04 30 22 17 1.47 49 9 13.1 1.28 73 14 9.5 2.2 87 9 7.8 1.8 Note t h a t except f o r the i n i t i a l v a l u e s , the e q u a t i o n s p r o v i d e good p r e d i c t i o n s of the numbers of s n a i l s found a t a l l o t h e r s a m p l i n g p e r i o d s . I g n o r i n g the i n i t i a l v a l u e s , f o r b a t c h A,p>.5 f o r b a t c h B,p>.l 49 F i g u r e 9. S i m u l a t i o n s from b a s a l t i c s h e l f model t h a t show e f f e c t of s u r v i v a l r a t e and r e c r u i t m e n t r a t e of l a r g e s n a i l s on t h e i r s u r v i v a l a t low t i d a l l e v e l s . Note e f f e c t of b i r t h r a t e ( J = l , 3 ) on the s u r v i v a l of l a r g e s n a i l s . In these s i m u l a t i o n s , D2=2, D3=.25. The model i s o u t l i n e d i n Appendix I I I . Legend: G2= p r o p o r t i o n of s m a l l s n a i l s t h a t grow i n t o l a r g e s n a i l s R = r v a l u e i n e x p o n e n t i a l e q u a t i o n J ; e v e r y y e a r , #small= J * ( # l a r g e ) 51 e f f e c t s of • s n a i l growth and s u r v i v a l r a t e s on the l a r g e s i z e d s n a i l p o p u l a t i o n . Large s n a i l s s u r v i v e d under a l l c o n d i t i o n s , but a t much lower d e n s i t i e s when s n a i l growth and s u r v i v a l r a t e s were low (Table X V I I I ) . However, i t seems t h a t the lower s u r v i v a l r a t e s o b s e r v e d i n the f i e l d a t the low s t a t i o n a r e not s u f f i c i e n t , under the c o n d i t i o n s of the model, t o account f o r the absence of l a r g e l i t t o r i n e s . To s e p a r a t e wave a c t i o n m o r t a l i t y from p r e d a t i o n , I put out 60 l a r g e s n a i l s on l e a s h e s t h a t were a t t a c h e d t o cemented b a r s . The r e s u l t s a r e summarized i n Ta b l e IX. Seven of the 12 b a r s d i s a p p e a r e d i n two months. T h i s shows t h a t wave a c t i o n i s s t r o n g a t low t i d a l l e v e l s of b a s a l t i c s h e l v e s . However, wave f o r c e a c t i n g on a cemented bar i s not e q u i v a l e n t t o t h a t a c t i n g on a s n a i l . The s n a i l s t h a t were c o m p l e t e l y wrapped around the bar may have been v i c t i m s of wave a c t i o n . S n a i l s m i s s i n g c o u l d have been eaten or c o u l d have escaped from t h e i r l e a s h e s . A f t e r two weeks, t h e r e were 11/50 ( e q u a l t o an i n s t a n t a n e o u s m o r t a l i t y r a t e of -.018) p o s s i b l e v i c t i m s of wave a c t i o n and o n l y 5/50 (r=-.007) s n a i l s were m i s s i n g . T h i s experiment s u g g e s t s t h a t wave a c t i o n m o r t a l i t y o c c u r s and t h a t p r e d a t i o n r a t e s a r e much lower than on pebble beaches. I put out 12 l e a s h e d s n a i l s on pe b b l e beach A and most were m i s s i n g a f t e r one h i g h t i d e ; a l s o , s h e l l fragments remained a t t a c h e d t o some l e a s h e s . From the above d a t a , I s u s p e c t wave a c t i o n i s r e s p o n s i b l e f o r poor s u r v i v a l of s n a i l s a t low l e v e l s of b a s a l t i c s h e l v e s . I do not expect m o r t a l i t y from wave a c t i o n t o be c o n s t a n t . I t 52 Tab l e IX. F a t e s of 60 l e a s h e d s n a i l s put out a t low t i d a l l e v e l s of B a s a l t i c S h e l f A on August 2,1980. date checked August 18 October 27 FATES Number of s n a i l s Numbers s t i l l on l e a s h 34 2 wrapped around bar - a l i v e 8 5 -dead 3 6 s n a i l s m i s s i n g 5 9 s n a i l s a t t a c h e d 10 20 t o m i s s i n g b a r s of s n a i l s 60 42 53 s h o u l d v a r y w i t h t i d a l h e i g h t , sea sur g e , and season. S u r v i v a l r a t e s s h o u l d be lower i n the w i n t e r when t h e r e a r e more storms. T h i s c o u l d e x p l a i n why my obse r v e d summer s u r v i v a l r a t e s of marked s n a i l s were not s u f f i c i e n t t o e x p l a i n the absence of l a r g e s n a i l s from low l e v e l s of b a s a l t i c s h e l v e s . I c o n c l u d e t h a t s u r v i v a l i s h i g h e r a t h i g h t i d a l l e v e l s than a t low l e v e l s on b a s a l t i c s h e l v e s . T h e r e f o r e , s u r v i v a l i s an i m p o r t a n t mechanism a f f e c t i n g the s n a i l s i z e g r a d i e n t s on both b a s a l t i c s h e l v e s and p e b b l e beaches. Reduced s u r v i v a l a t low l e v e l s i s p r o b a b l y due t o p r e d a t i o n on p e b b l e beaches, and wave a c t i o n on b a s a l t i c s h e l v e s . Movement Pebble Beach E x p e r i m e n t s The movement e x p e r i m e n t s on pebble beach A t e s t e d i f t r a n s p l a n t e d s n a i l s c o u l d move homeward. Large s n a i l s t r a n s p l a n t e d downwards showed s i g n i f i c a n t movement up. The s n a i l s t h a t were t r a n s p l a n t e d l a t e r a l l y t o s e r v e as c o n t r o l s showed no p r e f e r r e d d i r e c t i o n of movement. The r e s u l t s a re summarized i n Table X and sample d a t a s e t s a r e shown i n F i g u r e 10. S m a l l s n a i l s t r a n s p l a n t e d upwards showed s t r o n g d i r e c t i o n a l movement down. However, the b e h a v i o u r of the c o n t r o l s was not c l e a r c u t . The p o s i t i o n s of the s n a i l s i n 3 of 6 c o n t r o l s were randomly d i s t r i b u t e d . The r e m a i n i n g t h r e e c o n t r o l s showed s i g n i f i c a n t movement up, but showed more 54 Ta b l e X. R e s u l t s of R a y l e i g h t e s t on p o s i t i o n s of s n a i l s 24 hours a f t e r r e l e a s e f r o m . o r i g i n on Pebble Beach A. The n u l l h y p o t h e s i s f o r the R a y l e i g h t e s t i s t h a t the d i s t r i b u t i o n s of p o s i t i o n s a re u n i f o r m or random. The a l t e r n a t e h y p o t h e s i s i s t h a t t h e r e i s a p r e f e r r e d d i r e c t i o n of movement ( i n t h i s example, up or down). Large S n a i I s S m a l l s n a i l s c o n t r o l s t r a n s p l a n t s c o n t r o l s t r a n s p l a n t s # of r e p l i c a t e s 4 8 6 9 d i r e c t i o n moved -random 4 2 3 1 -up 0 6 3 0 -down 0 0 0 8 mean v e r t i c a l d i s t a n c e moved(cm) -1 43.5' 20.5 -22.6 s.e. 9.7 3.5 5.1 3.7 Tab l e X b. Mean a n g u l a r d e v i a t i o n s of the mean d i r e c t i o n v e c t o r s e x p r e s s e d i n degr e e s ; the number of v e c t o r s i s i n p a r e n t h e s e s . A n g u l a r d e v i a t i o n i s the c i r c u l a r a n a l o g of s t a n d a r d d e v i a t i o n ( B a t s c h e l e t 1965). Large s n a i l s S m a l l s n a i l s c o n t r o l s -random -up 68 (4) (0) 66 (3) 54 (3) t r a n s p l a n t s -random -up -down 66 (2) 37*(6) (0) 67 (1) (0) 37*(8) * Note t h a t the v a l u e s a r e s m a l l e s t f o r the v e c t o r s of t r a n s p l a n t e d s n a i l s t h a t moved homeward. 55 F i g u r e 10. R e p r e s e n t a t i v e s c a t t e r diagrams and mean v e c t o r s of p o s i t i o n s of l a r g e and s m a l l s n a i l s 24 hours a f t e r r e l e a s e on Pebble Beach A. LARGE SNAILS '6 control -random n=4 transplant-up n = 6 transplant-random n=2 40cm SMALL SNAILS control -random • n^3 control -up n =3 transplant -down n=8 57 s c a t t e r about the mean d i r e c t i o n than the t r a n s p l a n t s d i d (Table Xb and F i g u r e 1 0 ) . T h e r e f o r e , the t r a n s p l a n t s showed s t r o n g e r d i r e c t i o n a l movement than the c o n t r o l s . The r e s u l t s demonstrate t h a t t r a n s p l a n t e d s n a i l s show s t r o n g d i r e c t i o n a l movement homeward. Movement c o u l d be c o n t r i b u t i n g t o the s i z e g r a d i e n t p a t t e r n on pebble beaches. Movement on B a s a l t i c S h e l v e s As l i t t o r i n e s become l a r g e r , do they move up from low l e v e l s of b a s a l t i c s h e l v e s ? T h i s s c e n a r i o i s not s u p p o r t e d by the movements of the s n a i l s marked a t the low t i d e p o o l of b a s a l t i c s h e l f A t o measure s u r v i v a l and growth ( T a b l e X I ) . There was l i m i t e d movement b o t h up and down from the low t i d e p o o l . The s n a i l s p a i n t e d i n May t o measure growth, showed a tendency t o move down. For example, 22 of the 81 r e c a p t u r e d medium s n a i l s moved down, but o n l y 2 moved up. The t r a n s p l a n t e d s n a i l s d i d not show any marked d i f f e r e n c e s . For example, 9 of the 117 l a r g e t r a n s p l a n t s moved up, but 8 moved down. There may be l i m i t e d movement upwards from low a r e a s of b a s a l t i c s h e l v e s , but the o b s e r v e d r a t e s a re not s u f f i c i e n t t o e x p l a i n the absence of l a r g e s n a i l s from low t i d a l l e v e l s . L i t t o r i n a s i t k a n a p o p u l a t i o n s , s t u d i e d on b a s a l t i c s h e l v e s and p e b b l e beaches, showed s i m i l a r w i t h i n - h a b i t a t v a r i a t i o n i n s u r v i v a l and growth. However, o n l y on pe b b l e beaches d i d t r a n s p l a n t e d s n a i l s show s t r o n g d i r e c t i o n a l movement homeward. T a b l e X I . P o s i t i o n s of marked s n a i l s r e l e a s e d a t the low t i d e p o o l of B a s a l t i c S h e l f A. Growth S n a i l s r e l e a s e d : d a t e checked May 23 May 30 Aug 8 Aug 20 T o t a l s May 13 ( b a t c h A) 8 0 s m a l l , 50 medium May 27 ( b a t c h B) 50 medium s i z e #found #up #down s i z e #found #up #down s 34 1 3 m 21 1 4 s 14 0 2 m 13 1 4 m 20 0 1 s 1 0 1 m 3 0 0 m 14 0 9 s 1 0 1 m 1 0 0 m 9 0 4 s 50 1 7 m 38 2 8 m 43 0 14 T r a n s p l a n t S n a i l s r e l e a s e d : June 26: August 2: 25 l a r g e , 25 medium 50 l a r g e , 50 medium date checked Aug 8 Aug 20 s i z e #found #up #down s i z e #found #up #down m 16 0 0 m 45 1 1 1 15 1 0 1 49 2 3 m 14 3 0 m 34 2 2 1 19 4 0 1 34 2 5 T o t a l s m 30 3 0 m 79 3 3 1 34 5 0 1 83 4 8 59 L i t t o r i n a . s c u t u l a t a L i t t o r i n a s c u t u l a t a was found w i t h L . s i t k a n a on pebble beaches and b a s a l t i c s h e l v e s i n B a r k l e y Sound. I c o n c e n t r a t e d my f i e l d e f f o r t s on L . s i t k a n a because i t i s e a s i e r t o mark and a l l l i f e h i s t o r y s t a g e s a r e p r e s e n t on the s h o r e . L . s c u t u l a t a showed s i m i l a r s i z e g r a d i e n t p a t t e r n s t o L . s i t k a n a . In b oth h a b i t a t s , s n a i l s i z e i n c r e a s e d w i t h t i d a l h e i g h t and the l a r g e s t s n a i l s were found a t h i g h l e v e l s of b a s a l t i c s h e l v e s ( F i g u r e 11). Both s p e c i e s were c o l l e c t e d t o g e t h e r i n the q u a d r a t s sampled. S i n c e I was p r i m a r i l y i n t e r e s t e d i n L . s i t k a n a , I d i d not i n c r e a s e sample s i z e where the d e n s i t y of L . s i t k a n a was h i g h but the d e n s i t y of L . s c u t u l a t a was low. To see i f L . s c u t u l a t a was a l s o v u l n e r a b l e t o p i l e p e r c h p r e d a t i o n at low t i d a l l e v e l s of pebble beaches, I put out 20 l a r g e L . s c u t u l a t a and 20 l a r g e L . s i t k a n a i n the f e n c e s on p e b b l e beach A. T a b l e X I I shows t h a t both s n a i l s p e c i e s had low s u r v i v a l r a t e s a t low t i d a l l e v e l s . A two way ANOVA showed no s p e c i e s p r e f e r e n c e (p>.75) but a g a i n showed s i g n i f i c a n t d i f f e r e n c e s between t i d a l h e i g h t s (p<.001). Both s p e c i e s may have no l a r g e i n d i v i d u a l s on low l e v e l s of p e b b l e beaches because of f i s h p r e d a t i o n . 60 F i g u r e 11. S i z e frequency d i s t r i b u t i o n s of L . s c u t u l a t a sampled i n 1980 on main study s i t e s . Sample s i z e i s g i v e n on each s i z e f r e q u e n c y h i s t o g r a m . Note t h a t i n b o t h h a b i t a t s , the l a r g e s t s i z e c l a s s e s are p r e s e n t o n l y a t h i g h t i d a l l e v e l s . T a b l e XIV i n Appendix I I I shows s i m i l a r t r e n d s f o r Pebble Beach B and B a s a l t i c S h e l f B; mean s n a i l s i z e i n c r e s e d w i t h t i d a l h e i g h t . 62 T a b l e X I I . Mean s u r v i v a l of 20 l a r g e L . s i t k a n a and 20 l a r g e L . s c u t u l a t a a f t e r one h i g h t i d e . The s n a i l s were p l a c e d i n e n c l o s u r e s at two t i d a l l e v e l s on f o u r o c c a s i o n s . mean # a l i ve s. e. t o t a l # fragments found t i d a l h e i g h t (m) low (1.5) m i d d l e (2.2) s n a i l s p e c i e s L . s i t k a n a L . s c u t u l a t a 11.3 1.6 44 12.4 0.9 35 s n a i l s p e c i e s L . s i t k a n a L . s c u t u l a t a 19.0 0.3 19.4 0.2 63 DISCUSSION T a b l e X I I I summarizes the s i z e g r a d i e n t p a t t e r n s observed i n L . s i t k a n a , and the way t h a t growth, s u r v i v a l and movement v a r i e d w i t h t i d a l h e i g h t . I t i s c l e a r t h a t s i z e g r a d i e n t s cannot be m a i n t a i n e d by v a r i a t i o n i n growth r a t e s . My d a t a s u p p o r t the h y p o t h e s i s t h a t poor s u r v i v a l a t low t i d a l l e v e l s m a i n t a i n s the s i z e g r a d i e n t s i n both h a b i t a t s . In a d d i t i o n , s n a i l movement h e l p s t o m a i n t a i n s i z e g r a d i e n t s on pebble beaches, but not on b a s a l t i c s h e l v e s . I w i l l d i s c u s s how s i z e g r a d i e n t s a r e m a i n t a i n e d i n each h a b i t a t and then compare the l i t t o r i n e s i n the two h a b i t a t s w i t h each o t h e r and w i t h s t u d i e s on s i z e g r a d i e n t s i n o t h e r i n t e r t i d a l a n i m a l s . Pebble Beaches On pe b b l e beaches,- l a r g e s n a i l s a re absent from low t i d a l l e v e l s and s m a l l s n a i l s a r e absent from h i g h t i d a l l e v e l s . My p r e d a t i o n and movement ex p e r i m e n t s s t r o n g l y suggest t h a t l a r g e s n a i l s a r e absent from low l e v e l s because of s i z e s e l e c t i v e p r e d a t i o n and s n a i l movement. The f i s h p r e d a t i o n model showed t h a t p r e d a t i o n a l o n e i s s u f f i c i e n t t o account f o r the absence of l a r g e s n a i l s from low t i d a l l e v e l s ( F i g u r e 6, Appendix I I I ) . P i l e p e r c h a t e b o t h l a r g e L . s i t k a n a and L . s c u t u l a t a t h a t were t r a n s p l a n t e d t o low l e v e l s . The f i s h showed no s p e c i e s p r e f e r e n c e (Table X I I ) . A l t h o u g h the model of f i s h p r e d a t i o n i n c l u d e d o n l y L . s i t k a n a , the h i g h d e n s i t y s i m u l a t i o n s c o u l d have d e s c r i b e d p r e d a t i o n on both l i t t o r i n e s p e c i e s . In a d d i t i o n , work by Behrens (1971) s u g g e s t s t h a t L . s c u t u l a t a 64 T a b l e X I I I . Summary of s i z e g r a d i e n t p a t t e r n s and r e s u l t s of e x p e r i m e n t s . PEBBLE BEACHES t i d a l h e i g h t s i z e c l a s s e s p r e s e n t r e s u l t s of e x p e r i m e n t s * h i g h m i d d l e low l a r g e a l l s m a l l Growth poor good v e r y good Pre d a t i o n . v e r y low low h i g h * movement- t r a n s p l a n t e d s n a i l s moved homeward BASALTIC SHELVES t i d a l h e i g h t s i z e c l a s s e s p r e s e n t r e s u l t s of e x p e r i m e n t s * Growth S u r v i v a l Wave A c t i o n h i g h a l l v e r y good h i g h low low s m a l l v e r y good low h i g h * movement- t r a n s p l a n t e d and l o c a l s n a i l s showed l i t t l e upward movement 65 grows s l o w e r and l i v e s l o n g e r than L . s i t k a n a . U s i n g growth and r e c r u i t m e n t r a t e s f o r L . s i t k a n a would o v e r e s t i m a t e the r e s i l i e n c e of the L . s c u t u l a t a p o p u l a t i o n t o p r e d a t i o n . T h e r e f o r e , I b e l i e v e t h a t p i l e p e r c h p r e d a t i o n a t low t i d a l l e v e l s can e x p l a i n the low abundance of l a r g e i n d i v i d u a l s of both l i t t o r i n e s p e c i e s f o r the f o l l o w i n g r e a s o n s : 1. The f i s h have enough a l t e r n a t e prey t o keep them v i s i t i n g the low t i d a l l e v e l a r e a s ; 2. The f i s h p r e f e r l a r g e s n a i l s and a r e a b l e t o f i n d and eat l a r g e s n a i l s a t low d e n s i t y . T h i s s i t u a t i o n i s s i m i l a r t o the p r e d a t o r - p r e y system s t u d i e d by C o n n e l l (1970). At low t i d a l l e v e l s , every b a r n a c l e was eaten w i t h i n two y e a r s of s e t t l i n g i n the study a r e a . The p r e d a t o r s ( s i x s p e c i e s of T h a i s ) p r e f e r r e d l a r g e b a r n a c l e s and had many a l t e r n a t e p r e y . They d i d not eat many b a r n a c l e s a t h i g h l e v e l s because the submergence p e r i o d was too s h o r t f o r them t o a t t a c k and eat a b a r n a c l e . The s i z e g r a d i e n t s o b s e r v e d i n t h e s e b a r n a c l e p o p u l a t i o n s were a l s o m a i n t a i n e d by s i z e s e l e c t i v e p r e d a t i o n a t low t i d a l l e v e l s . P i l e p e r c h may not fe e d a t h i g h t i d a l l e v e l s because the water l e v e l s a r e t o o s h a l l o w f o r them t o e n t e r and t h e r e a r e fewer a l t e r n a t i v e prey a v a i l a b l e . S m a l l s n a i l s c o u l d be s c a r c e a t h i g h t i d a l l e v e l s of pebb l e beaches because: (1) l a r g e s n a i l s do not l a y egg masses a t h i g h t i d a l l e v e l s ; (2) s m a l l s n a i l s do not move up and remain a t h i g h t i d a l l e v e l s ; (3) s m a l l s n a i l s t h a t move up i n t o h i g h t i d a l l e v e l s d i e q u i c k l y . My r e s u l t s suggest t h a t o n l y the 66 f i r s t two mechanisms a r e o p e r a t i n g . The t h i r d i s u n l i k e l y because the m o r t a l i t y r a t e s of s m a l l s n a i l s i n growth bags a t h i g h t i d a l l e v e l s were not h i g h ( T a b l e V I I ) . Most egg masses were o b s e r v e d a t mi d d l e t i d a l l e v e l s of peb b l e beaches ( T a b l e I I ) . They were c o n c e n t r a t e d i n damp a r e a s . Perhaps c o n d i t i o n s a t h i g h t i d a l l e v e l s a r e not s u i t a b l e f o r egg mass s u r v i v a l . T h i s h y p o t h e s i s c o u l d e a s i l y be t e s t e d e x p e r i m e n t a l l y s i n c e egg masses a r e found on the u n d e r s u r f a c e of r o c k s . The r o c k s c o u l d be t r a n s p l a n t e d t o h i g h and low l e v e l s and egg mass s u r v i v a l r e c o r d e d . S m a l l s n a i l s may a v o i d moving up t o h i g h a r e a s because growth c o n d i t i o n s a re poor. Slow growth r a t e s c o u l d lower the r e p r o d u c t i v e output of s n a i l s i f f e c u n d i t y i s d i r e c t l y r e l a t e d t o s n a i l s i z e , as i n L i t t o r i n a p l a n a x i s ( S c h m i t t , 1974). Why do l a r g e s n a i l s move up t o h i g h a r e a s ? Absence of f i s h p r e d a t i o n and reduced d e n s i t i e s may be p o s i t i v e f e a t u r e s of t h i s a r e a . However, growth r a t e s were poor a t h i g h t i d a l l e v e l s f o r a l l s n a i l s i z e s even a t the low d e n s i t i e s ( F i g u r e 6 ) . A l s o , s u r v i v a l of l a r g e and medium-sized s n a i l s was lo w e s t i n the h i g h t i d a l l e v e l growth bags ( T a b l e V I I ) . These r e s u l t s seem t o negate any p o s s i b l e a m e l i o r a t i v e e f f e c t s of reduced d e n s i t y . I t would be i n t e r e s t i n g t o know i f th e s e s n a i l s move down t o - l o w e r t i d a l l e v e l s t o l a y t h e i r egg masses. The m i d d l e t i d a l h e i g h t s of the peb b l e beaches s t u d i e d c o u l d be the most f a v o u r a b l e s n a i l h a b i t a t r e p r e s e n t i n g a good t r a d e o f f between growth ( F i g u r e .5) and s u r v i v a l ( T a b l e s I I I , V I I ) . The b u l k of egg masses were l a i d a t t h i s l e v e l . Yet 67 t h e r e were many s m a l l s n a i l s found at low t i d a l l e v e l s . T h i s s u g g e s t s t h a t egg mass and j u v e n i l e s u r v i v a l i s h i g h e s t a t low t i d a l l e v e l s and/or j u v e n i l e s m i g r a t e down t o low t i d a l l e v e l s . I found no marked d i f f e r e n c e s i n j u v e n i l e s u r v i v a l a t low and m i d d l e t i d a l h e i g h t s ( T a b l e V I I ) . Because few egg masses were found a t low t i d a l h e i g h t s , I suspect the h i g h d e n s i t y of j u v e n i l e s a t low t i d a l l e v e l s i s caused by j u v e n i l e s m i g r a t i n g down t o low t i d a l l e v e l s . The movement e x p e r i m e n t s show t h a t s n a i l s can move homeward when d i s l o d g e d . However, they do not d i f f e r e n t i a t e between a s n a i l r e t u r n i n g t o a t i d a l l e v e l t h a t i t i s h a b i t u a t e d t o and a s n a i l s e l e c t i n g a t i d a l l e v e l on the b a s i s of i t s s i z e . To d i f f e r e n t i a t e between these a l t e r n a t i v e s , l a r g e and s m a l l s n a i l s from m i d d l e h e i g h t s c o u l d be d i s l o d g e d both up and down the beach. I f the s n a i l s a r e j u s t h a b i t u a t e d t o a c e r t a i n t i d a l h e i g h t , I would e x p e c t no d i f f e r e n c e i n the s t r e n g t h of response between s n a i l s t r a n s p l a n t e d down and up from the r e l e a s e p o i n t . I f , on the b a s i s of t h e i r s i z e , s n a i l s a r e s e l e c t i n g t i d a l h e i g h t s I p r e d i c t : 1. Large s n a i l s t r a n s p l a n t e d down s h o u l d show s t r o n g e r homeward movement than l a r g e s n a i l s t r a n s p l a n t e d up t o h i g h l e v e l s ; 2. S m a l l s n a i l s t r a n s p l a n t e d up s h o u l d show s t r o n g e r homeward movement than s m a l l s n a i l s t r a n s p l a n t e d down t o low l e v e l s . D e s p i t e the a l t e r n a t e e x p l a n a t i o n f o r my t r a n s p l a n t e d s n a i l s moving homeward, I t h i n k the l i t t o r i n e s a r e s e l e c t i n g a 68 t i d a l h e i g h t on the b a s i s of t h e i r s i z e . A l l s n a i l s i z e s and egg masses a r e o n l y found t o g e t h e r at m i d d l e t i d a l l e v e l s . S m a l l s n a i l s are found a t m i d d l e and low l e v e l s , but a r e absent from h i g h l e v e l s . They may a v o i d h i g h l e v e l s because of poor growth c o n d i t i o n s . Large s n a i l s a r e found a t m i d d l e and h i g h t i d a l l e v e l s and may a v o i d low l e v e l s because of p r e d a t i o n . I n d i v i d u a l s t h a t a v o i d h i g h t i d a l l e v e l s when they a r e s m a l l and low t i d a l l e v e l s when they a r e l a r g e s h o u l d have h i g h e r s u r v i v a l and r e p r o d u c t i v e r a t e s than those t h a t do not a v o i d t h e s e a r e a s . F i s h p r e d a t i o n was an unexpected s t r u c t u r i n g agent on the pebble beaches t h a t I s t u d i e d . The r o l e of f i s h i n s t r u c t u r i n g temperate i n t e r t i d a l communities s h o u l d be more t h o r o u g h l y i n v e s t i g a t e d . For example, i n w a t c h i n g the low t i d a l l e v e l s t a t i o n of pebble"beach A throughout submergence, I noted t h a t most Hemigrapsus o r e g o n e n s i s and H. nudus were a c t i v e when the water l e v e l s were t o o s h a l l o w f o r the f i s h t o e n t e r , t h a t i s , when the t i d e was j u s t f l o o d i n g and r e c e d i n g . F i s h p r e d a t i o n may be one f a c t o r i n f l u e n c i n g the a c t i v i t y p a t t e r n s of these c r a b s . A l s o , Reimchen (1979) found t h a t the b l e n n y , B l e n n i u s  p h o l i s , s e l e c t i v e l y a l t e r e d c o l o r morph f r e q u e n c i e s i n B r i t i s h p o p u l a t i o n s of L i t t o r i n a m a r i a e . 69 B a s a l t i c S h e l v e s On b a s a l t i c s h e l v e s , a l l s n a i l s i z e s a r e found on the h i g h h o r i z o n t a l s h e l v e s . But o n l y s m a l l s n a i l s a r e found a t the low t i d a l l e v e l s . Growth ( F i g u r e 5) and movement (Table XI) cannot e x p l a i n t h i s p a t t e r n . S u r v i v a l v a r i e d w i t h t i d a l l e v e l i n the p r e d i c t e d d i r e c t i o n ( F i g u r e 7 ) . Reduced s u r v i v a l a t low l e v e l s c o u l d be due t o p r e d a t i o n or wave a c t i o n . P o s s i b l e p r e d a t o r s i n c l u d e s t a r f i s h , f i s h , and s h o r e b i r d s . The s t a r f i s h , P i s a s t e r o c h r a c e u s , i s common on the mussel zone of b a s a l t i c h e a d l a n d s . The mussel zone i s below the shoreward e x t e n s i o n of the l i t t o r i n e s . Menge (1972) found t h a t l i t t o r i n e s were a s m a l l component of the d i e t of t h i s s t a r f i s h . I never o b s e r v e d any P.ochraceus f e e d i n g i n the stu d y a r e a . Large p i l e p e r c h may f e e d i n the s e exposed a r e a s on the mussel beds. I t i s d o u b t f u l t h a t they would v e n t u r e up h i g h e r t o the wave break zone t o v i s i t a l e s s abundant food s o u r c e . A l s o , i f they d i d v i s i t t h i s a r e a , I would e x p e c t the l o s s r a t e of l a r g e l e a s h e d s n a i l s t o be much h i g h e r ( T a b l e I X ) . In the f a l l and s p r i n g , s h o r e b i r d s pass t h r o u g h B a r k l e y Sound. I obser v e d s e v e r a l f l o c k s of b l a c k t u r n s t o n e s ( A r e n a r i a melanocephala) and s u r f b i r d s ( A p h r i s a v i r g a t a ) a t low t i d a l l e v e l s of a few b a s a l t i c s h e l v e s . Smith (1952) sampled t h e s e s h o r e b i r d s i n a r e a s where L . s c u t u l a t a was p r e s e n t and found t h i s l i t t o r i n e i n b i r d s ' d i e t s . F e e d i n g s t u d i e s of r e l a t e d B r i t i s h s h o r e b i r d s have shown t h a t they may eat s m a l l (<5mm) l i t t o r i n e s ( P e t t i t t 1 975). S h o r e b i r d s may have a s e a s o n a l and l o c a l impact on low l e v e l s n a i l p o p u l a t i o n s . But b i r d p r e d a t i o n a l o n e cannot 70 e x p l a i n the l a r g e s c a l e absence ( p e r s o n a l o b s e r v a t i o n ) of l a r g e s n a i l s from low l e v e l s of a l l b a s a l t i c s h e l v e s . However, waves do a f f e c t the low t i d a l l e v e l s of a l l b a s a l t i c s h e l v e s . The h i g h l e v e l s a r e spared most of the wave f o r c e except d u r i n g severe w i n t e r storms. Many o t h e r l i t t o r i n e s t u d i e s have shown a n e g a t i v e c o r r e l a t i o n between exposure and s h e l l s i z e (James 1968, H e l l e r 1976, S t r u h s a k e r 1968). No one has c l e a r l y demonstrated t h a t t h i s c o r r e l a t i o n i s due t o s i z e s e l e c t i v e m o r t a l i t y by wave a c t i o n . S t r u h s a k e r (1968) and N o r t h (1954) both showed t h a t l a r g e r s n a i l s were l o s t i n f l o w tube e x p e r i m e n t s a t s i g n i f i c a n t l y h i g h e r r a t e s than s m a l l ones. However, o t h e r s argue t h a t a l a r g e f o o t s i z e makes l a r g e s n a i l s b e t t e r a b l e t o w i t h s t a n d wave a c t i o n ( H y l l e b e r g and C h r i s t e n s e n 1978). Perhaps t h i s e f f e c t i s . o v e r r i d d e n by the b e h a v i o u r a l mechanism of h i d i n g i n c r e v i c e s . S m a l l s n a i l s appear t o have a c c e s s t o more c r e v i c e s (and t h e r e f o r e p r o t e c t i o n from wave a c t i o n ) than l a r g e s n a i l s . On exposed coasts,- the s i z e d i s t r i b u t i o n and abundance of c r e v i c e s has been e x p e r i m e n t a l l y demonstrated t o a f f e c t the s i z e d i s t r i b u t i o n and abundance of B r i t i s h l i t t o r i n e s (Emson and F a l l e r - F r i t s c h 1976, R a f f a e l l i and Hughes 1978, Hughes and R o b e r t s 1981). In t h i s s t u d y , l a r g e and s m a l l e r - s i z e d s n a i l s d i s a p p e a r e d at s i m i l a r r a t e s ( F i g u r e 8 ) . E i t h e r wave a c t i o n was not s i z e s e l e c t i v e or the s m a l l e s t s n a i l s t r a n s p l a n t e d (4-5 mm) were above the s a f e s i z e t h r e s h o l d . The m a r k - r e c a p t u r e and l e a s h e x p e r i m e n t s suggest t h a t movement and p r e d a t i o n were not r e s p o n s i b l e f o r the reduced s u r v i v a l r a t e s a t low t i d a l l e v e l s 71 of b a s a l t i c s h e l f A. However, r e p e t i t i o n of a l l the above e x p e r i m e n t s under a v a r i e t y of weather c o n d i t i o n s i s needed t o draw f i r m e r c o n c l u s i o n s . On h i g h t i d a l l e v e l s of b a s a l t i c s h e l v e s , b oth growth and s u r v i v a l c o n d i t i o n s are good. The l a r g e s t i n d i v i d u a l s of L . s i t k a n a t h a t I found were i n t h i s h a b i t a t . These l a r g e s n a i l s were once thought t o be a s e p a r a t e s p e c i e s from the s m a l l e r s i z e d L . s i t k a n a p o p u l a t i o n s found i n o t h e r h a b i t a t s (Urban, 1962) . I o b s e r v e d l i t t l e upward movement of marked s n a i l s a t low t i d a l l e v e l s of b a s a l t i c s h e l f A. B a s a l t i c s h e l v e s a r e more heterogeneous s n a i l h a b i t a t than pebble beaches because the t i d e p o o l s are not always i n t e r c o n n e c t e d by s t r e a m s . A l s o , the d i r e c t i o n - i n which b a s a l t i c s h e l f A was f i r s t f l o o d e d v a r i e d w i t h sea surge and the wind d i r e c t i o n . T h e r e f o r e , I suggest t h a t the s n a i l s may not have c o n s i s t e n t o r i e n t i n g cues a v a i l a b l e t o e n a b l e them t o move homeward w h e n . d i s p l a c e d . G e n e r a l D i s c u s s i o n The s i z e d i s t r i b u t i o n of L . s i t k a n a a t low t i d a l l e v e l s i s a f f e c t e d by p r e d a t i o n on p e b b l e beaches, and wave a c t i o n on b a s a l t i c s h e l v e s . S i z e g r a d i e n t s o ccur because t h e r e a r e s p a t i a l r e f u g e s from t h e s e m o r t a l i t y agents a t h i g h t i d a l l e v e l s of b oth h a b i t a t s . The h y p o t h e s i s proposed by V e r m e i j (1972) t h a t j u v e n i l e m o r t a l i t y d r i v e s s i z e g r a d i e n t s i s not s u p p o r t e d by t h i s s t u d y . The common f e a t u r e of l i t t o r i n e s i z e g r a d i e n t s i n the two h a b i t a t s s t u d i e d i s the s e l e c t i v e 72 d i s t r i b u t i o n of a d u l t s . On b a s a l t i c s h e l v e s , egg masses and young were found a t a l l t i d a l l e v e l s ( T a ble I I , F i g u r e 4 ) . But on p e b b l e beaches, egg masses and young were absent from h i g h t i d a l l e v e l s . C e r t a i n l y , j u v e n i l e s u r v i v a l may c o n t r i b u t e t o t h e s e s i z e g r a d i e n t s and remains a l i k e l y h y p o t h e s i s f o r why b o t h young L . s i t k a n a and L . s c u t u l a t a were absent from h i g h l e v e l s of pebble beaches ( F i g u r e s 3,12). V e r m e i j ' s s y n t h e s i s of the l i t e r a t u r e and h y p o t h e s i s have s t i m u l a t e d r e s e a r c h on how and why s i z e g r a d i e n t s a r e m a i n t a i n e d . Recent s t u d i e s have not found j u v e n i l e s u r v i v a l t o be i m p o r t a n t i n m a i n t a i n i n g s i z e g r a d i e n t s ( B e r t n e s s 1977, B u t l e r 1979, R a f f a e l l i and Hughes 1978, Ma r k o w i t z 1980). P a i n e (1969) and M a r k o w i t z (1980) have l o o k e d a t why s i z e g r a d i e n t s occur i n Tegula f u n e b r a l i s . But the mechanism of how they a r e m a i n t a i n e d i s s t i l l not c l e a r . B u t l e r (1979) proposed a model f o r how s i z e g r a d i e n t s are m a i n t a i n e d i n s e v e r a l s p e c i e s of T h a i s , but i t has y e t t o be t e s t e d . He found t h a t he c o u l d not repeat the r e s u l t s of B e r t n e s s (1977). B e f o r e we can propose a r o b u s t t h e o r y f o r why s i z e g r a d i e n t s e x i s t , we must know how they a r e m a i n t a i n e d . S e v e r a l s t u d i e s c i t e d p r e v i o u s l y ( C o n n e l l 1970, James 1968, H e l l e r 1976, K i t c h i n g e t a l . 1959, S t r u h s a k e r 1968) and the p r e s e n t study, suggest t h a t the major f a c t o r m a i n t a i n i n g s i z e g r a d i e n t s i n upper shore a n i m a l s i s poor s u r v i v a l a t low t i d a l h e i g h t s . F u r t h e r m o r e , they suggest t h a t the the major m o r t a l i t y f a c t o r i s p r e d a t i o n i n s h e l t e r e d h a b i t a t s and wave a c t i o n i n exposed h a b i t a t s . Many o t h e r s t u d i e s have shown t h a t t h e e f f e c t of i n t e r t i d a l p r e d a t o r s on t h e i r prey i s much l e s s 73 on wave exposed area s than on s h e l t e r e d h a b i t a t s ( e . g . C o n n e l l 1972, Hughes and E l n e r 1979, Menge 1978a,b). More work needs t o be done on how s i z e g r a d i e n t s a r e m a i n t a i n e d i n lower shore g a s t r o p o d s . I t would be i n t e r e s t i n g t o compare the f a c t o r s t h a t m a i n t a i n s i z e g r a d i e n t s i n h e r b i v o r o u s and p r e d a t o r y g a s t r o p o d s . 74 REFERENCES B a t s c h e l e t , E. 1.965. S t a t i s t i c a l methods f o r the a n a l y s i s of problems i n a n i m a l o r i e n t a t i o n and c e r t a i n b i o l o g i c a l rhythms. AIBS monograph. 57pp. Behrens, S. 1971. The d i s t r i b u t i o n and abundance of the i n t e r t i d a l p r o s o b r a n c h s L i t t o r i n a s i t k a n a and L. s c u t u l a t a . M.Sc. 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P h y s i o l o g i c a l e c o l o g y and i n t e r t i d a l z o n a t i o n i n l i m p e t s (Acmaea): A c r i t i c a l l o o k a t l i m i t i n g f a c t o r s . B i o l . B u l l . 145: 389-422. Z i p s e r , E., G.J. V e r m e i j 1978. C r u s h i n g b e h a v i o r of t r o p i c a l and temperate c r a b s . J . Exp. Mar. B i o l . E c o l . 31:155-172. 79 APPENDIX Appendix I . Summary of mean s n a i l s i z e s of L. s i t k a n a and L . s c u t u l a t a sampled i n r e p l i c a t e p e b b l e beach and b a s a l t i c s h e l f h a b i t a t s i s shown i n T a b l e XIV. Appendix 11 . R e l a t i o n s h i p between l i p and l e n g t h L . s i t k a n a i s shown i n F i g u r e 12. increment i n 80 T a b l e XIV. Summary of mean s n a i l s i z e s i n r e p l i c a t e h a b i t a t s . Means a r e i n mm and sample s i z e s a r e i n p a r e n t h e s e s . PEBBLE BEACH B L . s i t k a n a L . s c u t u l a t a t i d a l h e i g h t (m) t i d a l h e i g h t (m) 1.5 2.2 2.8 1.5 2.2 2.8 J a n . 4.5 7.4 5.1 6.2 10.0 (185) (22) (0) (205) (89) (2) A p r i l 4.0 5.5 11.0 5.2 6.0 10.0 (220) (28) (2) (164) (181) (3) J u l y 4.5 6.0 8.0 5.1 5.5 8.3 (148) (31) (2) (221) (240) (17) BASALTIC SHELF B L . s i t k a n a L . s c u t u l a t a t i d a l h e i g h t (m) t i d a l h e i g h t (m) 3.3 3.8 3.3 3.8 A p r i l 3.5 6.2 5.3 8.2 (35) (259) (268) (143) J u l y 4.2 6.7 5.4 8.2 (18) (330) (167) (101) 81 F i g u r e 12. R e l a t i o n s h i p between l i p increment and l e n g t h i n c r e m e n t . A l l s n a i l s i z e c l a s s e s were p l o t t e d t o g e t h e r because r e g r e s s i o n s done s e p a r a t e l y were a l l h i g h l y s i g n i f i c a n t (p<.001) and a l l had s i m i l a r s l o p e s (p<.01). L E N G T H I N C R E M E N T (mm) 83 Appendix I I I . S i m u l a t i o n Models P r e d a t i o n r a t e s were h i g h e s t a t the low t i d a l l e v e l s of p e b b l e beach A. S u r v i v a l r a t e s were low e s t a t the low t i d a l l e v e l s of b a s a l t i c s h e l f A. To see i f the o b s e r v e d d i f f e r e n c e s were s u f f i c i e n t t o account f o r the low abundance of l a r g e s n a i l s a t low t i d a l l e v e l s of these h a b i t a t s , I made two s i m u l a t i o n models. The f i r s t e x p l o r e s t h e e f f e c t of f i s h p r e d a t i o n a t low t i d a l l e v e l s of pebble beaches. The second model e x p l o r e s the e f f e c t of reduced s u r v i v a l r a t e s a t low t i d a l l e v e l s of b a s a l t i c s h e l v e s . F i s h P r e d a t i o n Model The dynamics of the l a r g e s i z e c l a s s were s i m u l a t e d f o r a 40 by 4 m a r e a which i s e q u i v a l e n t t o the a r e a on pebble beach A t h a t the f i s h were most a c t i v e . The model makes the f o l l o w i n g a s s u m p t i o n s : 1. No l i t t o r i n e movement up t o h i g h l e v e l s ; 2. The d e n s i t y of l a r g e and s m a l l s n a i l s i s s i m i l a r t o t h a t o b s e r v e d a t 2.2m above datum on pebble beach A; 3. The number of young p r e s e n t i s d i r e c t l y r e l a t e d t o the number of a d u l t s . Every year-, the number of young i s se t e q u a l t o the number of a d u l t s a l i v e ; 4. The f i s h show a type two f u n c t i o n a l response t o s n a i l d e n s i t y ( H o l l i n g 1959). Growth, n a t u r a l m o r t a l i t y , and p r e d a t i o n r a t e s were d e t e r m i n e d from f i e l d d a t a and then v a r i e d f o r a range of i n i t i a l s n a i l 84 d e n s i t i e s . T a b l e XV o u t l i n e s the model. Each i t e r a t i o n i s e q u a l t o two months. Every 12 months (n=6), a p r o p o r t i o n of the l a r g e s i z e c l a s s d i e s due t o o l d age. A l s o , the number of young i s s e t e q u a l t o the number of a d u l t s t o p r o v i d e some feedback between the number of young produced and the number of a d u l t s p r e s e n t on the s h o r e . T h i s assumption may a r t i f i c i a l l y lower the r e s i l i e n c e of the l a r g e s n a i l p o p u l a t i o n t o p r e d a t i o n because the young may a c t u a l l y have i n c r e a s e d s u r v i v a l a t low d e n s i t i e s of a d u l t s and so may p r o v i d e more r e c r u i t s i n t o the l a r g e s i z e c l a s s . V a r i a b l e Range D2 and D3 both had v a l u e s of 2.5 i n the J u l y sample. I v a r i e d them both from 2-3. G2 was v a r i e d from .05 t o .2 k was v a r i e d from 2 t o 12 s was v a r i e d from 500 t o 10000 S2 and S3 were both v a r i e d s u b j e c t t o the c o n s t r a i n t t h a t they both add t o 0.6 m2 was v a r i e d from 0-0.5 m3 was v a r i e d from 0-0.8 T a b l e XVI g i v e s the r e s u l t s of v a r y i n g s e v e r a l of the p a r a m e t e r s . I f the p o p u l a t i o n d e c l i n e d , the i t e r a t i o n a t which i t went e x t i n c t i s g i v e n . The lower l i m i t s of the number of f i s h and t h e i r maximum f e e d i n g r a t e h i g h t i d e needed t o e l i m i n a t e the l a r g e s n a i l s (by MO=36) ar e a l s o g i v e n f o r a l l T a b l e XV. Model of the e f f e c t of f i s h p r e d a t i o n on s u r v i v a l of l a r g e s n a i l s a t low t i d a l l e v e l s of p e b b l e beaches. 10 d2=density of (5-7mm) s n a i l s i n a 10cm qu a d r a t 20 d3=density of (7-llmm) s n a i l s i n a 10cm qu a d r a t 30 g 2 = p r o p o r t i o n of (5-7mm) s n a i l s growing i n t o (7-llmm) s n a i l s / i t e r a t i o n 35 k= maximum # of s n a i l s e a t e n / f i s h / v i s i t 40 s= s n a i l d e n s i t y a t which f i s h eat K/2 s n a i l s / v i s i t 50 £2=#of f i s h i n a r e a 60 s 2 = p r o p o r t i o n of (7-llmm) s n a i l s t h a t a r e 1 year o l d 70 s 3 = p r o p o r t i o n of (7-llmm) s n a i l s t h a t a r e 1.5 y e a r s o l d 80 m 2 = p r o b a b i l i t y of d y i n g when 1 year o l d 90 m 3 = p r o b a b i l i t y of d y i n g when 1.5 y e a r s o l d 100 t= # of h i g h t i d e s per i t e r a t i o n 110 p r i n t D2,D3,G2,F2,S2,S3,m2,m3,s,T 120 sm=d2*16000=number of (5-7mm) s n a i l s i n a 40 by 4m a r e a 130 la=d3*16000=number of (7-llmm) s n a i l s i n a 40 by 4m ar e a 135 f l = k * l a / ( s + l a ) 140 f o r mo= 1 t o 20 150 n=n+l 160 i f n=6 go t o 400 170 l a = l a + ( g 2 * s m ) - ( f l * f 2 * t ) 180 p r i n t mo,sm,la 190 go t o 500 400 n=0 410 sm=la 420 l a = l a + ( g 2 * s m ) - ( f l * f 2 * t ) - ( l a * s 2 * m 2 ) - ( l a * s 3 * m 3 ) 430 p r i n t mo,sm,la 440 i f la<0 go t o 510 500 next mo 510 s t o p 86 Table XVI. Res u l t s of f i s h predation model s i m u l t i o n s . The i t e r t i o n (MO) at which the l a r g e s n a i l p opulation population (LA) went e x t i n c t i s given f o r a v a r i e t y of f i s h predation and s n a i l recruitment r a t e s . V a r i a b l e s are d e f i n e d i n Table XIV. V a r i a b l e s unless otherwise s p e c i f i e d : s2=.4, s3=.2, m2=.5, m3=.8 LA e x t i n c t at M0= at d2=d3=2.0 m2=m3=0 k 10 10 10 at d2=d3=2.5 10 10 m2=m3=0 10 at d2=d3=3.0 10 10 m2=m3=0 10 f2 s 22= .05 .10 .15 .20 6 500 6 7 9 13 6 10000 7 9 13 6 500 6 8 11 15 6 500 7 9 13 25 6 10000 8 12 19 inc 6 500 8 11 18 inc 6 500 8 12 18 inc 6 10000 10 14 29 inc 6 500 9 14 inc inc Lower l i m i t s of k,f2 r e q u i r e d f o r LA to go e x t i n c t by MO=36 (s=500) LA e x t i n c t at MO= at d2=d3=2.0 at f2=6, k> at k=10, f2> at d2=d3=2.5 at f2=6, k> at k=10, f2> at d2=d3=3.0 at f2=6, k> at k=10, f2> q2= .05 .10 1.5 1 2 1 2 2 4 2 4 3 5 3 ,15 6 4 7 4 8 5 20 / .05 .10 .15 .20 8 5 10 6 12 7 5 0 5 .232 24 24 25 30 31 18 21 32 34 29 25 36 23 34 25 36 31 34 25 23 31 30 36 87 i n i t i a l d e n s i t y and s n a i l growth r a t e s . T h i s model shows t h a t f i s h p r e d a t i o n has a s t r o n g e f f e c t on the l a r g e s i z e c l a s s . Under most c o n d i t i o n s (which were e x t r a p o l a t e d from f i e l d o b s e r v a t i o n s ) the l a r g e s n a i l p o p u l a t i o n went e x t i n c t . B a s a l t i c S h e l f Model T h i s model e x p l o r e s the e f f e c t of reduced r e c o v e r y r a t e s a t low t i d a l l e v e l s of b a s a l t i c s h e l v e s . I t i s o u t l i n e d i n Tabl e X V I I . The dynamics of a l a r g e s i z e c l a s s (6-8mm) was s i m u l a t e d f o r a 6m2 a r e a . Every 2 months, a p r o p o r t i o n of l a r g e s n a i l s d i s a p p e a r s , but a p r o p o r t i o n of s m a l l s n a i l s grows i n t o l a r g e s n a i l s . Every 12 months, t h e number of s m a l l s n a i l s i s a d j u s t e d t o the number of l a r g e s n a i l s p r e s e n t on the sho r e . V a r i a b l e Range D2 and D3 were c a l c u l a t e d from obse r v e d v a l u e s o b t a i n e d i n J u l y , October s a m p l i n g a t 3.3m. D2-observed 0.5,4.5; v a r i e d 2-4 D3-observed 0.2,0.2; v a r i e d 0.25-0.5 G2- • the mean l i p increment o b s e r v e d was 2.2mm which c o r r e s p o n d s t o 0.6mm i n c r e a s e i n s h e l l h e i g h t ( F i g u r e 1 7 ) . T h e r e f o r e , G 2 =proportion of (4-6mm) t h a t were >5.5mm; observed .125 i n J u l y and .36 i n O c t o b e r : v a r i e d .1-.4 R-from e q u a t i o n s shown i n F i g u r e 9;observed -.01,-. 0 3 , v a r i e d -.01,-.04 Table X V I I . Model of the e f f e c t of s u r v i v a l r a t e on the abundance of l a r g e s n a i l s a t low t i d a l l e v e l s of b a s a l t i c s h e l v e s . 10 d 2 = d e n s i t y of (4-6mm) s n a i l s per 10cm quadr a t 20 d 3 = d e n s i t y of (6-8mm) s n a i l s per 10 cm quadr a t 30 g 2 = p r o p o r t i o n of (4-6mm) s n a i l s growing i n t o (6-8mm) s n a i l s / i t e r a t i o n 40 r=r v a l u e f o r n e g a t i v e e x p o n e n t i a l 50 t= # of days per i t e r a t i o n 60 j=every 6 t h i t e r a t i o n , sm=j*la 70 p r i n t d 2,d3,g2,r,j , t 80 sm=d2*3600=number of (4-6mm) s n a i l s i n a 6m by 6m ar e a 90 la=d3*3600=number of (6-8mm) s n a i l s i n a 6m by 6m ar e a 105 f o r mo=l t o 20 110 n=n+l 120 i f n=6 go t o 400 130 r r = l a * e x p ( r * t ) 140 la=rr+(sm*g2) 150 p r i n t la,rr,sm,mo 160 next mo 400 n=0 410 sm=j*la 420 r r = l a * e x p ( r * t ) 430 la=rr+(sm*g2) 440 p r i n t la,rr,sm,mo 450 i f la<0 go t o 510 460 next mo 510 s t o p 89 T= number of days per i t e r a t i o n = 6 0 J= 1 or 3 T a b l e X V I I I shows the numbers of l a r g e and s m a l l s n a i l s p r e s e n t a t the 21st i t e r a t i o n of the model. I used another i n i t i a l d e n s i t y m i x t u r e (D2=2,D3=.5) which I d i d not i n c l u d e because i t gave v e r y s i m i l a r r e s u l t s t o D2=2,D3=.25. Under most c o n d i t i o n s , the l a r g e s n a i l p o p u l a t i o n d e c r e a s e d . But o n l y w i t h h i g h R and low G2 v a l u e s d i d i t become v e r y low. T h e r e f o r e , the observed d i s a p p e a r a n c e r a t e s of s n a i l s from low t i d a l l e v e l s of b a s a l t i c s h e l f A a r e not s u f f i c i e n t , under the a s s umptions of t h i s model, t o e x p l a i n the absence of l a r g e s n a i l s from low t i d a l l e v e l s of b a s a l t i c s h e l v e s . 90 Ta b l e X V I I I . R e s u l t s of s i m u l a t i o n s of b a s a l t i c s h e l f model showing numbers of l a r g e ( l a ) and s m a l l (sm) s n a i l s p r e s e n t a t MO=21. V a r i a b l e s a r e d e f i n e d i n T a b l e XVI.* J = 1 J= •1 J= 3 d2=2,d3=.25 d2=4,d3=.25 d2=2,d3=.2 l a sm l a sm l a sm r = - .01 4154 4378 8277 8724 174000 29883 r = - .02 552 788 1105 1576 30892 6173 r=- .04 132 269 265 540 9395 2183 r = - .01 259 346 515 688 7866 2054 r = - .02 26 52 52 103 1179 392 r = - .04 5 17 10 34 314 136 r = - .01 21 32 41 64 433 153 r = - .02 1.4 3.5 2.8 7 51 25 r = - .04 0.2 1 0.4 2 11 8 * the i n i t i a l v a l u e s f o r la,sm were la=900 a t d3 sm=7200 a t d2=2, sm=14400 a t d2=4. Note t h a t the B v a l u e s a r e = 2*A; t h e r e f o r e d2 j u s t e f f e c t s the magnitude and not the form of the response t o v a r i a t i o n i n s u r v i v a l and growth r a t e s . When the f e c u n d i t y of the l a r g e s n a i l s i s i n c r e a s e d (J=3), the l a r g e s n a i l p o p u l a t i o n a c t u a l l y i n c r e a s e s under most c o n d i t i o n s . 

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