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Characterization of a subtidal gastropod assemblage in the Strait of Georgia Cabot, Eric L. 1979

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OF A CHARACTERIZATION SUB-TIDAL GASTROPOD ASSEMBLAGE IN THE STRAIT OF GEORGIA BY ERIC L. CABOT B.A., Boston U n i v e r s i t y , 1974 THESIS SUBMITTED IN PARTIAL FULFILMENT THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Botany) We accept the t h e s i s as conforming to th r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1979 © E r i c L. Cabot, 1979 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. ERIC L. CABOT Department of „ BOTANY The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date A**/2K/?i-9 ABSTRACT A subtidal s i t e at Saturnina Island was sampled quantitatively to determine the seasonal and depth related trends of gastropod populations. The resulting density estimates were used to characterize the assemblage using d i v e r s i t y , niche-breadth, and cluster analyses. The relati o n s h i p between gastropod abundance and diatom density, as estimated from colonization of glass microscope s l i d e s , was investigatedi One experiment, which was conducted at the s i t e , was designed to determine whether the macrophytic algae s t i l l attracted the numerically dominant s n a i l after the algae had been either cleaned with hydrogen peroxide, or k i l l e d by immersion in 50° C. seawater. Most species attained t h e i r maximum development afte r recruitment i n spring or early summer. The greatest number of species was found at the shallowest station that was sampled, and abundances tended to decrease with depth with most species. Most of the s n a i l s had dispersion patterns that were simi l a r to those of the two dominant species Marqarites c o s t a l i s and Lacuna marmorata. Ninety-five percent of a l l the indi v i d u a l s c o l l e c t e d were found to belong to f i v e species. This high degree of numerical dominance severely affected the res u l t s of the d i v e r s i t y and the clus t e r analyses. Analyses of frequency vs. mean abundance, and niche-breadths revealed detailed information concerning the d i s t r i b u t i o n s of the gastropods that was not readily obtainable from graphs of mean density per quadrat. D i v e r s i t y , species richness, the t o t a l abundance of gastropods, and the densities of several species were correlated to the abundance of diatoms. Several species were also found to contain diatoms among t h e i r gut contents. Several other factors, including parasitism* predation* low s a l i n i t y - h i g h temperature water, and competition are discussed as factors, which, i n addition to diatom abundance, • A may have affected the gastropod dispersion patterns. i v TABLE OF CONTENTS ABSTRACT ............................................... i i LIST OF T ABL.ES ......................................... v i LIST OF FIGURES . . . . . . . . . . . . . ^ . . . . . . j . . . . . . . . . . . . . . . . . . . v i i ACKNOWLEDGEMENTS * . . . v i i i INTRODUCTION ............................................ 1 MATERIALS AND METHODS 5 S i t e D e s c r i p t i o n 5 Gastropod C o l l e c t i o n And Treatment ................... 6 Gut A n a l y s i s ........................................ 14 Diatom C o l l e c t i o n And Treatment 15 Ma r g a r i t e s c o s t a l i s Feeding Experiment . . i . . . . . . . . . . . . 15 A n a l y t i c a l Methods Used To C h a r a c t e r i z e The Gastropod Assemblage . ... . . i . ......... ........................ 20 Density , 20 T o t a l Abundance ................................... 20 Species Richness .................................. 20 Simpsons' Index .i ....................... . i . . , i . . . . . . 2 1 The Shannon-Wiener Index .......................... 22 C o n s i d e r a t i o n s Applying To Both D And H .... i . . . . . . . 23 The Evenness And Richness Components Of Heterogeneity 23 Sample S i z e , ± . . . . . . . . . . . . 23 Niche Breadth 25 C l u s t e r And Inverse C l u s t e r Analyses .............. 27 The Pair-group Method .......................... 27 The B r a y - C u r t i s S i m i l a r i t y Index 28 V RESULTS . . 30 The Gastropod assemblage ............................. 30 Abundance 30 Niche-breadth .. i . . . . . . . . . . . . . . . . . . . . . . . i . ; . .... . 40 Species Richness, T o t a l Abundance And Sample Heterogeneity . i . . . . , 49 C l u s t e r A n a l y s i s Of The Samples ............ i . . . . . ; 55 Inverse C l u s t e r A n a l y s i s Of The Species ......... 59 Gut Analyses 66 Diatom Abundance ..................................... 68 Feeding Experiment 73 DISCUSSION . . . . . . . i 75 C h a r a c t e r i z a t i o n Of. The Gastropod Assemblage ......... 75 The P o t e n t i a l Of Diatoms As A Gastropod Food Resource 80 The I n f l u e n c e Of Diatom Abundance On Gastropod D i s t r i b u t i o n s 83 P r e d a t i o n . . . . . . . . . . i . . . ' . 90 P a r a s i t i s m .1. ......... .. 93 S a l i n i t y And Temperature 96 The Role Of Substrate Requirements In H a b i t a t S e l e c t i o n 99 CONCLUSIONS .:. ............. i .. . 10 2 REFERENCES CITED .. ....... ...... i.k . . . . . . . i ...... 108 aPPENDIX A . ........................... I . . . . . . i . . . . . . . 12 2 APPENDIX B ........ ...123 APPENDIX C .............................................. 125 APPENDIX D ...... ..........................134 v i LIST OF TABLES Table I i Densities of 26 gastropod species i n 200 quadrats. ............ 31 Table I I . Occurrence and mean abundance of gastropod species. 33 Table I I I . Time-blocked analyses of variance of the mean densities of gastropods at four s t a t i o n s . ^ . . . . . . . i . . . . 35 Table IV. Differences i n s h e l l length increases i n mm as a function of time for Marqarites c o s t a l i s at stations 1 # 2, and 3. . ........... ................... 43 Table V. C l a s s i f i c a t i o n of gastropod species by INB values......................... ........ 47 Table VI. Spearman rank correlations and li n e a r regressions of t o t a l niche breadth to t o t a l density and occurrence 48 Table VII. Time blocked analyses of variance f o r assemblage parameters i n four sta t i o n s ; . . . . . . . . . . . . . . . 54 Table VIII. Gut content summary 67 Table IXi Time blocked analysis of variance f o r diatom density. 71 Table X. Spearman rank correlations of gastropod species and assemblage parameters to diatom density;...., 72 Table XI. One-way c l a s s i f i c a t i o n analysis of variance for the feeding experiments ........; 74 Table XII. Summary of parasite data........................ 94 V l l LIST OF FIGURES Fi g u r e 1 . L o c a t i o n of Saturnina I s l a n d i n the S t r a i t of Georgia, B.C...« . ............... 7 F i g u r e 2. C r o s s - s e c t i o n a l p e r s p e c t i v e view of the r e s e a r c h s i t e 9 F i g u r e 3. Schematic r e p r e s e n t a t i o n of the a i r l i f t sampler 11 F i g u r e 4., Schematic r e p r e s e n t a t i o n of submersible e n c l o s u r e used i n the f e e d i n g experiment.............. 17 Figure 5 . Mean d e n s i t y versus time............ . * . 3 8 F i g u r e 6; S h e l l l e n g t h versus time.........^............. 4 1 F i g u r e 7. INB (A) and t o t a l n i c h e breadth (B) f o r the gastropod s p e c i e s . . . . . . 4 5 F i g u r e 8: Species r i c h n e s s and the t o t a l abundance....... 50 F i g u r e 9.. Gastropod s p e c i e s d i v e r s i t y . ........ .i;,,± . 52 F i g u r e 1 0 . Dendrogram o f c l u s t e r a n a l y s i s . ... .. .1 1 ..... 5 7 F i g u r e 1 1 . Dendrogram of i n v e r s e c l u s t e r a n a l y s i s . . . . . . . . 60 F i g u r e 1 2 . Spearman rank c o r r e l a t i o n matrix of gastropod s p e c i e s . ............................. 6 2 F i g u r e 1 3 . Diatom d e n s i t y in.mm2 versus.date o f c o l l e c t i o n at s t a t i o n s 1 , 2 , and 3 . . . . . . . . . . . . . . . . . . . . . 6 9 F i g u r e 1 4 . Surface view of the Saturnina I s l a n d s i t e viewed from the E a s t . . . . . . . . . . . . 97 ACKNOWLEDGEMENT I am g r a t e f u l t o many people who helped i n the s u c c e s s f u l completion of t h i s study, but, above a l l , t o Dr. Ronald E. Foreman, who s u p e r v i s e d the work and provided space, m a t e r i a l s , and money, l a r g e l y through a grant from the F i s h e r i e s Research Board of Canada. I am e t e r n a l l y indebted to Mr. Barry D. Smith who was my d i v i n g buddy f o r most of the study; Dr. R.E. DeWreede a l s o helped with some of the d i v i n g and* along with Dr. P . a . Dehnel, reviewed the.manuscript. S p e c i a l thanks are due t o Dr. Mary Jo Duncan who o f f e r e d c o u n t l e s s u s e f u l suggestions as w e l l as moral support throughout the e n t i r e course of r e s e a r c h and w r i t i n g . Dr. H. Ching deserves mention f o r opening my eyes to the p o s s i b l e e c o l o g i c a l e f f e c t s of p a r a s i t i c i n f e s t a t i o n s . Mr. Tom N i e o l a s s i s t e d with computer programming, and Dr. Foreman s u p p l i e d s e v e r a l of the FORTRaN programs. 1 INTRODUCTION One approach to the study of organisms i n r e l a t i o n to t h e i r environment i s to c h a r a c t e r i z e the b i o t i c assemblage or community of a h a b i t a t i n terms of s p e c i e s composition and, i f p o s s i b l e , the p a t t e r n s of abundance and resource u t i l i z a t i o n . T h i s i n f o r m a t i o n i s an i n v a l u a b l e a i d i n f o r m u l a t i n g the course of f u r t h e r experimental i n v e s t i g a t i o n s i n t o s p e c i f i c i n t e r a c t i o n s between sp e c i e s and p a r t i c u l a r b i o t i c and p h y s i c a l components of the environment ( S h e l f o r d and Towler 1925, E l t o n 1927, Odum 1971, Whittaker 1975). In order t o a s c e r t a i n the e c o l o g i c a l r e l a t i o n s h i p s between c o - o c c u r r i n g s p e c i e s , d e t a i l e d i n f o r m a t i o n r e g a r d i n g the number of i n d i v i d u a l s , t h e i r d i s t r i b u t i o n s , and t h e i r resources held i n common must be obtai n e d . I t i s a l s o important to determine whether any shared resources are i n s h o r t supply, f o r i f not, then other f a c t o r s must c o n t r o l the abundances o f the s p e c i e s : Taxonomic a f f i n i t y provides a convenient and meaningful b a s i s f o r p a r t i t i o n i n g assemblages i n t o u n i t s of workable s i z e when i t i s not f e a s i b l e t o c o n s i d e r a l l organisms at once. The gr e a t e s t degree of o v e r l a p i n the resource u t i l i z a t i o n by co-o c c u r r i n g s p e c i e s i s expected f o r those organisms which have s i m i l a r p h y s i o l o g i c a l and s t r u c t u r a l f e a t u r e s , and, t h e r e f o r e , groups of ta x o n o m i c a l l y r e l a t e d organisms are of p a r t i c u l a r e c o l o g i c a l i n t e r e s t (King 1964). The gastropods, which are conspicuous upon rock and seaweed s u r f a c e s i n c o a s t a l marine environments are an e c o l o g i c a l l y d i v e r s e group of organisms (Morton and Yonge 1964, 2 Yonge and Thompson 1977), making them w e l l s u i t e d f o r i n i t i a l s t u d i e s of marine assemblages. Due to a wide range of morphological s p e c i a l i z a t i o n , gastropods are a b l e to u t i l i z e many consumer nich e s ; a d d i t i o n a l l y , there are s e v e r a l known examples of h a b i t a t d i v e r s i t y (Kohn 1959, 1968, 1971, 1976, T e s t 1945, H y l l e b e r g and F e n c h e l 1978), l i f e - c y c l e d i v e r s i t y (Spight 1976, Grahame 1976), and p h y s i o l o g i c a l d i v e r s i t y (Kingston 1968, Test 1945, Fenchel 1975a, 1975b, H y l l e b e r g 1975, Bertness and Schneider 1976) among gastropod congeners. E a r l y n a t u r a l i s t s were aware of the d i v e r s e gastropod fauna of the West coast of North america (see e.g. Darwin 1860, D a l l 1927, Olroyd 1928, 1935) but i n america, the few d i s p e r s i o n p a t t e r n s of marine gastropods t h a t are a c t u a l l y known are f o r i n t e r t i d a l s p e c i e s . In other areas, such as Danish f j o r d s (Fenchel 1975b, H y l l e b e r g 1975), and t r o p i c a l r e e f s (Kohn 1959, 1968, 1971, L e v i t e n 1978, Kohn and L e v i t e n 1976) , s t u d i e s of gastropod assemblages have: rev e a l e d the phenomenon of resource p a r t i t i o n i n g , whereby r e l a t e d s p e c i e s u t i l i z e d i f f e r e n t foods* or d i f f e r e n t s i z e s of the same foods i n order t o reduce c o m p e t i t i o n . Marine gastropod d i s p e r s i o n s have a l s o been s t u d i e d upon the e a s t e r n c o n t i n e n t a l s h e l f of North america (Franz 1977), the a t l a n t i c Ocean f l o o r (Rex 1977) , and the f j o r d s of Greenland (Thorson 1933). The m a j o r i t y of s t u d i e s of marine benthic assemblages have been conducted i n the more e a s i l y a c c e s s i b l e i n t e r t i d a l zone (see e.g. Southwood 1958, Nybakken 1978, Holland and Polgar 1976). E a r l y s u b t i d a l s t u d i e s were commonly r e s t r i c t e d t o areas 3 of r e l a t i v e l y soft and f l a t substrata, since animals and plants dwelling on rocky substrata cannot be e f f i c i e n t l y c o l l e c t e d with grabs or similar equipment. Therefore, since the nineteenth century, studies of i n t e r t i d a l and soft-substrate communities have flourished, but detailed quantitative studies of rocky subtidal assemblages were rare u n t i l after the advent of SCOBA i n 1943. With diving equipment, b i o l o g i s t s are able to obtain accurate ecological data, as well as d i r e c t observations as to the d i s t r i b u t i o n and behavior of marine organisms i n s i t u . It i s noteworthy that as early as 1930, Gislen u t i l i z e d hard-hat diving equipment and quadrat frames to enumerate the f l o r a and fauna of subtidal communities. Host workers have emphasized the roles of subtidal invertebrates as grazers (Saito and Nakamura 1961, C a r l i s l e et a l . 1964, North 1971, Paine and Vadas 1969, Forster 1959, L. Jones 1971, Leighton 1971, Carefoot 1967, Powell 1964), prey of f i s h (Quast 1968), and other animals (Paine 1965, R o b i l l i a r d 1971, Menge 1972), or as useful indicators of the severity of human-induced environmental perturbations such as o i l s p i l l s (D. Jones 1971, North et a l . 1964), kelp harvesting (Clendenning 1971) and sedimentation (Foreman 1975), but there have been few attempts to elucidate dispersion patterns of the animals as a means of understanding basic e c o l o g i c a l i n t e r r e l a t i o n s h i p s . The objectives of t h i s study are (1) to determine the seasonal and depth related dispersion patterns of shell-bearing gastropods i n a subtidal seaweed-dominated habitat i n the 4 S t r a i t of Georgia, and (2) to c h a r a c t e r i z e the e n t i r e assemblage by examining the d i s t r i b u t i o n of i n d i v i d u a l s among s p e c i e s and by comparing the i n d i v i d u a l d i s p e r s i o n patterns;. To c l a r i f y the d i s p e r s i o n p a t t e r n of the n u m e r i c a l l y dominant gastropod, Marqarites c o s t a l i s , the 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 o f t h i s s p e c i e s were examined. The u t i l i z a t i o n and abundance of b e n t h i c diatoms were i n v e s t i g a t e d under an i n i t i a l h y p o t h e s i s t h a t they are a major determinant of gastropod d i s p e r s i o n p a t t e r n s . T y p i c a l l y , s t u d i e s of animals on seaweed have been concerned with the number of s p e c i e s and i n d i v i d u a l organisms found on v a r i o u s p l a n t p a r t s ( i . e . , h o l d f a s t s , s t i p e s and blades) and s p e c i e s (Bergh 1871, Andrews 1925, Warmke and Almodovar 1963, Duffas 1969, G e h l a r d i 1971, Wing and Clendenning 1971, Nassichuck 1974, Smith 1973). T h i s approach presumes t h a t there i s some i n h e r e n t s u b s t r a t e s p e c i f i c i t y f o r a l l organisms which r e s i d e on p l a n t s i n the sea. In many cases t h i s may not be so, e s p e c i a l l y f o r motile animals* Since gastropods are o f t e n motile i t may be more d e s i r a b l e t o measure t h e i r a b s o l u t e d e n s i t i e s and d i s p e r s i o n p a t t e r n s through the use of guadrat c o l l e c t i o n s . 5 MATERIALS AND METHODS S i t e D e s c r i p t i o n A l l f i e l d work was conducted at a s h e l t e r e d s u b t i d a l s i t e adjacent to the southwestern shore of Sa t u r n i n a I s l a n d i n the c e n t r a l province o f the S t r a i t of Georgia, B.C., Canada, (49° 8 . 8 « N, 123° 40.3' W) (Figure 1 ) . Saturnina i s one of the F l a t Top I s l a n d s i n the Gulf I s l a n d s a r c h i p e l a g o . The F l a t Tops are d e s c r i b e d more f u l l y by Lindstrom and Foreman (1979). For a d e t a i l e d account o f the g e o l o g i c a l f e a t u r e s of t h i s p o r t i o n of the S t r a i t o f Georgia, r e f e r t o M u l l e r (1971). Appendix A l i s t s the common and dominant algae at the s i t e , most of which belong to a grouping t h a t has been named the shallow red a l g a l community by Lindstrom and Foreman (1979). The dense macrophytic cover harbors great numbers of minute gastropods. The s i t e s upper boundary i s approximately at Canadian datum (Lowest Lower Low Water), above which i s a narrow zone of barren rock. A substratum of sandstone bedrock o v e r l a i n by l a r g e f l a t b o u l d e r s (up to 4 sg. m i n area and 1 m i n height) extends i n t o the s u b t i d a l t o a depth of 3.5 m, a t which p o i n t t h e r e i s a change i n sl o p e with the boulders becoming more rounded and s m a l l e r (1 m or l e s s i n diameter). At 6 . 8 m below datum, the substratum a b r u p t l y changes t o a ge n t l y s l o p i n g * b r o k e n - s h e l l covered bottom, d e l i m i t i n g the lower boundary of 6 the s i t e . T h i s r e g i o n c o n t a i n s rocks (0.5 m or l e s s i n diameter) spaced at 4 t o 5 meter i n t e r v a l s . . At the lower s i t e boundary most a l g a l s p e c i e s are r e p l a c e d by a sparse e e l - g r a s s community; Gastropod C o l l e c t i o n And Treatment Within the s i t e , a permanent t r a n s e c t was p l a c e d p e r p e n d i c u l a r t o the shore to a po i n t 2.5 m past the change from boulder-covered bedrock slope t o s h e l l - c o v e r e d f l a t bottom. Four permanent s t a t i o n s were l o c a t e d at 10 m i n t e r v a l s along the t r a n s e c t l i n e (Figure 2) with the deepest s t a t i o n (no. 4) a t the t r a n s e c t endpoint. Boundary e f f e c t s and the i n f l u e n c e of t i d e s were minimized by p l a c i n g s t a t i o n 1 at a depth of 1.5 m below datum. Q u a n t i t a t i v e c o l l e c t i o n s were made at each s t a t i o n , i n an area extending 3 m on e i t h e r s i d e of the t r a n s e c t ; Each month, f o r a pe r i o d of one year beginning October, 1975, twenty 15 x 15 cm quadrat frames, f i v e at each s t a t i o n , were c o l l e c t e d with a SCUBA -operated a i r i i f t sampler (Foreman 1977) t h a t was f i t t e d with c o l l e c t i o n bags made from nylon s t o c k i n g s i n order t o r e t a i n minute gastropods ( F i g . 3 ) . A random numbers t a b l e was used to determine the placement o f quadrat frames along imaginary l i n e s extending p e r p e n d i c u l a r l y from the t r a n s e c t at each s t a t i o n . No frame was placed at a 7 F i g u r e 1- L o c a t i o n o f S a t u r n i n a I s l a n d i n t h e S t r a i t o f G e o r g i a , B.C. The s u b t i d a l r e s e a r c h s i t e i s denoted by an arrow; 8 gure 2. Cross-sectional perspective view of the research s i t e * Depths are r e l a t i v e to Canadian datum (LLLW) as recorded by a SCUBA diver with a hand-held depth gauge Rectangles i l l u s t r a t e sampling areas at each s t a t i o n . A-C correspond to substrate zones. A: gently-sloping bedrock overlain with f l a t boulders up to 4 m2 in area up to 1 m i n height. B: more steeply sloped bedrock overlain by round boulders up to 1 m in diameter. C: r e l a t i v e l y f l a t bottom of sand and broken s h e l l with small rocks of up to 0.5 m in diameter. 11 Figure 3 i Schematic r e p r e s e n t a t i o n of the a i r l i f t sampler developed by Foreman (1977)i The device i s c o n s t r u c t e d of PVC with brass f i t t i n g s . A threaded male PVC f i t t i n g with a s t e e l screw clamp (a) i s e a s i l y connected t o the a i l i f t which i s f i t t e d with a threaded female PVC j o i n t (b ) . Specimens enter the a i r l i f t a t c, p r o p e l l e d by compressed a i r from the f i r s t stage of a SCUBA r e g u l a t o r . A valve (d) c o n t r o l s the r e l e a s e of a i r . Specimens are r e t a i n e d i n a nylon s t o c k i n g ( e ) i A j -shaped brass f i t t i n g (f) d i r e c t s the a i r upwards. 10 cm 13 p r e v i o u s l y sampled point,. A l l animal and p l a n t m a t e r i a l l y i n g w i t h i n a guadrat was c o l l e c t e d . A group of r e p l i c a t e quadrats from a p a r t i c u l a r s t a t i o n and month i s r e f e r r e d t o , i n t h i s study, as a station-month sampling u n i t , or an SMU. Each monthly sample of twenty quadrats, t h e r e f o r e , r e p r e s e n t s f o u r SMU's. A f t e r two t o three weeks of d r y i n g i n the open a i r , the contents of the a i r l i f t bags were c a r e f u l l y brushed i n t o separate p l a s t i c bags f o r storage. Each sample was l a t e r passed through metal s i e v e s (U.S. Standard s i z e s : 0;84 mm, 0.46 mm, and 0.25 mm) and s h e l l - b e a r i n g gastropods were removed from the r e s u l t a n t f r a c t i o n s u sing a p a i r of f i n e f o r c e p s under a d i s s e c t i n g microcope. The presence o f t i s s u e or an operculum was used as a s i g n t h a t a specimen had been a l i v e at the time of c o l l e c t i o n . M a r g a r i t e s c o s t a l i s r e t a i n e d upon the l a r g e s t s i z e s i e v e were s i z e d by measuring the l o n g e s t s h e l l a x i s with a p a i r of v e r n i e r c a l i p e r s . These l e n g t h s were recorded to the n e a r e s t 0.5 mm. Smaller i n d i v i d u a l s were s i z e d a c cording t o which s i e v e they came to r e s t upon; S n a i l s p a s s i n g through a 0.25 mm s i e v e , f o r i n s t a n c e , were expected to have s h e l l l e n g t h s t h a t were l e s s than 0.25 mm. C a r e f u l examinations of the s i e v e - s o r t e d f r a c t i o n s o c c a s i o n a l l y r e v e a l e d the presence of i n d i v i d u a l s t h a t were too s m a l l f o r the given s i z e - c a t e g o r y . These i n d i v i d u a l s were then p r o p e r l y s i z e d with the c a l i p e r s ; A l l gastropod i d e n t i f i c a t i o n s were s u b s t a n t i a t e d by Dr. I. Mc Taggart Cowan' and the taxonomic nomenclature f o l l o w s Abbott 14 (1974) and C a r l t o n and Both (1975). Exact i d e n t i f i c a t i o n of some s p e c i e s ( i . e . C e r i t h i o p s i s sp., Odostomia sp., and an u n i d e n t i f i e d r i s s o i d gastropod) was i m p o s s i b l e , owing to inadequacies of the taxonomic l i t e r a t u r e . G O T A N A L Y S I S S n a i l s were c o l l e c t e d f o r gut a n a l y s i s on two o c c a s i o n s , June 10, and October 7, 1976, at p o i n t s 5 to 10 m north o f , and at the same depth as, s t a t i o n 1. Seaweed with s n a i l s on i t was placed i n t o p l a s t i c bags which were s e a l e d i n s i t u . The s n a i l s were removed from the seaweed and f i x e d i n 50% EtOH w i t h i n 10 minutes of c o l l e c t i o n . S h e l l s were softened by a 12 hour r i n s e i n Bouin's Pi c r o - F o r m o l f i x a t i v e (75 parts p i c r i c a c i d , 25 p a r t s f o r m a l i n , 5 p a r t s g l a c i a l a c e t i c acid) which was c l e a r e d with three 30 minute r i n s e s i n 50% EtOH. Specimens were st o r e d i n 100% EtOH u n t i l d i s s e c t i o n . S o f t t i s s u e s were exposed f o r d i s s e c t i o n by g e n t l e f l a k i n g of s h e l l m a t e r i a l with f o r c e p s . A s c a l p e l was used t o e x c i s e head and f o o t r e g i o n s , and the remainder of the t i s s u e was streched out upon a microscope s l i d e . Stomach contents were teased out of the s n a i l bodies under a d i s s e c t i o n microscope at 40x m a g n i f i c a t i o n . A f t e r t e a s i n g , l a r g e pieces of s n a i l remains were removed from the s l i d e , c o v e r g l a s s and d i s t i l l e d water were added, and the gut m a t e r i a l was viewed a t m a g n i f i c a t i o n s o f 400x and 970x. 15 'Diatom C o l l e c t i o n And Treatment On e i g h t o c c a s i o n s from July,1976 through March, 1977, concrete b r i c k s , upon which were fastene d three g l a s s microscope s l i d e s , were submerged a t s t a t i o n s 1,2, and 3 f o r 14 days (+- 4 h o u r s ) . S l i d e s were s e t i n place and r e t r i e v e d by SCUBA d i v e r s . Within minutes of r e t r i e v a l , a drop of mounting medium and a c o v e r s l i p were placed upon the exposed f a c e of each s l i d e . The mounting medium c o n s i s t e d of a 50% s o l u t i o n of corn syrup and 10% f o r m a l i n . The number of diatom c e l l s was recorded f o r f i f t e e n 0.177 sq. mm f i e l d s of each s l i d e at 400x m a g n i f i c a t i o n . Diatoms l a c k i n g pigmentation were not counted. The mean number of c e l l s per f i e l d was computed f o r each s l i d e and then f o r each s e t of t r i p l i c a t e s . I n d i v i d u a l f i e l d counts were transformed to c e l l s per sq. mm b e f o r e data a n a l y s i s . M ,a,.r , q_a r i . t .e^s. c_ o_ s_ t_a_ l _ i _ s _ Feeding Experiment An experiment was performed to determine i f the presence of e p i p h y t i c m i c r o f l o r a had any e f f e c t on the a f f i n i t y of M. c o s t a l i s t o seaweed; The experiment was conducted using t h r e e a r t i f i c i a l e n c l o s u r e s h e l d underwater at S a t u r n i n a I s l a n d during August and September, 1976. Twenty f r e s h l y c o l l e c t e d a d u l t M. c o s t a l i s 16 were placed i n each of the e n c l o s u r e s with f r o n d s of Plocamium c a r t i l a q i n e u m t h a t were e i t h e r dead, s u r f a c e - c l e a n e d , or i n the n a t u r a l s t a t e . Plocamium was s e l e c t e d because i t was h i g h l y abundant and easy to separate i n t o approximately equal s i z e d clumps; Algae f o r the experiment were t r e a t e d w i t h i n f i v e minutes of c o l l e c t i o n and immediately taken t o the e n c l o s u r e s . Fronds 10 cm i n l e n g t h and with approximately the same order of branching and t h i c k n e s s were used throughout; The P; c a r t i l a q i n e u m were a l l c o l l e c t e d from a s i n g l e patch t h a t was 10 m from, but a t the same depth as, s t a t i o n 2 on the t r a n s e c t . A f i v e minute bath i n 50° C seawater was used to k i l l the Plocamium and i t s e p i b i o n t s . C l e a n i n g was accomplished by f i v e minutes of immersion i n 10% hydrogen peroxide;. M i c r o s c o p i c examinations r e v e a l e d t h a t the s u r f a c e s of the heat k i l l e d algae s t i l l bore the remains of microrganisms but d i s p l a y e d severe c e l l u l a r damage; The c e l l s of the p e r o x i d e - t r e a t e d algae remained i n t a c t , but the p l a n t s u r f a c e s had become devoid of e p i b i o n t s . The untreated Plocamium, which was brought to the s u r f a c e and kept i n covered t r a y s of seawater, was found to support a dense cover of microorganisms* mostly diatoms. Each 'enclosure (Figure H) c o n s i s t e d o f a f l a t , round concrete base (27 cm i n diameter) with a removable cover o f 0.85 mm p l a s t i c mesh; A r t i f i c i a l h o l d f a s t s were c o n s t r u c t e d with two s i z e s of Nalgene tubing* one i n s i d e the other, with the outer segments v e r t i c a l l y embedded i n t o the c o n c r e t e ; P. c a r t i l a g i n e u m f r o n d s , were he l d by,, l o d g i n g , the s t i p e s f i r m l y ure 4 i Schematic r e p r e s e n t a t i o n of submersible e n c l o s u r e used i n the feeding experiment. A, a r t i c i a l h o l d f a s t ; B, top view of e n c l o s u r e base (not t o s c a l e ) ; C, s i d e view of e n c l o s u r e base; D s i d e view of e n c l o s u r e c o v e r i n g . L a b e l s : a, Plocamium c a r t i l a g i n e u m s t i p e ; b, Nalgene t u b i n g ; c, c o n c r e t e ; d r d e p r e s s i o n f o r v i a l ; e f p o l y e t h y l e n e rims cut from a bucket; f, p l a s t i c screen with 0.85 mm pores; h, a r t i f i c i a l h o l d f a s t ; i , f l a t i r o n bar handle. 18 19 between both p i e c e s of t u b i n g . The h o l d f a s t s were p o s i t i o n e d 1 cm from the base rim, i n f o u r l o c a t i o n s corresponding t o the major p o i n t s of a compass. Although the e n c l o s u r e s were themselves maintained i n l e v e l p o s i t i o n s , the general d i r e c t i o n of s l o p e at the s i t e was from west to e a s t , and, t h e r e f o r e , to confound any f i x e d d i r e c t i o n a l b i a s i n the movement of Mi c o s t a l i s , the choice of h o l d f a s t i n each t r i a l (N-S vs E-W) was determined by a c o i n t o s s . The a l l o c a t i o n o f algae to the v a r i o u s treatments was randomized by a l o t t e r y . S n a i l s were c a r r i e d t o the e n c l o s u r e s i n 16 dram p l a s t i c medicine v i a l s , each with f o u r e q u a l l y spaced 1x1 cm p o r t s c u t i n the s i d e ; The p o r t s were s e a l e d with c l o t h s t r i p s u n t i l the v i a l s were i n p l a c e i n the c e n t e r s of the bases with the p o r t s f a c i n g the h o l d f a s t s . S i l a s t i c (patented) s i l i c o n e adhesive was used to h o l d the v i a l s i n shallow depressions and to form a continuous s e a l with the c o n c r e t e s u r f a c e . In order to reduce any t o x i c i t y of the s i l i c o n e or c o n c r e t e , the e n c l o s u r e s and f i t t i n g s were seasoned by immersion, i n s i t u ; f o r three months p r i o r t o the experiment. A f t e r 24 hours, the number of s n a i l s i n c o n t a c t with P. c a r t i l a g i n e u m of d i f f e r e n t treatments was recorded, a l l s n a i l s removed, the e n c l o s u r e s cleaned by vigorous hand-generated water movement and a new experimental t r i a l begun. A one-way ANOVA was used t o t e s t whether the seaweed i n a l l t h r e e c o n d i t i o n s was contacted by t h e same number of s n a i l s . 20 A n a l y t i c a l Methods.Used To Characterize The Gastropod Assemblage DENSITY The basic abundance measure used, density, i s the mean number of ind i v i d u a l s of a given species per quadrat i n samples of several quadrats- Mean density (n) refers to the mean of 5 quadrats comprising each SMO, while t o t a l density refers to samples containing a l l the quadrats of the study. Density i s the product of two components, the percentage of quadrats i n the sample which contain the species, or the occurrence, and the mean number of individ u a l s per occurrence. TOTAL ABUNDANCE Total abundance (N) i s the sum of a l l of the species densities i n a sample. SPECIES RICHNESS The number of richness (S) . This (Peet 1974). species found i n a sample i s the simplest measure of i s the species species d i v e r s i t y 21 SIMPSONS' INDEX Simpson's index of c o n c e n t r a t i o n , or dominance, c o n t a i n s components r e l a t e d to both the s p e c i e s r i c h n e s s and the evenness of the d i s t r i b u t i o n of i n d i v i d u a l s among s p e c i e s , and i s an index of the apparent d i v e r s i t y or c o m p o s i t i o n a l h e t e r o g e n e i t y of a sample (Simpson 1919). T h i s index was o r i g i n a l l y borrowed from the f i e l d of l i n g u i s t i c s ( G i n i 1912) and i n an e c o l o g i c a l context i s used as a measure of the p r o b a b i l i t y t h a t two randomly s e l e c t e d i n d i v i d u a l s from a sample w i l l belong t o the same s p e c i e s . A sample t h a t i s e i t h e r s p e c i e s poor, or has most i n d i v i d u a l s concentrated among a few s p e c i e s , has l i t t l e "apparent d i v e r s i t y " s i n c e t h e r e i s a l a r g e chance o f two i n d i v i d u a l s of the same s p e c i e s being obtained by a random draw; P i e l o u (1.967) modified the o r i g i n a l form of Simpson's index t o d e r i v e the f o l l o w i n g unbiased e s t i m a t o r of sample h e t e r o g e n e i t y : s D=l-Z {[n. ( n . - l ) 7//"N(N-l) ]•} i = l 1 1 The value belong t o a rep r e s e n t e d by of D v a r i e s from s i n g l e s p e c i e s , one i n d i v i d u a l . z e r o , when a l l i n d i v i d u a l s t o u n i t y , when each s p e c i e s i s 22 THE SHANNON-WIENER INDEX The Shannon-Weiner index, H*, i s mathematically r e l a t e d to D, but has i t s o r i g i n s i n the i n f o r m a t i o n theory of communications (Shannon and Weaver 1949). Like D, H1 i s used to estimate the degree of c o m p o s i t i o n a l h e t e r o g e n e i t y o f a po p u l a t i o n from the r e l a t i v e abundances of s p e c i e s i n a r e p r e s e n t a t i v e sample* Stated f o r m a l l y , H* i s the average amount of u n c e r t a i n t y i n v o l v e d i n p r e d i c t i n g the i d e n t i t y o f a sp e c i e s i n a sample, and i t i s measured i n b i t s of i n f o r m a t i o n per i n d i v i d u a l as (Patten 1962) : s H ' = - Z { ( n . / N ) l o g 2 ( n . / N ) } i = l 1 1 An i n c r e a s e i n e i t h e r the number of s p e c i e s or the evenness of t h e i r p r o p o r t i o n a l r e p r e s e n t a t i o n s w i l l i n c r e a s e the uncertainty, i n p r e d i c t i o n , and thus H* i s r e l a t e d to he t e r o g e n e i t y i n the same manner as D. Onlike D, however, H* ranges from 0 t o i n f i n i t y . 23 CONSIDERATIONS APPLYING TO BOTH D AND H The Evenness And Richness Components Of Heterogeneity There are s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n s of the val u e s of D and H» to both of t h e i r r e s p e c t i v e components of r i c h n e s s and evenness (De Jong 1975), However i n each case, i t i s i m p o s s i b l e t o r e s o l v e which component has the gr e a t e r i n f l u e n c e over the h e t e r o g e n e i t y index without independent measurement. Although s e v e r a l evenness i n d i c e s have been proposed ( P i e l o u 1977), a l l are mathematically dependent upon s p e c i e s r i c h n e s s , and, t h e r e f o r e , can only be used f o r comparisons of the evenness of samples with the same number of s p e c i e s (Peet 1974), However, d i f f e r e n c e s i n evenness between a group o f samples can be q u a l i t a t i v e l y assessed by comparisons of the concomittant behavior of S^ N, and,the h e t e r o g e n e i t y i n d i c e s . Sample S i z e The evenness component of D depends e n t i r e l y on the most abundant s p e c i e s of sample (Peet 1974) , while the i n t e r m e d i a t e s p e c i e s ( s p e c i e s whose d e n s i t i e s approach N/2) c o n t r i b u t e the most i n f o r m a t i o n t o the H' evenness component (Farger 1972) . H' i s a l s o more s e v e r e l y a f f e c t e d by equal changes i n the abundance of r a r e s p e c i e s than of dominants, although dominant s p e c i e s are not completely ignored (Peet 1974). Thus, comparisons of D and H» may r e v e a l , i n some cases, whether evenness changes are due t o r a r e or common s p e c i e s . 24 Because the e x c l u s i o n of r a r e s p e c i e s has a l a r g e e f f e c t upon the r e l a t i v e magnitude of H« (Peet 1974, Whittaker 1972), the H 1 index i s s u s c e p t i b l e t o severe sampling e r r o r s when too small an area i s sampled ( P i e l o u 1977); To counter t h i s e f f e c t and yet permit comparisons between other s t a t i s t i c a l t echniques used* the f i v e r e p l i c a t e quadrats of each SMU are pooled f o r the c a l c u l a t i o n of h e t e r o g e n e i t y , r i c h n e s s , and t o t a l abundance. The use of a l a r g e r sample s i z e a l s o i n c r e a s e s the chance o f o b t a i n i n g a more r e p r e s e n t a t i v e accounting of the tr u e s p e c i e s p r o p o r t i o n s w i t h i n an assemblage. There are two reasons why i t i s b e t t e r to lump the guadrats f i r s t and then t o d e r i v e h e t e r o g e n e i t y and r i c h n e s s , r a t h e r than to average i n d i c e s c a l c u l a t e d from the i n d i v i d u a l r e p l i c a t e s ; F i r s t l y , h e t e r o g e n e i t y i n d i c e s are r a r e l y a d d i t i v e s i n c e each o f the pooled samples would have t o have completely unique s p e c i e s compositions, but the same degree of hete r o g e n e i t y - Furthermore, s p e c i e s r i c h n e s s can only be a d d i t i v e when r e p l i c a t e s have no s p e c i e s i n common. C a l c u l a t i o n e i t h e r before or a f t e r averaging produces the same r e s u l t s f o r t o t a l abundance; 25 NIGHE BREADTH According to Levins (1968), the breadth of a s p e c i e s ' niche can be assessed i n terms of the extent or evenness of i t s d i s t r i b u t i o n through a range of h a b i t a t s , or degrees of the a v a i l a b i l i t y of an important r e s o u r c e . A l l a n (1975), P i e l o u (1972), and C o l w e l l and Futayama (1971), f u r t h e r e l a b o r a t e d L e v i n s ' mathematical model, and extended i t t o permit simultaneous comparisons of the h e t e r o g e n e i t y of s p e c i e s abundances through s e v e r a l h a b i t a t s or re s o u r c e s . H a b i t a t can be p a r t i t i o n e d i n t o two dimensions i n t h i s study, time and depth; In terms of these two h a b i t a t dimensions, the t o t a l n i c h e breadth of i t h s p e c i e s i s ( A l l a n 1975) : q u B ( t ) . = - E £ { ( n . . , / n . . . ) l o g 2 ( n . . , / n . . . ) } 1 j = l k = l 1 3 1 i : , k 1 where i , j , and k, and, s, q, and u are s u b s c r i p t s f o r s p e c i e s , time, and depth, r e s p e c t i v e l y , ; A dot i n place o f a s u b s c r i p t i n d i c a t e s summation f o r a l l valu e s of t h a t s u b s c r i p t , as i s standard n o t a t i o n . In the same f a s h i o n as the p a r t i t i o n i n g of the squares i n an ANOVA model, the t o t a l n i c h e breadth i s separated i n t o two components whose magnitudes r e f l e c t the r e l a t i v e c o n t r i b u t i o n s of a s p e c i e s ' d i s t r i b u t i o n through depth and time. 2 6 The t i m e - r e l a t e d niche breadth o f the i t h s p e c i e s i s (A l l a n 1975) : q B(m).=-E { (n . . ./n.. . . ) l o g 2 (n . . ./n . . . ) } 1 j = 1 1D i i : i The nic h e breadth of the i t h s p e c i e s at the j t h time (the term i n brackets) i s weighted by the p r o p o r t i o n of i n d i v i d u a l s o c c u r r i n g at that time t o produce the average: d e p t h - r e l a t e d n i c h e breadth ( A l l a n 1975) : B (d) . =B (t) . -B (.m) . l l l = £ (n .../n ...):'.-£{ Hn ... /n ....) i o g 2 Cn ./n ...) 7 } j = 1 I D i k = 1 i ] k I D i ] k i3 Note t h a t s i n c e the depth component of B(t) i s nested w i t h i n the component f o r time, the niche breadth model r e q u i r e s that the same number of depths be sampled each month. The r a t i o of B(m) to B(t) prov i d e s an index (INB) of the deqree of v e r t i c a l r e s t r i c t i o n t h a t e x i s t s i n a s p e c i e s ranqe. When a ' s p e c i e s i s r e l a t i v e l y r e s t r i c t e d i n depth, most of the breadth of i t s t o t a l n i c h e , i s due to the he t e r o g e n e i t y of i t s d i s p e r s i o n through time, and i t w i l l have a value of INB t h a t i s c l o s e to u n i t y . When the i n d i v d u a l s of a s p e c i e s are approximately evenly apportioned between depth and time, the value o f INB i s nea r l y equal to ( t ) / ( t + d ) , where t and d re p r e s e n t the r e s p e c t i v e number of time and depth u n i t s i n use. 27 CLOSTEE AND INVEESE CLUSTER ANALYSES The Pair-group Method C l u s t e r a n a l y s i s i s a method of c l a s s i f y i n g e n t i t i e s i n t o s u c c e s s i v e l y l a r g e r h i e r a r c h i c a l groupings based upon m u l t i v a r i a t e i n f o r m a t i o n contained i n each e n t i t y . O r i g i n a l l y a p p l i e d to b e h a v i o r a l s c i e n c e s , c l u s t e r i n g techniques have been adapted and g r e a t l y e l a b o r a t e d by numerical taxonomists who use l i s t s o f morphometric a t t r i b u t e s to c l a s s i f y taxa of organisms (Sneath and Sokal 1976). The same methods are used to order e c o l o g i c a l data, except s i t e s or samples are compared i n s t e a d of taxonomic u n i t s , and the a t t r i b u t e s c o n s i s t of the abundance values of the s p e c i e s t h a t are i n the samples. The data matrix of s p e c i e s i n samples i s used to generate a hemi-matrix of s i m i l a r i t y values f o r every p o s s i b l e p a i r o f samples. This s i m i l a r i t y matrix i s u t i l i z e d i n the c l u s t e r i n g procedure to produce groups of samples with the h i g h e s t p o s s i b l e i n t e r n a l s i m i l a r i t y . The outcome of a c l u s t e r a n a l y s i s i s h i g h l y dependent upon the choice of both the c l u s t e r i n g s t r a t e g y and the s i m i l a r i t y index; The B r a y - C u r t i s s i m i l a r i t y index i s used i n t h i s study, and i s presented i n d e t a i l i n the next s e c t i o n . The unweighted pair-group method (Sokal and Michner 1958) used i n t h i s study i s among the most s t r a i g h t f o r w a r d and widely used c l u s t e r i n g procedures. In what i s known as the f i r s t c l u s t e r i n g c y c l e , the p a i r of samples with the h i g h e s t 28 s i m i l a r i t y i s fused as a c l u s t e r and the s i m i l a r i t y index i s saved as the c l u s t e r i n g l e v e l of the f i r s t group. The two rows of the s i m i l a r i t y matrix which correspond t o the p a i r a re r e p l a c e d by one row which c o n t a i n s the a r i t h m e t i c average o f the two rows, f o r every column. The same b a s i c procedure i s c a r r i e d out i n s u c c e s s i v e c y c l e s , with c l u s t e r s being t r e a t e d i n the same f a s h i o n as i n d i v i d u a l samples, u n t i l a l l of the samples have been fused i n t o a s i n g l e group. A dendrogram i s used to i l l u s t r a t e the h i e r a r c h i c a l amalgamation of the samples. I n v e r s i o n of the e n t i r e s i t e - s p e c i e s data matrix permits the c l u s t e r i n g of s p e c i e s on the b a s i s of some measure of abundance i n the v a r i o u s samples by the same procedure. T h i s technigue i s r e f e r r e d t o a i n v e r s e c l u s t e r a n a l y s i s (Sneath and Sokal 1976). The Bray- C u r t i s S i m i l a r i t y Index The B r a y - C u r t i s s i m i l a r i t y index i s used i n t h i s study not only because i t has been employed i n a l a r g e number of e c o l o g i c a l s t u d i e s , but a l s o because i t i s r e l a t i v e l y simple and, thus, produces e a s i l y understood r e s u l t s , In g e n e r a l terms, a simple index of the minimum abundance shared between the h y p o t h e t i c a l samples A and B i s expressed as (Motyfca et a l . 1950) : s I =2 (S C . ) /N + N S . . , i A B 1 = 1 29 where c i s the minimum abundance'of the i t h s p e c i e s t h a t i s common to both s i t e s and N i s the t o t a l abundance i n each sample. Mean d e n s i t y i s the abundance measure used t o c a l c u l a t e the s i m i l a r i t y matrix f o r the gastropods o f t h i s study. A d o u b l e - s t a n d a r d i z a t i o n i s performed upon the matrix i n o r d e r t o reduce v a l u e s of d e n s i t y f o r a l l s p e c i e s t o s c a l e s of comparable range, and to prevent a s m a l l number of s p e c i e s o f wide-ranging abundance from a u t o m a t i c a l l y dominating the remainder (Williams 1971),. The maximum abundance value of each s p e c i e s i s r e s e t t o 100, and the remaining values s c a l e d a c c o r d i n g l y . Then the values i n every sample are ad j u s t e d so th a t the t o t a l abundance i n each sample i s 1.0. With double s t a n d a r d i z a t i o n the s i m i l a r i t y index reduces to (Bray and C u r t i s 1957): s I = 1 c . BC . , l i = l T h i s index ranges from 0 t o u n i t y when there i s 100% s i m i l a r i t y between samples; The B r a y - C u r t i s index i s i n s e n s i t i v e t o unique s p e c i e s and ze r o - z e r o matches; Because o f the s t a n d a r d i z a t i o n and the use of minimum* r a t h e r than average, shared abundance, the index reduces the numerical dominance of one s i t e over another. 30 RESULTS The Gastropod Assemblage Most of the a n a l y t i c a l methods employed i n the c h a r a c t e r i z a t i o n o f the gastropod assemblage r e q u i r e t h a t the same number of depths be sampled i n each month. Samples from October, S t a t i o n 4 and November, S t a t i o n 1 were wet-sorted and y i e l d e d s u s p i c i o u s l y low mean d e n s i t i e s f o r most s p e c i e s , presumably because the presence of l a r g e amounts of seaweed i n the c o l l e c t i o n s made i t d i f f i c u l t t o f i n d the s m a l l gastropods. These two samples were, t h e r e f o r e , omitted, and t h i s precluded the use of October and November samples i n most a n a l y s e s . Unless e x p l i c i t l y i n d i c a t e d , a l l a n a l y t i c a l r e s u l t s given below p e r t a i n only t o the 200 quadrats (40 SMUS) c o l l e c t e d over a 10 month p e r i o d from December, 1975 through September, 1976. ABUNDANCE The mean d e n s i t i e s of 16 s p e c i e s of high abundance are p l o t t e d as a f u n c t i o n s of s t a t i o n and month i n Appendix C. The time-depth d i s p e r s i o n p a t t e r n s of the remaining 10 s p e c i e s are c i t e d i n t a b u l a r form i n Appendix D. Table I l i s t s the t o t a l d e n s i t i e s of a l l 26 sp e c i e s found i n 200 pooled quadrats. The most abundant s p e c i e s , M a r g a r i t e s c o s t a l i s had a t o t a l d e n s i t y of approximately 3000 per square 31 T a b l e I . D e n s i t i e s o f t w e n t y - s i x g a s t r o p o d s p e c i e s i n two hund r e d q u a d r a t s c o l l e c t e d f r o m December, 1975 t h r o u g h September, 1976. DENSITY SPECIES RANK no./QUADRAT CUMULATIVE % Margarites costalis 1 130. 316 48 . 0 Lacuna marmorata 2 83 . 800 78 . 8 Alvania compacta 3 36. 455 92 . 3 Lacuna car inata 4 8 . 768 95 . 5 Granulina margaritula 5 6. 065 97 . 8 Odos tomia sp. 6 1 . 543 98 . 3 Lirularia lirulata 7 0. 865 98. 6 Cerithiopsis sp. 8 0. 790 98 . 9 Mitrella gouldii 9 0 . 670 99 . 2 Admete circumcincta 10 0 . 536 99 . 4 Margarites olivaceus 11 0 . 480 99 . 7 Notoacmea scutum 12 0. 250 99 . 7 Diaphana californica 13 0. 185 99 . 8 Alvania carpenteri 14 0. 150 99 . 8 Amphissa columbiana 15 0. 135 99 . 8 Balcis micans 16 0. 115 99 . 9 Collisella pelta 17 0. 075 99. 9 Bittium eschrichtii 18 0. 055 99. 9 Cerithiopsis stejnegeri 19 0. 050 99 . 9 Ocenebra inter fossa 20 0. 040 99 . 9 Nassarius mendicus 21 0. 030 99 . 9 Velutina laevigata 22 0. 030 99 . 9 Crepipatella lingulata 23 0. 020 99 . 9 Acmaea mitra 24 0. 015 99 . 9 u n i d e n t i f i e d sp. 25 0. 010 99. 9 Turboni1 la Vancouverensis 26 0. 005 100 32 meter; Ma r g a r i t e s c o s t a l i s , . Lacuna marmorata, and A l y a n i a compacta comprised 92% of the i n d i v i d u a l s taken from December, 1975 through September, 1976, while Lacuna c a r i n a t a and G r a n u l i n a m a r q a r i t u l a c o n t r i b u t e d another 5%. T a b l e I I r e v e a l s the v a r i a b i l i t y i n the r e l a t i v e c o n t r i b u t i o n s ..of mean abundance and occurrence to d e n s i t y . M a r g a r i t e s c o s t a l i s was u s u a l l y the most abundant s p e c i e s i n a quadrat. Lacuna marmorata was, on the average, t h r e e times more abundant, per occurrence, than A l v a n i a compacta yet L. marmorata occurred, l e s s f r e g u e n t l y than e i t h e r A> compacta or M. c o s t a l i s . A l l but the f i v e dominant s p e c i e s occurred i n l e s s than h a l f o f the samples. Three s p e c i e s had extremely low abundances. Acmaea m i t r a , an u n i d e n t i f i e d r i s s o i d and T u r b o n i l l a vancouverensis were represented by one, two* and three i n d i v i d u a l s , r e s p e c t i v e l y . A c l a s s i f i c a t i o n scheme based upon the r e l a t i v e values, .of mean occurrence and abundance i s shown in . t h e f o u r t h column.of Table I I * Species of the f i r s t group occurred f r e g u e n t l y i n high numbers,; Those which are placed i n t o the second category had r e l a t i v e l y high f r e g u e n c i e s . y e t low mean abundances. The t h i r d .type, of s p e c i e s were numerous w i t h i n r a r e clumps. The f o u r t h grouping i s a c a t c h - a l l f o r s p e c i e s of low d e n s i t y which are e i t h e r t r u l y r a r e * . o r c o u l d be i n c l u d e d i n e i t h e r the second o r t h i r d . c a t e g o r i e s , but with l i t t l e c o n f i d e n c e . For most of the s p e c i e s , d e n s i t y decreased through the winter t o minimal val u e s i n A p r i l , f o l l o w e d by r e c r u i t m e n t , i n 33 T a b l e I I . O c c u r r e n c e and mean abundance o f t w e n t y - s i x g a s t r o p o d s p e c i e s f o u n d i n two h u n d r e d q u a d r a t s c o l l e c t e d f r o m December, 1975 t h r o u g h September, 1976. See t e x t f o r an e x p l a n a t i o n o f s p e c i e s g r o u p i n g s b a s e d upon o c c u r r e n c e and abundance. OCCURRENCE SPECIES MEAN ABUNDANCE GROUP RANK n o ' o f QUADRATS Margarites costalis 1 189 137 . 90 I • Alvania compacta 2 185 39. 40 I Lacuna marmorata 3 161 104. 10 I Granulina margaritula 4 131 9. 26 I Lacuna carinata 5 128 13 . 70 I Odostomia sp. 6 98 3 . 15 I I I Mitrella gouldii 7 57 2 . 35 I I Lirularia lirulata 8 56 3 . 09 ' I I I Admete circumcincta 9 47 2 . 28 I I Cerithiopsis sp. 10 35 4. 51 I I I Margarites olivaceus 11 30 3 . 20 I I I Notoacmea scutum 12 23 2. 17 I I Diaphana californica 13 23 1. 61 I I Balcis micans 14 20 1. 15 I I Amphissa columbiana 15 16 1. 69 IV Alvania carpenteri 16 10 3 . 00 I I I Cerithiopsis stejnegeri 17 10 1. 00 IV Collisella pelta 18 7 2 . 14 I I I Bittium eschrichtii 19 5 2 . 20 I I I Ocenebra interfos sa 20 5 1. 60 IV Velutina laevigata 21 5 1. 20 IV Nassarius mendicus 22 4 1. 50 IV Crepipatella lingulata 23 3 1 . 33 IV Acmaea mitra 24 2 1. 00 IV u n i d e n t i f i e d sp. 25 2 1. 00 IV Turboni1 la Vancouver ensis 26 1 1. 00 IV 34 May and June and peak d e n s i t i e s through September- Nass a r i u s mendicus was the only s p e c i e s not found during the summer. F i v e s p e c i e s , C o l l i s e l l a p e l t a , V e l u t i n a l a e v i g a t a , Acmaea m i t r a , T u r b o n i l l a vancouverensis, and the u n i d e n t i f i e d r i s s o i d were absent from the winter c o l l e c t i o n s . Another seven, L i r u l a r i a l i r u l a t a , Admete c i r c u m c i n c t a , Diaphana c a l i f o r n i c a , Amphissa columbiana, Notoacmea scutum* B a l e i s - micans, and C e r i t h i o p s i s sp. were r a r e l y encountered i n the winter. Twelve s p e c i e s were found at a l l depths. They were: M a r g a r i t e s c o s t a l i s . Lacuna marmorata, Alv a n i a compacta. Lacuna c a r i n a t a , G r a n u l i n a m a r g a r i t u l a , Odostpmia sp., L i r u l a r i a l i r u l a t a , Admete c i r c u m c i n c t a , M a r g a r i t e s o l i v a c e u s marginatum, B a l c i s micans, and C e r i t h i o p s i s s t e j n e g e r i ; The d i s p e r s i o n p a t t e r n s of the 11 most important s p e c i e s are b r i e f l y d e s c r i b e d below. Species of l e s s e r abundance were too s p o r a d i c a l l y d i s t r i b u t e d f o r meaningful d e s c r i p t i o n s . Time-blocked analyses of v a r i a n c e were performed on M. c o s t a l i s , L. marmorata, A. compacta, L. c a r i n a t a , and G. m a r g a r i t u l a using data from the 200 quadrats (Table I I I ) . The other s i x s p e c i e s d e s c r i b e d occurred i n l e s s than h a l f of the quadrats and.so ANOVA could.not be used. M a r g a r i t e s c o s t a l i s and Lacuna marmorata had s i m i l a r d i s p e r s i o n p a t t e r n s (Appendix C ). Both s p e c i e s g e n e r a l l y showed a decrease i n numbers with depth but no s i g n i f i c a n t d i f f e r e n c e s between s t a t i o n s 1 and 2, (Duncan's New M u l t i p l e Range Test p=0.01, . Table I I I ) . L. marmorata had i t s maximum d e n s i t y at s t a t i o n 2 i n J u l y and M^ . c o s t a l i s peaked, a t s t a t i o n 2 in,. J u l y , T a b l e I I I . T i m e - b l o c k e d a n a l y s e s o f v a r i a n c e o f mean d e n s i t y a t f o u r s t a t i o n s u s i n g t e n m o n t h l y b l o c k s . E a c h s t a t i o n was sampled w i t h f i v e r e p l i c a t e q u a d r a t s . D e n s i t y was t r a n s f o r m e d a s : Y=lo g (n+1). p 1 HOMOGENEOUS GROUPS OF STATIONS WITH DEPTH TIME DUNCAN'S NMR T E S T 2 Margarites costalis , 001 . 001 (1 2) (3) (4) Lacuna marmorata < .  001 < , . 001 (1 2) (3) (4) Alvania compacta NS NS Lacuna carinata . 001 < . 01 (1) (2 3) (4) Granulina margaritula . 01 NS (1) (2) (3 4) P r o b a b i l i t y o f t h e d i f f e r e n c e between mean d e n s i t i e s b e i n g due to c h a n c e . 2 ot=0.01 36 August, and September, The mean d e n s i t y of A l y a n i a compacta v a r i e d l i t t l e between s t a t i o n s 2 and 3; During most of the winter, most A. compacta were found at s t a t i o n 2, but the maximum abundance occ u r r e d at s t a t i o n 1 i n J u l y (Table I I I , Appendix C) . Lacuna c a r i n a t a was most abundant at s t a t i o n 1 i n the s p r i n g and summer (Table I I , Appendix C) but had a winter peak at s t a t i o n 2 i n February. Few L. c a r i n a t a were found at s t a t i o n 4 or during samples from October through January.' There i s a noteworthy d i s s i m i l a r i t y between the d i s p e r s i o n p a t t e r n of L> c a r i n a t a and those of L. marmorata- and M. c o s t a l i s . Recruitment of Lacuna c a r i n a t a o c c u r r e d e a r l i e r and at a shallower depth than the other two s p e c i e s . The abundance of G r a n u l i n a m a r g a r i t u l a decreased c o n t i n u o u s l y with depth although the d i f f e r e n c e between s t a t i o n s was grea t e r i n the summertime: There was r e l a t i v e l y l i t t l e month-to-month v a r i a t i o n a t s t a t i o n s 1 and 2 (Appendix C) . Except f o r a s l i g h t peak a t s t a t i o n 3 i n December, the d e n s i t y of Odostomia was g r e a t e s t at s t a t i o n 2, throughout the year. The d i f f e r e n c e s between s t a t i o n s 1, 2, and 3 were s l i g h t except du r i n g September when the s p e c i e s a t t a i n e d i t s maximum abundance of 11 per quadrat a t s t a t i o n 2. The l a t e summer peak was the only major seasonal change observed f o r Odostomia sp. L i r u l a r i a l i r u l a t a was mostly l i m i t e d to s t a t i o n s 1, 2 and 3 between June and August, with a d e f i n i t e peak a t s t a t i o n 2 i n J u l y (Appendix C). 37 C e r i t h i o p s i s sp. was r e s t r i c t e d to s t a t i o n s 1 and 2 / # and was r a r e l y found i n the f a l l samples (Appendix C) . The d e n s i t y peak was at s t a t i o n 2 i n A p r i l , but a f t e r June, c e r i t h i p p s i s sp. was only observed a s t a t i o n 1. M i t r e l l a g o u l d i i was more f r e g u e n t l y encountered than C e r i t h i o p s i s sp. but u s u a l l y at a lower d e n s i t y (Table I I ) . There was a f a l l peak f o r M. g o u l d i i a t s t a t i o n 2 and a summer peak o f s l i g h t l y g r e a t e r magnitude at s t a t i o n 1. M. g o u l d i i was r a r e a t s t a t i o n s 3 and never found a t 4. Admete c i r c u m c i n c t a was found a t s t a t i o n 3 i n December, but only had a p p r e c i a b l e abundance from A p r i l through September. Most of the i t c i r c u m c i n c t a were a t s t a t i o n 2. M a r g a r i t e s o l i v a c e u s was the only s p e c i e s which had g r e a t e r abundance a t s t a t i o n s 3 and 4 than the s h a l l o w e r s t a t i o n s . T h i s s p e c i e s was only numerous i n J u l y , August; and September. The M a r g a r i t e s c o s t a l i s p o p u l a t i o n was p a r t i t i o n e d i n t o t h r e e generations on the b a s i s of s h e l l l e n g t h . There were only s l i g h t d e p t h - r e l a t e d d i f f e r e n c e s i n importance f o r the o l d e s t ( f i r s t ) g e n e r a t i o n (Figure 5 ) . No f i r s t g e n e r a t i o n s n a i l s were c o l l e c t e d a f t e r February, 1976, so the maximum age of M. c o s t a l i s a t S a t u r n i n a I s l a n d i s a p p a r e n t l y 2 2 months. M a r g a r i t e s c o s t a l i s was c o n s i s t e n t l y l e s s abundant at s t a t i o n 3 than at s t a t i o n s 1 and 2 , f o r a l l t h r e e g e n e r a t i o n s . Values of abundance were so low a t s t a t i o n 4 t h a t they are not i n c l u d e d i n F i g u r e 5 . 38 F i g u r e 5; Mean d e n s i t y v e r s u s t i m e f o r M a r g a r i t e s c o s t a l i s o f t h r e e g e n e r a t i o n s a t s t a t i o n s 1, 2, a n d 3 i N u m e r a l s o n t h e g r a p h r e l a t e t h e s t a t i o n s a t w h i c h s n a i l s w e r e c o l l e c t e d . S m a l l d a s h e s , f i r s t g e n e r a t i o n ; s o l i d l i n e s , s e c o n d g e n e r a t i o n ; l a r g e d a s h e s , t h i r d g e n e r a t i o n * 3 9 40 Most of the Mo c o s t a l i s c o l l e c t e d from October, 1975, through May, 1976, belonged to the second g e n e r a t i o n . During the f a l l and winter months, t h e r e was l i t t l e d i f f e r e n c e i n the number of second generation M. c o s t a l i s at s t a t i o n 1,2, and 3. From March through September, however, there: was a g r e a t e r decrease i n numbers with depth f o r the second ge n e r a t i o n . T h i r d g e n e r a t i o n M. c o s t a l i s were most numerous a t s t a t i o n 2 du r i n g the summer. -This caused s p e c i e s abundance to be maximal a t s t a t i o n 2 at times when the g r e a t e s t numbers of a d u l t s n a i l s were found at s t a t i o n 1. There were no d i f f e r e n c e s between the mean s h e l l l e n g t h s of second gener a t i o n M. c o s t a l i s from s t a t i o n s 1 and 2, but s h e l l s were s l i g h t l y s m a l l e r (Figure 6 ) , and t h e . r a t e of l e n g t h increase.was s i g n i f i c a n t l y slower a t s t a t i o n 3 (Table I V ) . S i z e d i f f e r e n c e s between s t a t i o n s were not s i g n i f i c a n t f o r the other two g e n e r a t i o n s of Margarites c o s t a l i s . NICHE-BREADTH Niche-breadth i n d i c e s , which were computed f o r a l l s p e c i e s o c c u r r i n g i n the 10 month c o l l e c t i o n of 200 guadrats, are presented as f u n c t i o n s of s p e c i e s rank i n Fig u r e 7. A l v a n i a compacta had the l a r g e s t t o t a l n i c h e - b r e a d t h i n d i c a t i n g t h a t t h i s s p e c i e s was the most evenly d i s p e r s e d . Some s p e c i e s , such as Baeis micans, A l y a n i a c a r p e n t e r i , and C e r i t h i o p s i s ste-jnegeri, o c c u r r e d f r e q u e n t l y yet possessed r e l a t i v e l y broad n i c h e s because of co n s t a n t , though s l i g h t 41 Figure 6; S h e l l l e n g t h versus time f o r Ma r g a r i t e s c o s t a l i s of three generations at s t a t i o n s 1, 2, and 3. Lengths were recorded i n m i l l i m e t e r s with a v e r n i e r c a l i p e r . Numerals on the graph r e l a t e to the s t a t i o n s a t which s n a i l s were c o l l e c t e d . Small dashes, f i r s t g e n e r a t i o n ; s o l i d l i n e s , second g e n e r a t i o n ; l a r g e dashes, t h i r d g e n e r a t i o n . 4 2 43 T a b l e IV. D i f f e r e n c e s i n s h e l l l e n g t h i n c r e a s e i n mm as a f u n c t i o n o f t i m e f o r M. costalis a t s t a t i o n s 1, 2, and 3. GENERATION STATION LINEAR REGRESSION HOMOGENEOUS GROUPS OF STATIONS WITH SHEFFE'S T E S T 3 1 Y=2.835+.291X 2 Y=2.875+.285X 3 Y=2.866+.218X NS (1 2 3) 1 Y=l.609+.226X 2 Y=l.439+.213X 3 Y=l.315+.244X 0003 (1 2) (3) 1 Y=-3.51+.405X 2 Y=-2.43+.397X 3 Y=-3.62+.402X NS (1 2 3) 1 (SHELL LENGTH)=b+a(MONTH) w i t h O c t o b e r , 1975 c o n s i d e r e d month 1. 2 F-TEST f o r e q u a l i t y o f s l o p e s . 3 S u b s e t s a r e b a s e d upon s l o p e s o f r e g r e s s i o n s . 44 average abundances per occurrence (Table I I , Appendix C). These s p e c i e s are r e l a t i v e l y evenly d i s p e r s e d (Figure 7b),. The v a l u e s of INB are p l o t t e d f o r a l l 26 s p e c i e s (Figure 7a). There are 10 time c a t e g o r i e s and 4 f o r depth, so t h e r e f o r e INB should approach 0.71 (=10/10+4) when a s p e c i e s i s as evenly d i s t r i b u t e d among the time as the depth dimension. On the b a s i s of INB values, s i x s p e c i e s groupings can be d i s t i n g u i s h e d (Table V),. The members of group I were the seven most important s p e c i e s and were found i n most of the SMDs although n i c h e - s i z e s and d i s p e r s i o n p a t t e r n s v a r i e d c o n s i d e r a b l y . The f i v e s p e c i e s of group I I had INB v a l u e s s l i g h t l y below 0.71, i n d i c a t i n g t h a t - t h e s m a l l depth and time r e s t r i c t i o n s i n the d i s p e r s i o n of these s p e c i e s were of approximately e q u i v a l e n t magnitudes. The sp e c i e s of group I I I occurred i n l e s s than four s t a t i o n s but the r e l a t i v e r e s t r i c t i o n s o f these s p e c i e s through time were l e s s severe than f o r depth and thus the INB values were s l i g h t l y g r e a t e r than 0.71. Group IV c o n s i s t s of s i x s p e c i e s which were found i n s e v e r a l monthly c o l l e c t i o n s but a t onl y one depth per month. T u r b o n i l l a v a n c o u v e r e n s i s : o c c u r r e d i n a s i n g l e SMU and had the narrowest p o s s i b l e niche (B (t) =B (d) =B (m) =0) . The only s p e c i e s which d i s p l a y e d a value of B (d) t h a t was g r e a t e r than B,(m) was C o l l i s e l l a p e l t a . T h i s s p e c i e s was present f o r only t h r e e months (33% of the time c a t e g o r i e s c o n s i d e r e d i n the c a l c u l a t i o n o f niche-breadth) but occurred at t h r e e d i f f e r e n t s t a t i o n s (75% of the sampled v e r t i c a l range). Because o f the severe r e l a t i v e time r e s t r i c t i o n i n i t s range through time. 45 F i g u r e 7. INB (A) and t o t a l n i c h e breadth (B) f o r the gastropod s p e c i e s . The gastropods were c o l l e c t e d i n 200 quadrats i n 10 monthly samples. See t e x t f o r e x p l a n a t i o n s of the n i c h e breadth i n d i c e s . 46 INB TURBVAN uuuuuuu ACMAMIT CREPLIN VELULAE NASSMEC OCENINT CERISTE B I T T E S C COLLPEL BALSMIC AMPHCOL ALVACAR DIAPCAL NOTOSCU MARGOLI ADMECIR MITRGOU CERISPP L I R U L I R ODOSTOM GRANMAR LACUCAR ALVACOM LACUMAR MARGCOS v £ Z I HlQV3rja 3H0IN 1V101 4 7 T a b l e V. C l a s s i f i c a t i o n o f g a s t r o p o d s p e c i e s by t h e r a t i o B ( m ) / B ( t ) = I N B , w h i c h i s e x p l a i n e d i n t e x t . GROUP I INB-.7 S p e c i e s w h i c h span a l l d e p t h s a t most t i m e s Margarites costalis Lacuna marmorata Lacuna carinata Lirularia lirulata Alvania compacta Granulina margaritula Odostomia s p . GROUP I I INB<.7 S p e c i e s as r e s t r i c t e d i n d e p t h as t i m e Notoacmea scutum Margarites olivaceus Balcis micans Cerithiopsis stejnegeri Amphissa columbiana GROUP I I I INB>.7 S p e c i e s more d e p t h t h a n t i m e r e s t r i c t e d Cerithiopsis sp. Mitrella gouldii Alvania carpenteri Admete circumcincta Diaphana californica Velutina laevigata GROUP IV INB=1. S p e c i e s r e s t r i c t e d t o one d e p t h p e r month Bittium eschrichtii Nassarius mendicus Ocenebra interfossa u n i d e n t i f i e d sp. Acmaea mitra Crepipatella lingulata GROUP V INB<.5 S p e c i e s more t i m e t h a n d e p t h r e s t r i c t e d Collisella pelta GROUP VI INB=0. S p e c i e s r e s t r i c t e d t o one d e p t h and t i m e Turhoni11a Vancouverensis 48 T a b l e v i . Spearman r a n k c o r r e l a t i o n s and l i n e a r r e g r e s s i o n s o f t o t a l n i c h e b r e a d t h , B ( t ) , t o t o t a l d e n s i t y and" o c c u r r e n c e f o r t w e n t y - s i x g a s t r o p o d s p e c i e s . INDEPENDENT VARIABLE Y Q t f o r y 1 t f o r r e g r e s s i o n ' TOTAL DENSITY 0.89 9.68 3 4.61 3 OCCURRENCE 0.94 13.64 3 8.41 3 1 2 t=y M 2 5 ) / ( l - y 2 ) 7 s s The r e g r e s s i o n e q u a t i o n i s B(t)=b+ a [ l o g (X+l)7 10 3 P r o b a b i l i t y o f t h e o b s e r v e d c o r r e l a t i o n o r s l o p e o f t h e r e g r e s s i o n l i n e b e i n g due t o chance £ .001. 49 C p e l t a ranks 24th i n t o t a l n i c h e - b r e a d t h compared to a d e n s i t y rank of 17. T o t a l niche-breadth was more s t r o n g l y c o r r e l a t e d to frequency of occurrence than t o s p e c i e s d e n s i t y (Table V I ) . Ranked by d e c r e a s i n g values of B ( t ) , the f i r s t seven s p e c i e s are A. compacta, G. m a r g a r i t u l a , Odostomia sp., M. c o s t a l i s , L. c a r i n a t a , L. marmorata, and M. g o u l d i i , which i s d i f f e r e n t from the order by frequency of occurrence (Table I I ) . T h i s d i s c r e p a n c y can be a t t r i b u t e d to the f a c t t h a t B(t) i s dependent upon both the number of encounters and the evenness i n the abundance of a s p e c i e s among the encounters (Levi n s 1968). SPECIES RICHNESS, TOTAL ABUNDANCE AND SAMPLE HETEROGENEITY The mean d e n s i t i e s of gastropods from a l l 46 SMUs were used to c a l c u l a t e S, N, D, and H'. These r e s u l t s are shown as contour maps i n F i g u r e s 8 and 9. Species r i c h n e s s (Figure 8a) and t o t a l abundance (Figure 8b) were g r e a t e r i n the summer than i n the w i n t e r , a t a l l depths. The highest values of S and N were found at s t a t i o n s 1 and 2. Time-blocked ANOVAS and Duncan New M u l t i p l e Range t e s t s (Table VII) showed t h a t values of S and N at s t a t i o n 1, 2, and 3 were s i g n i f i c a n t l y d i f f e r e n t (p=0.01) from values a t s t a t i o n 4. The contour maps of D (Figure 9a) and H" (Figure 9b) are s i m i l a r , and can be t r e a t e d t o g e t h e r , Although analyses of 50 F i g u r e 8. Species r i c h n e s s and the t o t a l abundance of the gastropod assemblage. Contoured a g a i n s t the s t a t i o n and month of c o l l e c t i o n . T o t a l abundance transformed by: Y = l o g l o ( N + l ) .51 TOTAL ABUNDANCE 52 F i g u r e 9.; Gastropod s p e c i e s d i v e r s i t y e x p r e s s e d at Simpson's D (A) and H (B) . V a l u e s are c o n t o u r e d a g a i n s t t h e s t a t i o n and month o f c o l l e c t i o n . 54 T a b l e V I I . Time b l o c k e d a n a l y s e s o f v a r i a n c e f o r a s s e m b l a g e p a r a m e t e r s i n f o u r s t a t i o n s u s i n g t e n m o n t h l y b l o c k s . ' R e p l i c a t e q u a d r a t s were p o o l e d as d i s c u s s e d i n t e x t . DEPENDENT VARIABLE MEAN P 1 DEPTH TIME HOMOGENEOUS GROUPS OF STATIONS WITH DUNCAN'S NMR T E S T 2 S 9. 35(1.01) 3 . 001 . 0035 (1 2 3) (4) N 2 4 8 ( 2 . 4 0 ) 3 .001 NS (1 2 3) (4) D .601 NS NS H ' 3 . 09 NS NS 1 P r o b a b i l i t y o f t h e d i f f e r e n c e between sample means b e i n g due t o c h a n c e . 2 a=0.01 3 P a r e n t h e s e s f o r t r a n s f o r m e d means where Y=log (Y+l) were 10 u s e d . 55 v a r i a n c e f o r these i n d i c e s i n d i c a t e no s i g n i f i c a n t changes with s t a t i o n and month (Table VII) , d e f i n i t e trends are v i s i b l e . Sample h e t e r o g e n e i t y was minimal i n the f a l l at a l l depths, at s t a t i o n 4 i n the winter and s p r i n g , and a t s t a t i o n 2 i n the l a t e summer. High values of D and H' were found i n the s p r i n g at s t a t i o n 1, and i n the summer at s t a t i o n 4. The remainder of the SMOs possesed roughly the same in t e r m e d i a t e degree of h e t e r o g e n e i t y i n the apportionment of i n d i v i d u a l s among spec i e s . CLUSTER ANALYSIS OF THE SAMPLES C l u s t e r a n a l y s i s of the SMUs using the B r a y - C u r t i s index was performed upon 21 of the gastropod s p e c i e s i n a l l 46 SMUs. Four o f the s p e c i e s c o l l e c t e d , C r e p i p a t e l l a l i n g u l a t a , T. vancouverensis, A. m i t r a , and the u n i d e n t i f i e d r i s s o i d occurred i n fewer than f o u r SMUs, and were excluded from the a n a l y s i s . C. p e l t a , was a l s o e l i m i n a t e d because of i t s narrow nich e (Figure 7b). A l l but three SMUs on the dendrogram (Figure 10) are grouped i n t o e i t h e r a shallow or a deep s u p e r - c l u s t e r . 22 SMUs from s t a t i o n s 1 and 2 are i n c l u d e d i n the l e f t - h a n d s u p e r c l u s t e r ( L ) , which c o n s o l i d a t e s at a s i m i l a r i t y l e v e l of 0.33. A l l but one of the 17 samples grouped i n t o the r i g h t - h a n d s u p e r c l u s t e r (R) at 0.45 are from s t a t i o n s 3 and 4. The 0.25 s i m i l a r i t y between R and L suggests t h a t there was a l a r g e degree of o v e r l a p i n the s p e c i e s present i n shallow and deep 56 sampling u n i t s . Seasonal tr e n d s are i e s s apparent, but at every depth, summertime SMOs are placed w i t h i n d i s t i n c t c l u s t e r s . W i t hin L, the summer samples from s t a t i o n s 1 and 2 are separated i n t o two sub-groups of c l u s t e r 1. These SMUs had g r e a t e r numbers of i n d i v i d u a l s and species than any other samples, but d i f f e r e d most i n the r e l a t i v e abundance of Ma r g a r i t e s c o s t a l i s , which was most numerous a t s t a t i o n 2 i n the summer. The remainder of L i s comprised of s i x c l u s t e r s which are chained together i n order of decreasing s i m i l a r i t y with the shallow summer samples. The SMOs i n c l u s t e r 4 (December, s t a t i o n s 2 and 3, January and February s t a t i o n 1, and June s t a t i o n 2) had about h a l f the number of M. c o s t a l i s , L. marmorata, and A. compacta t h a t were encountered i n the c l u s t e r 1 samples. The December SMO from s t a t i o n 1, which i s alone i n c l u s t e r 5, was low i n M. c o s t a l i s , but was otherwise s i m i l a r to the samples i n c l u s t e r 4, t o which i t i s l i n k e d ; Most of the samples of c l u s t e r -6 - (March* A p r i l , May at s t a t i o n 1 and May a t s t a t i o n 2) are high i n M. c o s t a l i s , L. marmorata, and L. c a r i n a t a , but are low i n ft. compacta. The May s t a t i o n 2 sample had fewer L. c a r i n a t a , but more marmorata than the other samples i n c l u s t e r 6, and t h e r e f o r e j o i n s the group at a lower l e v e l o f s i m i l a r i t y . The samples i n c l u d e d i n B have few s p e c i e s and low values of t o t a l abundance. Within c l u s t e r 8b, samples from s t a t i o n 4 i n March, May, and June c o n t a i n e d s p e c i e s with very low abundances. The remainder of c l u s t e r 8b are SMUs th a t a l s o had few s n a i l s , but on the average more than ten A. compacta per 57 F i g u r e 10. Dendrogram of c l u s t e r a n a l y s i s of SMOs on the b a s i s o f s p e c i e s abundance; Each of the 46 SMOs t h a t were con s i d e r e d c o n s i s t e d of f i v e pooled quadrats. Numerals on the dendrogram correspond to groupings which are de s c r i b e d i n the t e x t . BRAY-CURTIS INDEX 1 . 0 . 7 5 . 5 0 . 2 5 ' 1 Wit ft) 01 ft) 59 quadrat. C l u s t e r 8a c o n t a i n s a l l of the summertime samples from s t a t i o n 3, as w e l l as SMOs from s t a t i o n 4 i n November and s t a t i o n 3 i n February. The p r i n c i p a l s p e c i e s i n these samples are A. compacta, L. marmorata, and M. c o s t a l i s , but the t o t a l abundance values t h a t were recorded were low, and so c l u s t e r 8a j o i n s c l u s t e r 8b, before the other c l u s t e r s l o c a t e d i n L. S t a t i o n 4 s i t e s from June, J u l y , and August are c l a s s i f i e d s e p a r a t e l y on the dendrogram and fuse with a l l other samples at a l e v e l of l e s s than 0,20. INVERSE CLUSTER ANALYSIS OF THE SPECIES Inverse c l u s t e r a n a l y s i s was performed with the same data as were used f o r the r e g u l a r c l u s t e r a n a l y s i s . As a means o f i n t e r p r e t i n g the r e l a t i o n s h i p s between the gastropod s p e c i e s expressed i n the i n v e r s e - c l u s t e r dendrogram (Figure 11), a Spearman rank c o r r e l a t i o n c o e f f i c i e n t was computed f o r every p o s s i b l e p a i r of the 21 s p e c i e s (Figure 12), and comparisons were made between the contour maps of s p e c i e s d e n s i t y vs. s t a t i o n and month (Appendix C). C o r r e l a t i o n c o e f f i c i e n t s were c a l c u l a t e d with s p e c i e s abundance values from i n d i v i d u a l guadrats r a t h e r than from means of r e p l i c a t e s , . The i n c r e a s e i n sample s i z e and degrees of freedom a l l o w s the Spearman c o e f f i c i e n t to become more c r i t i c a l of i n t e r s p e c i f i c c o r r e l a t i o n s ( S i e g e l , 1956). There are 13 s p e c i e s - c l u s t e r s shown on the dendrogram i n F i g u r e 11. The f i v e s p e c i e s of lowest and most s p o r a d i c 60 F i g u r e 11w Dendrogram of i n v e r s e c l u s t e r a n a l y s i s ; A n a l y s i s was performed on 21 gastropod s p e c i e s i n 46 SMOs. Numerals on the dendrogram correspond t o groupings which are d e s c r i b e d i n the t e x t . t o CO M 0> 0 0 m r 0 r-SZ OS • 0 • T BITTESC MARGOLI BALSMIC NOTOSCU AMPHCOL LIRULIR DIAPCAL CERISPP LACUCAR GRANMAR MITRGOU ADMECIR LACUMAR MARGCOS ALVACOM ODOSTOM ALVACAR VELULAE NASSMEC OCENINT CE RIS TE X3QNI suyno-Avya 62 Figu r e 12. Spearman rank c o r r e l a t i o n matrix of gastropod s p e c i e s ; C o r r e l a t i o n s were based upon v a l u e s of abundance i n 200 quadrats, and were performed f o r a t o t a l of 21 s p e c i e s , but only 14 which showed s i g n i f i c a n t c o r r e l a t i o n s were i n c l u d e d i n the matrix. P r o b a b i l i t y was determined from the o n e - t a i l e d p r o b a b i l i t y of t-_ (see Table X) . 63 PS H ll 3 D O a os EH PS 1 CO O Pi D U s o u s o EH CO O a o OS u D P < a u OS D w H O o EH o co u co ME TO HI a Q O < Z < u co CQ L I R U L I R M I T R G O U G R A N M A R M A R G C O S L A C U M A R A L V A C O M O D O S T O M L A C U C A R C E R I S P P A D M E C I R N O T O S C U A M P H C O L C E R I S T E B A L S M I C p £ .001 .001 < p < .01 ,01 < p <. .05 .05 < p £ .10 64 importance, C e r i t h i o p s i s stejnegeri, Ocenebra i n t e r f o s s a , Nassarius mendicus, Velutina laevigata, and Bittium e s c h r i c h t i i , clustered seperately from a l l of the other species, except Alvania carpenteri. Since there are no greater than two j o i n t occurrences f o r any pair of these f i v e species (table- in- Appendix B) , the calculated s i m i l a r i t i e s are of guestionable v a l i d i t y . Alyania carpenteri i s grouped with Velutina l a e v i g t a , but the two species only occurred together i n two samples. The remaining 15 species are grouped in t o 10 c l u s t e r s which are joined i n a stepwise—or "chained"— fashion, in d i c a t i n g a large amount of overlap i n dispersion patterns. This r e s u l t agrees favorably with the i n t e r s p e c i f i c c orrelations and the contour maps. Five of the 10 c l u s t e r s contain pairs of species while the other f i v e contain one species each. The pairing of A. compacta with Odostomia-sp.,-M. c o s t a l i s with L. marmorata, and L. carinata with C e r i t h i o p s i s sp. are reasonable, considering the c o r r e l a t i o n and contour map information. There i s a highly s i g n i f i c a n t c orrelation between H. g o u l d i i and G. marqaritula, and the two species have coincident peaks of importance at station 1 throughout the year, but M. g o u l d i i was seldom found deeper than station 2, while G. margaritula, which appears to be more sim i l a r to A. compacta and odostomia sp., occurred in a l l four stations. In contrast, the contour maps of Notoacmea scutum and Amphissa columbiana which are grouped together on the dendrogram, are 65 q u i t e s i m i l a r , but there i s no s i g n i f i c a n t c o r r e l a t i o n between the two s p e c i e s . . . Admete c i r c u m c i n t a occurs i n a separate c l u s t e r which i s fused to a group c o n t a i n i n g both c l u s t e r s 4 and 5. T h i s s p e c i e s c o r r e l a t e s s i g n i f i c a n t l y with a l l four s p e c i e s t h a t are i n the group, as w e l l as t o G r a n u l i n a m a r g a r i t u l a and Notoacmea scutum, but the highest c o r r e l a t i o n s and the g r e a t e s t amount of contour map overlap are with A l v a n i a compacta and Odostomia sp. L i r u l a r i a l i r u l a t a i s a l s o c l u s t e r e d s o l i t a r i l y , but c o r r e l a t e s . to, M a r g a r i t e s c o s t a l i s , Odostomia sp., A l v a n i a compacta, and most s t r o n g l y , .to G r a n u l i n a m a r g a r i t u l a . The contour map of L. l i r u l a t u s bears p a r t i a l resemblance to a l l of the above-mentioned s p e c i e s , as w e l l as to t h a t of Diaphana c a l i f o r n i c a . Although i t i s r e s t r i c t e d almost e n t i r e l y t o s t a t i o n s 1 and 2, and d i s p l a y s no i n t e r s p e c i f i c c o r r e l a t i o n s , the contour map of D. c a l i f o r n i c a has some s i m i l a r i t i e s to those of M. c o s t a l i s and L. marmorata, as w e l l ; B a l e i s micans of c l u s t e r 12 i s grouped to Amphissa columbiana and Notoacmea scutum on the dendrogram. But B. micans i s . c o r r e l a t e d only with A. compacta, and at a low l e v e l of s i g n i f i c a n c e ; M a r g a r i t e s o l i y a c e u s , which was the only s p e c i e s i n c l u s t e r 13, showed no c o r r e l a t i o n s with other s p e c i e s , and had a unique d i s t r i b u t i o n a l p a t t e r n . . - A l v a n i a c a r p e n t e r i was only found at s t a t i o n 1, and had a time-depth d i s p e r s i o n which wa s . s i m i l a r t o Lacuna c a r i n a t a and C e r i t h i o p s i s sp. of. cluster..8. 66 Gut Analyses Qualitative observations of the gut contents of a l l nine species that were examined revealed the presence of great numbers of diatoms with few c e l l s which could d e f i n i t e l y be attributed to other organisms (Table VIII).. The s i z e of ingested diatoms ranged from under 10 um (e.g. Cocconeis scutellum) to over 150 um i n length (Synedra fasciculata) . Easily digested organisms and organic debris were probably ingested to some degree by a l l diatom-grazing species, but could not be i d e n t i f i e d . I t i s p a r t i c u l a r l y d i f f i c u l t to separate ingested bacteria from the resident micro-flora of the gastropod gut (Calow 1975, G a l l i and Giese 1959). Some specimens of L. marmorata also contained portions of cyanophyte and rhodophyte filaments as well as bryozoan fragments. Two large adult M. g o u l d i i were found that had consumed an encapsulated mass of veligers belonging to the opisthobranch Styl i g e r f u s c o v i t t a t a i n addition to diatoms; 67 T a b l e V I I I . Gut c o n t e n t summary. The p r e s e n c e o f a t y p e o f • f o o d i s d e n o t e d by an X. MATERIAL FOUND AS GUT CONTENTS SPECIES N DIATOM RHODOPHYTE CYANOPYHTE OTHER Margarites costalis 30 X Lacuna marmorata 30 X X X X 1 Alvania compacta 10 X Lacuna carinata 10 X Granulina margaritula 10 X Cerithiopsis s p . 2 X X 2 Mitrella gouldii 2 X Lirularia lirulata 10 X Margarites olivaceus 10 X B r y o z o a n f r a g m e n t s i n two i n d i v i d u a l s 2 E n c a p s u l a t e d o p i s t h o b r a n c h v e l i g e r s . 68 Diatom Abundance F i g u r e 13 shows the d e n s i t i e s of diatoms on s l i d e s r e t r i e v e d from J u l y 22, 1976, through March 14, 1977, a f t e r two week immersion p e r i o d s a t e i t h e r s t a t i o n 1,2, or 3. The average number of c e l l s per square mm decreased s i g n i f i c a n t l y with depth (Table I X ) . At a l l times, except i n J u l y , the r e l a t i v e trends i n d e n s i t y were s i m i l a r a t a l l t h r e e s t a t i o n s (Figure 13). In J u l y , d e n s i t i e s were high a t s t a t i o n 1 and r e l a t i v e l y low a t s t a t i o n 2 and 3. Diatom abundance f l u c t u a t e d d u r i n g the l a t e summer with peaks.on August 5 and September 9, and a dep r e s s i o n i n between on August 19. The lowest diatom d e n s i t i e s occurred i n December .followed by an i n c r e a s e t o March 1 a t a l l three s t a t i o n s on June 18, 1976, abundances were so great as t o render counting i m p o s s i b l e . T h e r e f o r e , diatom abundance was probably c o n t i n u o u s l y high throughout the rec r u i t m e n t p e r i o d s of most of the gastr o d s . Diatom abundance was compared to the mean d e n s i t i e s of e i g h t gastropod s p e c i e s , N, S, and H', using the Spearman rank c o r r e l a t i o n c o e f f i c i e n t with p o r t i o n s of the p r e v i o u s l y d e s c r i b e d data (Table X). Since the gastropods from the f a l l were c o l l e c t e d i n 1975, and the diatoms were from the f a l l of 1976, these c o r r e l a t i o n s might have l i m i t e d a p p l i c a b i l i t y ; Diatom d e n s i t y was s t r o n g l y c o r r e l a t e d t o N, S, H•, and the d e n s i t i e s of A. compacta, L. c a r i n a t a , and L. l i r u l a r i a . The r e l a t i o n s h i p s between the d e n s i t i e s of diatoms and the two 69 F i g u r e 13. Diatom d e n s i t y i n mm2 versus date of c o l l e c t i o n a t s t a t i o n s 1,2, and 3. Numerals on the graph correspond to s t a t i o n numbers. Diatom d e n s i t i e s i n June were too great t o be ennumerated at a l l three s t a t i o n s . 70 71 T a b l e i x . T i m e - b l o c k e d a n a l y s i s o f v a r i a n c e f o r d i a t o m d e n s i t y on g l a s s s l i d e s a t t h r e e s t a t i o n s i n e i g h t m o n t h l y b l o c k s . HOMOGENEOUS GROUPS OF STATIONS SOURCE p 1 WITH DUNCAN'S NMR T E S T 2 STATION <.001 (1) (2) (3) MONTH <.001 1 P r o b a b i l i t y o f t h e d i f f e r e n c e between mean d e n s i t i e s b e i n g due t o c h a n c e . 2 a=.01 7 2 T a b l e X. Spearman r a n k c o r r e l a t i o n s o f g a s t r o p o d s p e c i e s and ass e m b l a g e p a r a m e t e r s t o d i a t o m d e n s i t y . A l l d a t a from g a s t r o p o d samples c o l l e c t e d a t t h e same s t a t i o n s and months as t h e d i a t o m s were used r e g a r d l e s s o f t h e y e a r o f c o l l e c t i o n . VARIABLE d f P 1 N 19 < . 005 S 19 < . 05 H 1 19 < . 005 M . costalis ( a l l g e n e r a t i o n s ) 19 < . 025 M. costalis ( e x c l u d i n g j u v e n i l e s ) 19 NS L . marmorata 19 < . 05 L . carinata 19 <.0005 A . compacta 19 <.0005 G. mar garitula 16 NS M. go uldii 15 NS L. l i r u l a t a 14 < . 025 M. olivaceus 8 NS 1 The o n e - t a i l e d p r o b a b i l i t y o f p o s i t i v e c o r r e l a t i o n b e i n g due t o chance where p i s d e r i v e d f r o m t and: t = Y s / " ( d f ) / ( l - y s 2 ) 7^  7 3 dominant gastropods, M. c o s t a l i s , and L. marmorata were also s i g n i f i c a n t , but to a lesser degree. When juvenile M. c o s t a l i s were excluded from the analysis, the rel a t i o n s h i p no longer existed. The dispersions of M. g o u l d i i and M, pliyaceus were not correlated to diatom abundance. Feeding Experiment In eight experimental t r i a l s , more Margarites c o s t a l i s were found i n contact with l i v e Plocamium cartilagineum than with either heat or hydrogen peroxide treated .fronds (Table XI). The dead material averaged the least number of contacts. M. c o s t a l i s may be attracted to the l i v e algae because i t contained higher surface concentrations of diatoms. This view i s supported by the fact that an average of 10 s n a i l s per cage per t r i a l were found upon the p l a s t i c mesh cage covers which developed dense diatom growths during each experimental period of 24 hours. 7 4 T a b l e x i . One-way c l a s s i f i c a t i o n a n a l y s i s o f v a r i a n c e f o r t h e number o f M. costalis i n c o n t a c t w i t h Plocamium cartilagineum o f t h r e e t r e a t m e n t s . See t e x t f o r an e x p l a n a t i o n o f t h e t h r e e t r e a t m e n t s and t h e e x p e r i -m e n t a l d e s i g n . HOMOGENEOUS GROUPS SOURCE MEAN 2 d f F p OF TREATMENTS WITH DUNCAN'S NMR T E S T 1 2.0 0.5 2 5.91 <.01 (A) (B C) 0. 25 21 23 1 04=0.01 2 Mean number o f c o n t a c t s i n e i g h t t r i a l s . TREATMENT A ( l i v e ) B ( c l e a n ) C (dead) ERROR TOTAL 75 DISCUSSION C h a r a c t e r i z a t i o n Of The Gastropod Assemblage Maximum development of the gastropod assemblage o c c u r r e d i n the summertime. Large numbers of s m a l l , p o s t - l a r v a l i n d i v i d u a l s , seen i n the p e r i o d between A p r i l t o June f o r most s p e c i e s , caused i n c r e a s e s i n p o p u l a t i o n d e n s i t i e s and s p e c i e s r i c h n e s s . Only one s p e c i e s was found i n the winter t h a t was not a l s o present i n the summer, although a few were r e s t r i c t e d e n t i r e l y t o the summer c o l l e c t i o n s . The s p e c i e s which were encountered only s e a s o n a l l y were probably present year round but at such low d e n s i t i e s t h a t they c o u l d be detected only a f t e r r e c r u i t m e n t , when d e n s i t i e s were high. Species r i c h n e s s and the abundance of most . s p e c i e s decreased with depth. Thorson (1933) noted the.same t r e n d f o r M. c o s t a l i s and oth e r animals i n h a b i t i n g seaweed i n two f j o r d s of n o r t h e a s t e r n Greenland. The epifauna r a r e l y extended as deeply as the host macrophytes, Desmarestia and f i l a m e n t o u s red algae. In t h i s study, however, the ab s o l u t e depth t o which the fauna c o u l d reach was not determined s i n c e the the r e d - a l g a l zone at S a t u r n i n a I s l a n d i s i n t e r r u p t e d by a bed of e e l g r a s s a t an approximate depth of 6.5 m. The few gastropods c o l l e c t e d at s t a t i o n 4, on the f r i n g e of the e e l g r a s s community (Figure 2), were e n t i r e l y r e s t r i c t e d to the r e l a t i v e l y s m a l l boulders and 76 blades o f Aqarum cribosum. The assemblage was dominated year-round by f i v e s p e c i e s which ranged to a l l depths, but were more numerous at the shallower s t a t i o n s . Host other s p e c i e s were more r e s t r i c t e d v e r t i c a l l y . T h i s extreme numerical dominance had a heavy i n f l u e n c e upon values of N, D, H *, and the B r a y - C u r t i s s i m i l a r i t y index. Although S and N showed s i m i l a r t r e n d s through depth and time, the contour map of N bears even s t r o n g e r resemblance to those of M. c o s t a l i s and L. marmorata, due to these s p e c i e s ' high d e n s i t i e s . T o t a l importance, t h e r e f o r e , y i e l d e d l i t t l e i n f o r m a t i o n c o ncerning the o v e r a l l assemblage. C l u s t e r a n a l y s i s with the unweighted pair-group method and the B r a y - C u r t i s index grouped samples p r i n c i p a l l y by depth, but summer , samples were separated from a l l o t h e r s w i t h i n each s t a t i o n ; C l o s e r c o n s i d e r a t i o n of the contents of the SMO c l u s t e r s r e v e a l e d t h a t the a n a l y s i s was based almost completely upon the dominant three gastropods, and ignored any i n f o r m a t i o n concerning the time-depth d i s p e r s i o n s of the remaining s p e c i e s . A s i m i l a r problem was encountered by Rex (1977), who found t h a t c l u s t e r a n a l y s i s with a r e l a t e d index o f s i m i l a r i t y (a q u a n t i f i e d v e r s i o n of the J a c c a r d index, Sepkoski 1974) fused deep-sea samples by order of the abundance of the n u m e r i c a l l y dominant gastropod, A l v a n i a p e l a g i c a . The c l u s t e r a n a l y s i s was s u c c e s s f u l i n the sense t h a t a l a r g e (242 x 20) data matrix was reduced t o groupings of samples t h a t were based upon more than 80% of the i n d i v i d u a l 77 s n a i l s c o l l e c t e d . T h i s i s the primary purpose f o r using a m u l t i v a r i a t e approach to data a n a l y s i s (Sneath and Sokal 1976, Green and Vascotto 1 978). S u p e r f i c i a l l y , the a n a l y s i s seemed to r e p r e s e n t the e n t i r e assemblage -because many s p e c i e s had d i s p e r s i o n p a t t e r n s which were s i m i l a r to the the dominants. Had the l e s s e r s p e c i e s d i s p l a y e d more v a r i e d v e r t i c a l or temporal d i s t r i b u t i o n s , i t i s d o u b t f u l t h a t the c l u s t e r a n a l y s i s would have provided any i n d i c a t i o n . The i n v e r s e c l u s t e r a n a l y s i s was reasonably s e n s i t i v e to s i m i l a r i t i e s i n the d i s t r i b u t i o n s of s p e c i e s with high d e n s i t i e s * but those with i n t e r m e d i a t e t o low frequency of occurrence tended t o be j o i n e d when no r e a l s i m i l a r i t i e s e x i s t e d . T h i s s o r t of e r r o r i s common to h i e r a r c h i c a l schemes which are designed t o c l a s s i f y a l l of the u n i t s t h a t are i n c l u d e d i n an a n a l y s i s ( P i e l o u 1977, W i l l i a m s 1971). The process of i n t e r p r e t i n g the i n v e r s e a n a l y s i s l e d to d i r e c t comparisons of s p e c i e s d i s p e r s i o n s , which u l t i m a t e l y proved t o be more u s e f u l . The co-dominants, M. c o s t a l i s and L i marmorata, had the g r e a t e s t s i m i l a r i t y i n d i s p e r s i o n s , a f a c t r e f l e c t e d by i n v e r s e a n a l y s i s , the Spearman c o r r e l a t i o n c o e f f i c i e n t s and the s u p e r i m p o s i t i o n of the contour maps. No other speciejs had i. d i s t r i b u t i o n s which even remotely approached t h i s d e c r e e of o v e r l a p . The r e l a t i v e abundance of the s p e c i e s , i . e . the evenness, d i d not f l u c t u a t e g r e a t l y and most SMDs had e q u i v a l e n t values of the two.heterogeneity i n d i c e s , H' and D. Most changes i n h e t e r o g e n e i t y were -caused by f l u c t u a t i o n s i n the numbers of 78 s p e c i e s e n c o u n t e r e d . The o n l y e x c e p t i o n o c c u r r e d i n t h e summer s a m p l e s f r o m s t a t i o n s 1 a n d 2, when b o t h h e t e r o g e n e i t y i n d i c e s w e r e i n s e n s i t i v e t o t h e l a r g e n u m b e r o f s p e c i e s t h a t ,we,re p r e s e n t . T h i s was b e c a u s e o f a s e v e r e d r o p i n e v e n n e s s a s s o c i a t e d w i t h t h e l a r g e i n c r e a s e s i n L . m a r m o r a t a a n d M a r g a r i t e s c o s t a l i s . B o t h t h e d e p t h a n d t i m e c o m p o n e n t s o f n i c h e - b r e a d t h ^ w e r e p a r t i a l l y d e p e n d e n t u p o n a b u n d a n c e , i n t h a t v a l u e s w e r e g e n e r a l l y s l i g h t f o r s p e c i e s o f l o w d e n s i t y . Some s p e c i e s ( e . g . B a l c i s rnicahs a n d D i a p h a n a c a l i f o r n i e a ) h a d r e l a t i v e l y l a r g e v a l u e s f o r t h e n i c h e - b r e a d t h i n d i c e s d e s p i t e l o w mean d e n s i t y . T h e s e s p e c i e s h a d r e l a t i v e l y c o n s t a n t , t h o u g h l o w , d e n s i t i e s t h r o u g h t h e r e s o u r c e d i m e n s i o n s o f t i m e a n d / o r s p a c e . T h e r e l a t i o n s h i p b e t w e e n a b u n d a n c e a n d n i c h e - b r e a d t h was m o r e p r o n o u n c e d f o r t h e d e p t h c o m p o n e n t , B ( d ) , t h a t i s , s p e c i e s o f l o w d e n s i t y w e r e u s u a l l y more e v e n l y d i s p e r s e d t h r o u g h t i m e t h a n d e p t h , a n d c h a r a c t e r i s t i c a l l y d i s p l a y e d h i g h I N B v a l u e s . T h e c l a s s i f i c a t i o n s c h e m e s u s i n g f r e q u e n c y , mean a b u n d a n c e a n d n i c h e - b r e a d t h p r o v i d e d i n f o r m a t i o n t h a t was n o t a s e a s i l y a t t a i n a b l e f r o m t h e c o n t o u r maps o f mean d e n s i t y . S p e c i e s g r o u p i n g s b a s e d u p o n I N B ( T a b l e V ) , h o w e v e r , s h o w e d o n l y p a r t i a l a g r e e m e n t w i t h t h o s e u t i l i z i n g t h e r e l a t i v e v a l u e s o f o c c u r r e n c e a n d a b u n d a n c e ( T a b l e I I ) . T h i s i n d i c a t e s t h a t w i t h i n a g i v e n g r o u p w i t h a c e r t a i n a m o u n t o f t i m e o r d e p t h z o n a t i o n , t h e r e w e r e s p e c i e s d i s p l a y i n g v a r y i n g d e g r e e s o f c l u m p i n g . F o r e x a m p l e , c e r i t h i o p s i s s p . a n d M. g o u l d i i w e r e b o t h r e s t r i c t e d t o 3 d e p t h s , b u t t h e f i r s t s p e c i e s was i n f r e q u e n t l y 79 encountered, though i n r e l a t i v e l y high numbers, while the other, M. q o u l d i i , was present i n many samples ye t u s u a l l y at a low d e n s i t y . Very l i t t l e confidence can be placed upon any f i n d i n g s r e g a r d i n g s p e c i e s of low d e n s i t y . The sampling method was adeguate f o r s p e c i e s with high d e n s i t i e s , whose r e p r e s e n t a t i o n s v a r i e d l i t t l e between r e p l i c a t e guadrats, but as the abundances of s p e c i e s dropped they tended to become i n c r e a s i n g l y s p o r a d i c i n occurrence; T h i s c o u l d w e l l be an a r t i f a c t , s i n c e t h e r e are decreased p r o b a b i l i t i e s of encounter when s p e c i e s have low d e n s i t i e s or are d i s t r i b u t e d i n t o r a r e clumps, as could have been the case f o r s p e c i e s only observed d u r i n g or j u s t a f t e r p e r i o d s of r e c r u i t m e n t ; Nonetheless, i t i s i m p o s s i b l e to determine i f a " r a r e " s p e c i e s was genuinely uncommon or had simply been undersampled without a d d i t i o n a l , more i n t e n s i v e sampling. In a d d i t i o n , r a r e l y encountered s p e c i e s may have been l o c a l i z e d i n t o non-sampled areas between the s t a t i o n s or i n t o m i c r o - h a b i t a t s such as c r e v i c e s or the undersides of r o c k s . I t i s l i k e l y t h a t the l i m p e t s Acmaea m i t r a , C o l l i s e l l a p e l t a , Notoacmea scutum, and the s n a i l s V e l u t i n a l a e v i g a t a , C r e p i p a t e l l a l i n g u l a t a , which were low i n abundance, are space comp e t i t o r s of macrophytic algae. These f i v e gastropods were u s u a l l y observed upon bare rock at Saturnina I s l a n d . Within a kelp-bed at Bath I s l a n d , 150 m d i s t a n t (Figure 1), t h e r e i s more rock than a l g a l s u r f a c e a v a i l a b l e at t h e bottom and l i m p e t s are r e l a t i v e l y common. 8 0 The P o t e n t i a l Of Diatoms As A Gastropod Food Resource Diatoms comprised a major p o r t i o n o f the gut contents f o r the nine s p e c i e s t h a t were examined. Foods which are are 'eaten on an i n t e r m i t t e n t b a s i s , or d u r i n g non-sampled p e r i o d s , may have been overlooked* s i n c e samples f o r gut analyses were obtained on only two o c c a s i o n s . Most of the diatom s p e c i e s observed on the microscope s l i d e s were found i n the gut p r e p a r a t i o n s . P r o p o r t i o n a t e l y few of the diatom c e l l s found c o n t a i n e d c y t o p l a s m i c m a t e r i a l , i n d i c a t i n g t h a t the gastropods which i n g e s t diatoms are able to d i g e s t them as w e l l . T h i s agrees with N i c o t r i ' s (1977) f i n d i n g t h at most diatom d i g e s t i o n by C o l l i s e l l a p e l t a and Notoacmea scutum - was accomplished by chemical d i g e s t i o n through pores i n the c e l l w a l l s r a t h e r than by mechanical d e g r a d a t i o n . C e l l damage could a l s o be caused by the a c t i o n of the r a d u l a d u r i n g f e e d i n g . Measurements of the p r o p o r t i o n a l r e p r e s e n t a t i o n of empty c e l l s among the gut c o n t e n t s , while p o s s i b l e , were not attempted because d i g e s t i v e c a p a b i l i t i e s would have been overestimated f o r any s p e c i e s which consumed p r o p o r t i o n a t e l y l a r g e amounts of empty f r u s t u l e s . In a d d i t i o n , the r a t i o of f u l l t o empty diatom c e l l s should vary during the p r o g r e s s i o n of a bolus from mouth to anus* but the d i s s e c t i o n technique used d i d not allow determinations of the exact l o c a t i o n s of observed m a t e r i a l s i n the s n a i l s 1 a l i m e n t a r y t r a c t s . Using r a d i o t r a c e r s , Calow (1975) 81 has s u c c e s s f u l l y q u a n t i f i e d d i g e s t i o n by microphagous prosobranchs. Enzymes capable of degrading the a l g a l c o n s t i t u e n t s amylose and c e l l u l o s e are widespread among the gastropoda (Owen 1966b) and have even been found among such s p e c i e s as the d e t r i t i v o r e N a s s a r i u s r e t i c u l a t u s and the predator N u c e l l a l a p i l l u s . There i s not n e c e s s a r i l y any correspondence between enzymatic complement and d i e t (Owen 1966b), but presumably any i n g e s t e d m a t e r i a l t h a t i s d i s s o l v e d or mechanically reduced to p a r t i c l e s i n the f o r e g u t and stomach i s a v a i l a b l e f o r d i g e s t i o n i f s u i t a b l e enzymes are present. An experiment run by Powell (1964) showed t h a t Lacuna sp. ingested C o n s t a n t i n e a s u b u l i f e r a but M a r g a r i t e s sp. d i d not. The same design was used by Roland and Druehl ( p e r s o n a l communication) who found d e f i n i t e c i r c u l a r l e s i o n s on d i s c s c u t from blades of N e r e p c y s t i s luetkeana corresponded to weight l o s s e s over 72 hour p e r i o d s i n which L. marmorata were present i n the j a r s . These f i n d i n g s are i n keeping with the r e s u l t s of the gut analyses performed upon L. marmorata- and M. c o s t a l i s i n t h i s study. That i s , L. marmorata guts were found to c o n t a i n a v a r i e t y of m a t e r i a l such as b i t s of bryozoan s k e l e t o n and polysiphonous red algae as w e l l as diatoms* while only diatoms were ever found w i t h i n M. c o s t a l i s . Apparently, the f e e d i n g n i c h e s of the two dominant s n a i l s only p a r t i a l l y o v e r l a p ; S e v e r a l s p e c i e s of Lacuna r e p o r t e d l y eat macrophytic algae ( F r a l i c k e t a l 1974, F r e t t e r and Graham 1962, Ankel 1936) but p u b l i s h e d l i s t s of the foods known to be 82 u t i l i z e d by Margarites spp. do not i n c l u d e macrophytes ( F r e t t e r and Graham 196 2) . Two e x p l a n a t i o n s f o r d i e t a r y divergence are the d i f f e r e n c e s i n r a d u l a r morphology and mode of o p e r a t i o n e x h i b i t e d by the two genera. The r h i p i d i o g l o s s a n radulae of M a r g a r i t e s , and a l l other Trochidae have f a n - l i k e a r r a y s of marginal t e e t h which sweep smal l p a r t i c l e s of food i n t o the paths of the short and f l a t t e n e d r h a c h i d i a l and l a t e r a l t e e t h which then c a r r y the food i n t o the mouth (Markel 1966, F r e t t e r and Graham 1962). Lacuna has a t a e n i o g l o s s a n r a d u l a with fewer marginals f o r brushing and l o n g e r and sharper l a t e r a l and c e n t r a l t e e t h that can rasp e i t h e r s m a l l e n c r u s t i n g organisms or the s u r f a c e s of macrophytes (Yonge and Thompson 1977, F r e t t e r and Graham 1962).. Some members of the genus A l v a n i a can consume red al g a e , i n c l u d i n g C o r a l l i n a , and d e t r i t u s . The t a e n i o g l o s s a n C e r i t h i o p s i s t u b e r c u l a r i s i s an eater o f sponges and d e t r i t u s ( F r e t t e r and Graham 1962), but C e r i t h i o p s i s sp. o f t h i s study ingested mainly diatoms and had a w e l l developed c r y s t a l l i n e s t y l e , which i s an organ known to produce a m a l y t i c enzymes (Owen 1966b) . D i e t s were not determined f o r 17 of the 26 gastropod s p e c i e s encountered at Saturnina I s l a n d and must be i n f e r r e d from p u b l i s h e d accounts. For many s p e c i e s t h e r e are no d i e t a r y r e c o r d s and the f e e d i n g h a b i t s o f congeners must be used. The foods consumed by the lim p e t s Aemaea m i t r a , C o l l i s e l l a p e l t a , and Notoacmea scutum have been repo r t e d elsewhere and i n c l u d e a wide v a r i e t y of both micro- and macro-algae (C r a i g 1968, Test 83 1945, Castenholz 1961, N i c o t r i 1977). V e l u t i n a l a e v i g a t a i s known as a predator of s o l i t a r y a s c i d i a n s (Diehl 1956). B i t t i u m r e t i c u l a t u m and Nassarius spp. are g e n e r a l l y c o n s i d e r e d d e t r i t i v o r e s but Nassarius eat c a r r i o n as w e l l . Ocenebra s p e c i e s are c a r n i v o r e s t h a t bore through the s h e l l s of a d u l t b a r n a c l e s , b i v a l v e s and s n a i l s or eat j u v e n i l e s e n t i r e i Three s p e c i e s , Odostomia sp., B a l c i s micans, and T u r b o n i l l a vancouverensis are members of the opisthobranch o r d e r P y r a s i d e l l i d a e which are probably a l l s u c t i v o r o u s e c t o -p a r a s i t e s of b i v a l v e s * s n a i l s , s e s s i l e polychaetes and c o e l e n t e r a t e s with v a r i o u s degrees of h o s t - s p e c i f i c i t y ( F r e t t e r and Graham 1949, C l a r k 1971, Thompson 1976). The a c t u a l hosts of these s p e c i e s are unknown; The I n f l u e n c e Of Diatom Abundance On Gastropod D i s t r i b u t i o n s A major f e a t u r e of the diatom d i s p e r s i o n p a t t e r n , one h e l d i n common with the gastropods, was a decrease i n numbers with depth. Aleem (1949) and C astenholz (1963) have r e p o r t e d the r e v e r s e trend f o r b e n t h i c diatoms i n a temperate i n t e r t i d a l zone, where chances of d e s i c c a t i o n i n c r e a s e with h e i g h t on the shore. Because o f sampling d i f f i c u l t i e s , there have been few q u a n t i t a t i v e s t u d i e s o f e p i p h y t i c and e p i l i t h i c diatoms i n the s u b t i d a l h a b i t a t s (Round 1971). Montgomery e t a l . (1977) found evidence of depth-zdnation on one of three c o r a l r e e f s t h a t 84 were i n v e s t i g a t e d . The decrease i n i n c i d e n t i l l u m i n a t i o n with depth i s u s u a l l y c i t e d as the f a c t o r r e s p o n s i b l e f o r the c h a r a c t e r i s t i c d e p t h - p r o f i l e s of diatoms (Chandler 1944, Lund 1949, G r u e n d l i n g 1971). The r a t e of p h o t o s y n t h e s i s i s d i r e c t l y p r o p o r t i o n a l to the i n t e n s i t y of l i g h t , but o n l y when l i g h t i s l i m i t i n g (Rabinowitch and Govindjee 1969). I t i s , t h e r e f o r e , p o s s i b l e t h a t the r a t e of p r o d u c t i o n of b e n t h i c diatoms at S a t u r n i n a I s l a n d i s being i n f l u e n c e d by temperature ( P a t r i c k 1969, Round 1968, Hutchinson 1967, Wallace 1955), turbu l e n c e ( P a t r i c k 1969, Gruendling 1971), s a l i n i t y ( Williams 1964, Kain and Fogg 1958, Mc I n t i r e and Overton 1971, C u r l and Mc Leod 1961, G u i l l a r d and Ryther 1962, Simmons 1957), and n u t r i e n t a v a i l a b i l i t y (Lee et a l . 1975, Round 1971), as w e l l as other p h y s i c a l and c h e m i c a l parameters which may vary with depth (Parsons, Takahashi, and Hargrave 1977). Glass s l i d e s were used to estimate diatom abundance, but the d e n s i t i e s a c t u a l l y r e l a t e t o the r a t e s of c o l o n i z a t i o n over two-week p e r i o d s , r a t h e r than to s t a n d i n g crop on the n a t u r a l s u b s t r a t a , , rock and seaweed. Presumably, r a t e d i f f e r e n c e s on s l i d e s a t the v a r i o u s depths and times are r e p r e s e n t a t i v e of the p r o p o r t i o n a l d i f f e r e n c e s i n c o l o n i z a t i o n upon the n a t u r a l s u b s t r a t a . T h i s assumption i s , i n p a r t , j u s t i f i e d by C a s t e n h o l z ^ (1961) f i n d i n g t h a t l i t t l e d i f f e r e n c e e x i s t e d i n the i n t e r t i d a l diatom assemblages upon var i o u s types of g l a s s and the s u r f a c e s of wood and rock s t h a t were i n c l o s e p r o x i m i t y . 85 L i t t l e i s known regarding the r a t e s of c o l o n i z a t i o n on rocks versus macro-algae, although Round (1971) i n d i c a t e d t h a t e p i p h y t i c diatoms probably e x h i b i t e d some degree of host-s p e c i f i c i t y . De F e l i c e and Lynts (1978) noted that c h a r a c -t e r i s t i c and separate a s s o c i a t i o n s of diatoms oc c u r r e d on T h a l a s s i a testudinum and the mud substratum of Upper F l o r i d a Bay. Montgomery et a l . (1977) reported s i m i l a r d i f f e r e n c e s i n the f l o r a of c o r a l * c o r a l sand and T. testudinum i n the F l o r i d a Keys. Attachment problems vary g r e a t l y between s o f t and hard s u b s t r a t a (Round 1971). Since rocks and macrophytes are both r e l a t i v e l y hard, there may be c o n s i d e r a b l e s i m i l a r i t y between the diatom f l o r a s of these two s u b s t r a t a . Main et a l . (1974) found t h i s t o be the case when e p i l i t h i c assemblages were compared to those on nearby Zostera marina and Ulva, sp. growing i n t e r t i d a l l y i n an Oregon e s t u a r y . Lee et a l . (1975) have d e s c r i b e d a g e n e r a l s u c c e s s i o n a l sequence f o r b e n t h i c diatoms, beginning with s o l i t a r y and c l o s e l y adhering forms such as Coeconeis and Achnanthes, and proceeding to a v e r t i c a l l y l a y e r e d micro-community with an o v e r s t o r y of chain-forming diatoms such as M e l o s i r a . The r a t e of p r o d u c t i v i t y determined the degree of m i c r o - f l o r a l development on the g l a s s s l i d e s , s i n c e only the s l i d e s from s t a t i o n 1, and o c c a s i o n a l l y s t a t i o n 2, d u r i n g the bloom p e r i o d s , were able to a t t a i n the m u l t i - l a y e r e d stage. N i c o t r i (1977) found t h a t the morphology and means of attachment of v a r i o u s b e n t h i c diatoms g r e a t l y a f f e c t e d a v a i l a b i l i t y t o the 8 6 g r a z e r s C o l l i s e l l a p e l t a and Notoacmea scutum - which showed strong p r e f e r e n c e s towards chain-formers; The diatoms at the upper s t a t i o n i n t h i s study were, t h e r e f o r e * not only more numerous, but p o s s i b l y more e a s i l y consumed, although the gut analyses r e v e a l e d no such s e l e c t i v i t y . M e l o s i r a , a c c o r d i n g to N i c o t r i (1977), was the l e a s t d i g e s t a b l e of e i g h t diatoms c o n s i d e r e d , probably owing to the f a c t t h a t t h i s s p e c i e s has r e l a t i v e l y few pores i n . i t s - c e l l . w a l l . Thus g r e a t e r consumption of chain-forming s p e c i e s does not n e c e s s a r i l y imply g r e a t e r n u t r i t i o n a l y i e l d ; The diatom and gastropod assemblages a l s o d i s p l a y e d s i m i l a r p a t t e r n s through time. The youngest observed b e n t h i c stages o f most gastropods were f i r s t noted e i t h e r d u r i n g or j u s t p r i o r t o the s p r i n g diatom bloom, which occurred i n May and June* The p o s i t i v e r e l a t i o n s h i p between j u v e n i l e r e c r u i t m e n t and the diatom blooms was demonstrated i n the case of M. c o s t a l i s by the l a c k of c o r r e l a t i o n when j u v e n i l e s were excluded from the a n a l y s i s (Table X). Whether the gastropod r e c r u i t m e n t i s a c t u a l l y t r i g g e r e d by the diatom bloom i s u n c e r t a i n . Synchrony between phytoplankton blooms and the spawning of s e v e r a l marine i n v e r t e b r a t e s with p e l a g i c l a r v a e has been observed by Thorson (1946,1950) and Himmelman (1975). On the b a s i s of these r e s u l t s , and the experimental i n d u c t i o n of gamete r e l e a s e by S t r o n q y l o c e n t r o t u s d r o b a c h i e n s i s , T o n i c e l l a l i n e a t a , and Tonice11a i n s i g n i s , Himmelman proposed t h a t phytoplankton blooms serve as spawning s t i m u l i through the production and l i b e r a t i o n of e c t o c r i n a l 87 substances. Since other f a c t o r s , such as temperature change (Korringa 1956, Loosanoff 1968, G a l t s o f f 1940) and the presence of sperm (Galtsoff.1938,.1940, Young 1945) may cause spawning, Himmelman suggested t h a t the phytoplankton. are i n d i c a t o r s of optimal environmental c o n d i t i o n s f o r j u v e n i l e s u r v i v a l , and t h a t other i n d i c a t o r s of these c o n d i t i o n s might e x i s t as w e l l . The s e l e c t i v e advantages t h a t would be c o n f e r r e d by such an a d a p t a t i o n are obvious, c o n s i d e r i n g t h a t the younger stages of many i n v e r t e b r a t e s are o f t e n l e s s t o l e r a n t t o extremes i n temperature than the a d u l t s of the same s p e c i e s (Kinne 1964, Thorson 1950). I t i s e q u a l l y l i k e l y t h a t the co-occurrence of diatom blooms and i n v e r t e b r a t e recruitment are caused by c o i n c i d e n t a l a d a p t a t i o n to the same p h y s i c a l f a c t o r s . One c o u l d f u r t h e r suggest t h a t the presence of a l a r g e amount of food i s a major determinant of gastropod l a r v a l and j u v e n i l e s u r v i v a l , s i n c e d u r i n g these phases, i n d i v i d u a l s grow more r a p i d l y (Spight et a l . 1974, North 1954) and have f e e d i n g reguiremeTits t h a t a r e 5-10 times higher than a d u l t s (Thorson 1950). B e n t h i c a l l y d e v e l o p i n g members of the assemblage, i n c l u d i n g the h i g h l y abundant M. c o s t a l i s , L. marmorata., and L. c a r i n a t a , cannot u t i l i z e suspended phytoplankton and should be p a r t i c u l a r l y dependent upon blooms of b e n t h i c diatoms f o r j u v e n i l e s u r v i v a l . In terms o f n u t r i t i o n , . t h e r e f o r e , t h e r e may be some s e l e c t i v e advantage to the s y n c h r o n i z a t i o n of spawning and diatom blooms. Contrary t o Himmelmans's pro p o s a l , perhaps i t i s the p h y s i c a l f a c t o r s which i n d i c a t e whether c o n d i t i o n s are o p t i m a l f o r s u r v i v a l . .. - ,.- . 88 88 I t i s reasonable t o assume t h a t the j u v e n i l e s of some non-herbivorous s p e c i e s take advantage of the l a r g e food r e s o u r c e a v a i l a b l e i n the form of b e n t h i c diatoms i n the s p r i n g . C e r t a i n l y a l l f e e d i n g p l a n k t o t r o p h i c v e l i g e r s consume plankton (Yonge and Thompson 1.976) and some probably continue to eat s m a l l organisms as young a d u l t s , even though there may be d i f f e r e n t d i e t s when mature ( F r e t t e r and Graham 1962). At l e a s t one gastropod, the A f r i c a n pulmonate A c h a t i n a f u l i c a , i s known to switch from h e r b i v o r y t o omnivory as i t grows o l d e r (Smith and van Reel 1950). The d i e t a r y s h i f t of A. f u l i c a corresponds to a decrease i n the r a t i o of a m a l y t i c to p r o t e o l y t i c a c t i v i t y i n the d i g e s t i v e gland d i v e r t i c u l a (Prosser and van Weel 1958). T h i s change of enzymatic complement could not be induced by an a l t e r a t i o n of d i e t and so i s probably a developmental response (van Weel 1959). The f e e d i n g experiment conducted a t S a t u r n i n a I s l a n d had the s u r p r i s i n g r e s u l t t h a t M. c o s t a l i s r e p e a t e d l y s i t u a t e d themselves i n an area o f high diatom d e n s i t y , i . e . the p l a s t i c mesh cage c o v e r s , i n preference to a n a t u r a l l y o c c u r r i n g s u b s t r a t e , Plocamium c a r t i l a g i n e u m , which normally s u s t a i n s l a r g e numbers of s n a i l s . Low d e n s i t i e s of M. c o s t a l i s on P. c a r t i l a q i n e u m a t the end of each 24 hour experimental t r i a l c o u l d have been due to a d e p l e t i o n of food organisms on the seaweed s u r f a c e caused e i t h e r by rough h a n d l i n g or by g r a z i n g e a r l y i n the t r i a l . The r e s u l t s suggest t h a t Margarites c o s t a l i s w i l l move toward l o c a l l y high c o n c e n t r a t i o n s of 89 diatoms c e l l s ; There i s , though, the p o s i b i l i t y t h a t f a c t o r s such as the degree of water movement or the a v a i l a b i l i t y of oxygen i n the cages acted as s t i m u l i f o r s n a i l movements. The f i e l d experiment a l s o demonstrated t h a t heat and hydrogen-peroxide treatments tended t o decrease the d e s i r a b i l i t y of seaweed to M. c o s t a l i s . Whether t h i s l a c k of a t t r a c t i o n i s due to the decreased presence of diatoms or microorganisms, or t o some other c o n d i t i o n of the seaweed s u r f a c e c e l l s i s not known. Lacuna c a r i n a t a and a t r o c h i d , L i r u l a r i a l i r u l a t a , were diatom g r a z e r s whose d i s p e r s i o n s c o r r e l a t e d t o diatom abundance. C o r r e l a t i o n s to diatom d e n s i t y were not found f o r some s p e c i e s whose guts contained diatoms (Tables VIII and X). During p e r i o d s when diatoms were present at r e l a t i v e l y low d e n s i t i e s , an adeguate supply might s t i l l have been a v a i l a b l e f o r these s p e c i e s , and, t h e r e f o r e , food would not have been an a b u n d a n c e - l i m i t i n g f a c t o r . Consumption of other foods, as i n the case o f M. g o u l d i i , which was found to eat egg c a p s u l e s c o n t a i n i n g opisthobranch v e l i g e r s , would free a p o p u l a t i o n from s t r i c t dependence on diatom abundance,. Adult M a r g a r i t e s c o s t a l i s , M. p l i v a c e u s , and G. m a r g a r i t u l a might p o s s i b l y u t i l i z e other foods as w e l l . F a c t o r s such as p r e d a t i o n , c o m p e t i t i o n , p a r a s i t i s m , h a b i t a t s e l e c t i o n , and p h y s i c a l c o n d i t i o n s of the environment which a l s o may be important i n determining d i s p e r s i o n p a t t e r n s , w i l l be c o n s i d e r e d i n f u r t h e r d e t a i l i n the f o l l o w i n g s e c t i o n s . 90 PREDATION Se v e r a l s p e c i e s which can be c o n s i d e r e d p o t e n t i a l gastropod p r e d a t o r s were found at S a t u r n i n a I s l a n d , and m o r t a l i t i e s due to predation may be r e s p o n s i b l e f o r some of the observed v a r i a t i o n s i n gastropod abundance* Although no d i r e c t s t u d i e s of p r e d a t i o n were attempted, what l i t t l e evidence i s a v a i l a b l e i s worthy of review. Bottom-dwelling f i s h capable of t a k i n g s n a i l s ( F i t c h and Lavenberg 1975) were common at S a t u r n i n a . The most abundant f i s h Rhacochilus vacea (the p i l e - p e r c h ) ranged i n s c h o o l s to a l l depths at the s i t e ; Most of the other f i s h a t the s i t e were more s o l i t a r y and t e r r i t o r i a l than the perch, and seemed more i n t i m a t e l y a s s o c i a t e d with the bottom. Copper r o c k f i s h , Sebastes c a u r i n u s , b l e n n i e s , E p i g e i c h t h y s atropurpureseens, k e l p g r e e n l i n g , Hexaqrammus deeagrammus, and s c u l p i n s , O l i g o c o t t u s maculesus and L e g t o c o t t u s armatus, were a l l commonly found. P i s a s t e r ochraceus and L e p t a s t e r i a s h e x a c t i s are two predatory a s t e r i o d s found high i n the s u b t i d a l zone at the Sat u r n i n a s i t e ; Menge (1972) repo r t e d that when these two s p e c i e s co-occurred, L e p t a s t e r i a s consumed s m a l l - s i z e d food such as Mar g a r i t e s spp,. and Lacuna spp. P i s a s t e r was found t o be an e f f i c i e n t p redator of l a r g e gastropods such as N. scutum and C. p e l t a when more h i g h l y p r e f e r r e d foods were u n a v a i l a b l e . Two h i g h l y favored foods of P i s a s t e r , mussels and b a r n a c l e s , were r a r e at S a t u r n i n a . 91 D i v i n g ducks ( M e l a n i t t a n i g r a america, M e l a n i t t a f u s c a d e q l e n d i , C l a n g u l a hyemnali, Ay.thy_a maxila, Bucephala a l b e o l a ) overwinter nearshore i n the S t r a i t o f Georgia (Godfrey 1966, Vermeer and Levings 1977) and great numbers of s e v e r a l c a r n i v o r o u s s p e c i e s can be found i n the F l a t Top I s l a n d s from mid-September through March. The gut contents of ducks a t Boundary Bay, B.C., have been found to i n c l u d e M a r g a r i t e s , Lacuna, A l v a n i a , Odostomia, M i t r e l l a , B j t t i u m , Nassarius, and T u r b o n i l l a (Vermeer and Levings 1977, H i l d a L. Ching, p e r s o n a l communication). S t o t t and Olson (1973) observed t h a t Lacuna y i n c t a was the dominant prey of Clangula i n New England. Vermeer and Levings (1977) rep o r t e d t h a t more s n a i l s were consumed i n the winter than i n the summer. The wintertime decrease i n gastropod abundance a t s t a t i o n 1 r e l a t i v e to s t a t i o n 2 (Figure 5) c o u l d be e x p l a i n e d by a decrease i n b i r d p r e d a t i o n with depth. Although such a phenomenon was not d i r e c t l y observed, i t i s l o g i c a l t o assume th a t d i v i n g b i r d s would expend l e s s energy i n o b t a i n i n g food i f they fed c l o s e t o shore where i t i s a s h o r t e r swim to the bottom. According t o Vermeer and Levings (1977), ducks p r e y i n g upon molluscs were seldom found i n areas where the depth exceeded 3 m, f u r t h e r s u p p o r t i n g t h i s hypothesis; D e p t h - r e l a t e d t r e n d s i n p r e d a t i o n might a l s o account f o r the r e l a t i v e l y low s t a t i o n 1 recruitment by s p e c i e s with b e n t h i c egg masses such as M. c o s t a l i s and L. marmorata. That i s , i f l a r g e numbers of the a d u l t s of a s p e c i e s are removed from any given l o c a t i o n , fewer egg masses would be d e p o s i t e d . 92 adult M. c o s t a l i s were, however, most numerous a t s t a t i o n 1 during the breeding p e r i o d i n e a r l y s p r i n g , and i n the case of t h i s s p e c i e s , at l e a s t , the e f f e c t s of b i r d p r e d a t i o n upon r e p r o d u c t i o n may be discounted. Recruitment may have been hindered p a r t l y by the consumption of eggs, v e l i g e r s , and j u v e n i l e s , although t h e r e was no d i r e c t evidence of t h i s o c c u r r i n g a t Sa t u r n i n a I s l a n d . T h i s form of p r e d a t i o n c o u l d be p r a c t i c e d by any members of the gastropod assemblage with the a b i l i t y t o i n g e s t food as l a r g e as gastropod eggs or egg masses, however only one sp e c i e s was found which consumed the eggs of other gastropods. The consumption o f gastropod eggs by other gastropods i s expected to be most i n t e n s e at s t a t i o n 1 where, d u r i n g the breeding season, the g r e a t e s t numbers of young s n a i l s were found. There were no o b s e r v a t i o n s of d i r e c t p r e d a t i o n on a d u l t s n a i l s by c a r n i v o r o u s gastropods such as Ocenebra, Odostomia, B a l c i s , and T u r b o n i l l a . The P y r a m i d e l l i d Odostomia, however, was h i g h l y abundant and co-occurred with s e v e r a l other s p e c i e s of both high and low abundance (Figure 12). Pub l i s h e d accounts of o t h e r Odostomia s p e c i e s r e v e a l the a b i l i t y t o u t i l i z e prosobranch hosts (Clark 1971, F r e t t e r and Graham 1949, Thompson 1976). R o b i l l i a r d (1971) observed t h a t the nudibranch Dirona a l b o l i n e a t a consumed, among other animals, Margarites p u p i l l u s , M a r g a r i t e s h e l c i n u s , and Lacuna c a r i n a t a a t v a r i o u s s u b t i d a l s i t e s i n the San Juan I s l a n d s ; S o l i t a r y i n d i v i d u a l s of D i a l b o l i n e a t a were o c c a s i o n a l l y observed d u r i n g the monthly 9 3 v i s i t s t o S a t u r n i n a I s l a n d * and were not r e s t r i c t e d to any p a r t i c u l a r depth; P r e d a t i o n by Dirona upon the dominant s n a i l s o f t h i s study i s , t h e r e f o r e , l i k e l y but u n q u a n t i f i e d i There i s some evidence t h a t E o b i l l i a r d m i s i d e n t i f i e d both s p e c i e s of M a r g a r i t e s . Specimens l a b e l l e d M. h e l c i n u s i n .the F r i d a y Harbor i n v e r t e b r a t e c o l l e c t i o n are i d e n t i c a l to the Mar g a r i t e s o l i v a c e u s marginatus of t h i s study (personal o b s e r v a t i o n ) . There i s some degree of c o n f u s i o n among workers at F r i d a y Harbor concerning M a r g a r i t e s p u p i l l u s , which ap p a r e n t l y i s used as a c a t c h - a l l taxon f o r a l l M a r g a r i t e s s p e c i e s with r i d g e d s h e l l s (E. K o z l o f f , . p e r s o n a l communication),. I t i s p o s s i b l e t h a t the M. p u p i l l u s of E o b i l l i a r d i s the M. c o s t a l i s o f t h i s study, however another l i k e l y taxon i s M. salmoneus (Ian McTaggart Cowan, p e r s o n a l communication.) PABASITISM During the course of t h i s study l a r v a l trematode i n f e s t a t i o n s were found among s e v e r a l gastropod p o p u l a t i o n s . Some o f the i n f e s t a t i o n s were so heavy t h a t p a r a s i t i s m must be con s i d e r e d as an important i n f l u e n c e upon gastropod d i s p e r s i o n p a t t e r n s ; A l l of the i d e n t i f i c a t i o n s and counts of trematode l a r v a e were performed by H i l d a L. Ching, whose data are c i t e d i n Table XII. M. c o s t a l i s and M. o l i v a c e u s were the most h e a v i l y p a r a s i t i z e d s p e c i e s , i n terms of the number of i n d i v i d u a l s 94 T a b l e X I I . Summary o f t r e m a t o d e p a r a s i t e s f o u n d t o i n f e c t g a s t r o p o d s a t S a t u r n i n a I s l a n d . D a t a from H. L. C h i n g , p e r s o n a l c o m m u n i c a t i o n . GASTROPOD PARASITE DATE STATION % INFECTED 1 HOST M. costalis Parvatrema sp. Nov. SAug . /76 1 76 t i 2 33 II 3 3 II 4 0 J a n . SApr . /77 1 " 77 L. marmorata Podocotyle Apr . /77 1 8 enophrysi Apr . /78 1 10 A. compacta HEMIURID Nov . 777 1 3 ACANTHOCOLPID Nov. /77 1 1 Nov . /78 1 1 Microphallus pirrim Nov . /77 1 14 Nov. /78 1 2 M. gouldii LEPOCREADID O c t . /77 1 4 1 R e f e r s t o p a r a s i t e s . t h e p e r c e n t a g e Sample s i z e s o f s n a i l s d i s s e c t e d t h a t c o n t a i n e d r a n g e d from 58 t o 357 s n a i l s . 95 i n f e c t e d . M u l t i p l e i n f e c t i o n s were common, and as many as 60 l a r v a l Parvatrema sp were found per i n d i v i d u a l M a r g a r i t e s . The Parvatreaa assume an unusual and unigue p o s i t i o n w i t h i n the e x t r a - p a l l i a l c a v i t y of a d u l t s n a i l s (Ching 1979a). The d i g e s t i v e glands o f i n d i v i d u a l s s u s t a i n i n g l a r g e numbers of trematodes were i n extremely poor c o n d i t i o n , suggesting t h a t the l a r v a l Parvatrema feed i n a manner s i m i l a r t o t h a t of a d u l t f l u k e s . Ching's data i n d i c a t e t h a t the Parvatrema i n f e c t i o n decreased with depth. I t i s q u i t e p o s s i b l e t h a t the r e l a t i v e l y low r e c r u i t m e n t of M. c o s t a l i s at s t a t i o n 1 i s the r e s u l t o f p a r a s i t i s m s i n c e s n a i l s with diminished n u t r i t i o n a l c a p a b i l i t i e s undoubtably have a reduced amount of energy a v a i l a b l e f o r gonad development. Lacuna marmorata was a l s o h e a v i l y p a r a s i t i z e d (Ching 1979b). Although there was no attempt a t a s s e s s i n g the r e l a t i o n s h i p between Podocotyle enophrysi i n f e c t i o n s of Lacuna marrnorata and depth, i t was found t h a t the i n f e c t i o n s were great e r i n A p r i l than i n January. The gonads o f i n f e c t e d i n d i v i d u a l s were v i r t u a l l y destroyed by l a r v a l Podocotyle and, thus, p a r a s i t i s m i s a p o s s i b l e cause of decreased r e p r o d u c t i v e output f o r t h i s s p e c i e s , as w e l l ; 96 SALINITY AND TEMPERATURE Kinne (1964) noted that an organism's s a l i n i t y t o l e r a n c e i s g r e a t l y m o d i f i e d by temperature. At low temperatures, animals seem t o be a b l e to t o l e r a t e s a l i n i t i e s t h a t would be l e t h a l at high temperature. Kinne f e l t t h a t t h i s response i s caused by an i n c r e a s e d metabolic demand placed upon t h e r m a l l y conforming s p e c i e s i n v o l v e d i n osmotic r e g u l a t i o n , coupled with the low oxygen t e n s i o n s i n high temperature water. Kinne (1964) and Thorson (1950) emphasized t h a t many marine and e s t u a r i n e animals t o l e r a t e narrower ranges o f s a l i n i t y v a r i a t i o n as eggs and l a r v a e than as a d u l t s . During the s p r i n g and e a r l y summer, the e a s t - f a c i n g shores of the Gulf I s l a n d s are p e r i o d i c a l l y exposed to the r u n - o f f plume of the F r a s e r R i v e r , f o r p e r i o d s of up to two weeks i n d u r a t i o n . When i t f i r s t reaches Saturnina I s l a n d , t h i s water mass i s e s s e n t i a l l y a 2 to 5m t h i c k tongue of warm, low s a l i n i t y water (Figure 14). Thus, the F r a s e r R i v e r plume may have caused a c o n d i t i o n t h a t was s u f f i c i e n t l y adverse to l i m i t the s u r v i v a l of M. c o s t a l i s a t s t a t i o n 1, during the s p r i n g and e a r l y summer. T h i s provides an a l t e r n a t i v e t o the n o t i o n t h a t l a r v a l trematodes cause a decrease i n r e c r u i t m e n t . 97 F i g u r e 14. Surface view of the Saturnina I s l a n d s i t e viewed from the East. Note the F r a s e r River plume extending a c r o s s the photograph from the l e f t . Photograph courtesey of Dr. R.E. Foreman. 98 99 THE ROLE OF SUBSTRATE REQUIREMENTS IN HABITAT SELECTION The d i s t r i b u t i o n s of c e r t a i n gastropods may be l a r g e l y c o n t r o l l e d by s p e c i f i c s u b s t r a t e requirements. T h i s form of h a b i t a t s e l e c t i o n may be e x e r c i s e d e i t h e r by the a d u l t s d u r i n q egg l a y i n g or through the s e t t l e m e n t p a t t e r n s of j u v e n i l e s . In many cases* j u v e n i l e s a r e known t o s e t t l e i n one area and l a t e r migrate to d i s t a n t spawning p o s i t i o n s . I t appears t h a t j u v e n i l e s of the common i n t e r t i d a l l i m p e t of C o l l i s e l l a p e l t a metamorphose from v e l i g e r s s u b t i d a l l y and then migrate upwards, s i n c e only young specimens of t h i s s p e c i e s were c o l l e c t e d s u b t i d a l l y , and these were r e s t r i c t e d t o only a few months i n the e a r l y summer, du r i n g which time t h e i r d i s t r i b u t i o n became p r o g r e s s i v e l y shallower. Frank (1965) found a s i m i l a r s i t u a t i o n f o r Acmaea d i g i t a l i s , and concluded t h a t j u v e n i l e s were l e s s able to withstand d e s i c c a t i o n i n the i n t e r t i d a l zone, while the a d u l t s were probably escaping from s u b t i d a l predators. The u t i l i z a t i o n of i n t e r t i d a l h a b i t a t s by C. p e l t a may be a u s e f u l a d a p t a t i o n f o r reducing p r e d a t i o n i n s i t u a t i o n s where s u b t i d a l p r e d a t o r s are abundant, but t h i s h a r d l y seems the case at Saturnina I s l a n d , where b i r d p redators are extremely p r e v a l e n t and there i s r e l a t i v e l y l i t t l e p r o t e c t i v e cover or shade a f f o r d e d by the i n t e r t i d a l a l g a e . The v e r t i c a l m igration behavior e x h i b i t e d , however, may be p a r t l y r e i n f o r c e d by i t s r o l e i n the r e d u c t i o n of competition f o r food and space, at l e a s t at Saturnina I s l a n d . A s i m i l a r s i t u a t i o n probably e x i s t s f o r a d u l t s of another common i n t e r t i d a l s p e c i e s 100 of limpet Notoacmea scutum, which were a l s o found s u b t i d a l l y . Notoacmea scutum, though, had s l i g h t l y g r e a t e r d e n s i t i e s s u b t i d a l l y , and i s u s u a l l y found lower i n t e r t i d a l l y than C. p e l t a (Test 1945), perhaps r e f l e c t i n g a lower t o l e r a n c e to d e s i c c a t i o n , or a g r e a t e r a b i l i t y t o compete f o r food when othe r gastropods are present; J u v e n i l e s of other s p e c i e s may i n h a b i t the same, l o c a t i o n s as the a d u l t s * i n cases where the young i n d i v d u a l s have p h y s i o l o g i c a l requirements t h a t are s i m i l a r t o the a d u l t s . In these cases there i s a s e l e c t i v e advantage to having b e n t h i c , r a t h e r than p l a n k t o t r o p h i c , development. The l a r v a e of M a r g a r i t e s , Lacuna, and L i r u l a r i a develop b e n t h i c a l l y , but l i t t l e i s known of the p h y s i o l o g y of these organisms when young. I t i s known, however* t h a t a d u l t t r o c h i d s are v i r t u a l l y l i m i t e d to the areas with hard s u b s t r a t e s because t h e i r p r i m i t i v e , e a s i l y f o u l e d g i l l s , allow only l i m i t e d exposure t o s i l t laden water (Yonge and Thompson 1976). The t r o c h i d s a l s o d i s p l a y low t o l e r a n c e t o d e s i c c a t i o n , and when found i n t e r t i d a l l y , are g e n e r a l l y r e s t r i c t e d t o shaded or moist p o s i t i o n s . Grahame (1978) reported t h a t Lacuna p a l l i d u l a e x c l u s i v e l y i n h a b i t s and d e p o s i t s egg masses upon Fucus s e r r a t u s , but no reason was o f f e r e d f o r t h i s r e l a t i o n s h i p . There was the p o s s i b i l i t y t h a t the r e l a t i o n s h i p was due only t o chance or t h a t the Fucus was c o i n c i d e n t a l l y l o c a t e d w i t h i n an area t h a t was optimal f o r the s u r v i v a l of the Lacuna. Another gastropod of... t h i s study,. V e l u t i n a . . i s d e f i n i t e l y . known to be a s s o c i a t e d 101 with a s c i d i a n s which i t feeds upon, and excavates t o form brood chambers f o r egg masses (Diehl 1956, F r e t t e r and Graham 1965). I t i s l i k e l y t h a t t h e r e i s some chemical b a s i s f o r s p e c i f i c r e l a t i o n s h i p s o f - t h i s kind* but few s t u d i e s have, focused upon marine gastropods i n t h i s r e gard. K r e i g s t e i n (1974) d i s c o v e r e d , however, t h a t the swimming y e l i g e r s of A p l y s i a c a l i f o r n i c a would only metamorphose a f t e r settlement upon f r o n d s o f L a u r e n c i a c a l i f o r n i c u m . The algae may have provided e i t h e r t a c t i l e s t i m u l i or a chemical cue. In t h i s study, no attempt was made t o expl o r e the r e l a t i o n s h i p s between the d i s p e r s i o n s of gastropod s p e c i e s and the presence of s p e c i f i c macrophytic algae. The f e e d i n g experiment, however, suggested t h a t the dominant s n a i l M a r g a r i t e s c o s t a l i s u t i l i z e s seaweed mainly as a s u r f a c e which c o n t a i n s food organisms. I f t h i s i s t r u e , then there should be a d e f i n i t e correspondence between the numbers of s n a i l s and the amount o f seaweed s u r f a c e present at Saturnina I s l a n d . T h i s hypothesis was not t e s t e d . 102 CONCLOSIONS The d i s t r i b u t i o n s of the i n d i v i d u a l s of most of the gastropod s p e c i e s at Saturnina I s l a n d d i s p l a y e d d e f i n i t e s i m i l a r i t i e s through both depth and time, suggesting t h a t environmental c o n d i t i o n s may have a f f e c t e d most of the s p e c i e s i n a uniform manner- There s t i l l remains, however, the p o s s i b i l i t y o f o b t a i n i n g the same r e s u l t from a group of organisms which respond independently t o d i f f e r e n t environmental f a c t o r s * as has been argued by B e r s t e i n e t a l . (1 978). There was some support f o r the i n i t i a l h y p o t h e s i s t h a t the a v a i l a b i l i t y of diatoms was a major f a c t o r a f f e c t i n g s n a i l d i s p e r s i o n s . At s t a t i o n s 3 and 4 d u r i n g the w i n t e r , and at a l l depths i n the- s p r i n g and summer, the t o t a l abundance and s p e c i e s r i c h n e s s of the gastropod assemblage corresponded c l o s e l y to the number of diatom c e l l s p r e s e n t . During the s p r i n g and summer, d i f f e r e n c e s between diatom p r o d u c t i o n between deep and shallow s t a t i o n s are more severe than i n the winter, perhaps as a r e s u l t of i n c r e a s e d shading by phytoplankton and p a r t i c u l a t e matter of t e r r e s t r i a l o r i g i n . Thus the s h a l l o w - d w e l l i n g s n a i l s would have more.food a v a i l a b l e and g r e a t e r chances of s u r v i v a l . Greater food abundance, i f important t o a s p e c i e s , could a l s o cause an i n c r e a s e d growth r a t e . For M. c o s t a l i s , the only s p e c i e s f o r which growth data are a v a i l a b l e , there i s l i t t l e i n d i c a t i o n t h a t the s t a t i o n 1 i n d i v i d u a l s grew any f a s t e r than those at s t a t i o n 2. There may have been some balance between the r a t e of food production and 103 the number of u t i l i z e r s present r e s u l t i n g i n no net d i f f e r e n c e i n the diatom a v a i l a b i l i t y between s t a t i o n 1 r which had more diatoms and consumers, and s t a t i o n 2, with fewer of each. At s t a t i o n 3, where t h e r e were fewer competitors and diatom c e l l s , H« c o s t a l i s were s i g n i f i c a n t l y s m a l l e r . The d i s t r i b u t i o n of j u v e n i l e Margarites c o s t a l i s cannot be e x p l a i n e d e n t i r e l y i n terms of the a v a i l a b i l i t y of diatoms. There i s a chance t h a t very young a d u l t M. c o s t a l i s consume foods which are s m a l l e r than most diatoms c e l l s , such as n a n n o f l a g e l l a t e s * yeasts and b a c t e r i a * among any of the diatom consuming gastropods, there could have been p r e f e r e n c e s f o r c e r t a i n s i z e s or s p e c i e s of foods ( N i c o t r i 1977), but no comparisons between the d i s p e r s i o n p a t t e r n s of i n d i v i d u a l gastropod and diatom s p e c i e s can be made because the diatom assemblage was not examined i n s u f f i c i e n t d e t a i l . There were a great many f a c t o r s , beside diatom abundance, which c o u l d have caused v a r i a t i o n s i n gastropod abundances through depth and time. From a s u b j e c t i v e p o i n t of view, some of the f a c t o r s which seemed p a r t i c u l a r l y important were: p r e d a t i o n * p a r a s i t i s m , h a b i t a t s e l e c t i o n , and the combined a f f e c t s o f of s a l i n i t y and temperature. P r e d a t i o n by b i r d s p r o v i d e s an e x p l a n a t i o n f o r the wintertime d e p r e s s s i o n i n the number of s n a i l s p e c i e s and i n d i v i d u a l s at s t a t i o n 1, r e l a t i v e to s t a t i o n 2. Although t h e r e i s r e a l l y no data with which to judge t h i s c o n t e n t i o n , s n a i l s at deeper s t a t i o n s might have g r e a t e r chances of s u r v i v a l due to i n c r e a s e d p r o t e c t i o n from predation by b i r d s . T h i s advantage 104 i s probably not important i n the summer when t h e r e are comparatively few b i r d s present at Saturnina I s l a n d . The f i e l d experiment demonstrated t h a t M. c o s t a l i s can t r a v e l a t l e a s t over short d i s t a n c e s to areas of g r e a t e r diatom d e n s i t y . Perhaps, then, M. c o s t a l i s and L. marmorata migrate upwards i n the s p r i n g , o b t a i n i n g more food and r e p l e n i s h i n g numbers diminished by b i r d p r e d a t i o n . T h i s would account f o r the r e l a t i v e l y h i g h numbers of a d u l t s of these s p e c i e s t h a t were found at the s h a l l o w e s t s t a t i o n . There i s no d i r e c t evidence t h a t t h i s phenomenon a c t u a l l y occurred* but examples of both upward (Gendron 1977, Lambert and F a r l e y 1969, Thorson 1950) and downward (Kain and Svendson 1969, Frank 1965) m i g r a t i o n s of j u v e n i l e gastropods have been found, and used to e x p l a i n depth z o n a t i o n . The use of h e t e r o g e n e i t y i n d i c e s i n t h i s study, as a means, of determining the evenness of s p e c i e s r e p r e s e n t a t i o n s , i s ! p e r f e c t l y j u s t i f i e d , but i t i s important t o note t h a t h e t e r o g e n e i t y i s a concept of human o r i g i n which does not a c t u a l l y correspond to the e c o l o g i c a l p r o p e r t i e s o f an assemblage. Various models have been proposed t o l i n k r i c h n e s s and evenness to the nature of resource p a r t i t i o n i n g i n communities (Mac Arthur 1960, Motomura 1932, Preston 1948). In p r a c t i c e , few assemblages have been found f o r which s p e c i e s abundance r e l a t i o n s h i p s f i t e i t h e r of the r e s o u r c e u t i l i z a t i o n / c o m p e t i t i o n models (Whittaker 1975, King 1964). One o f the models s t a t e s t h a t r e l a t i v e s p e c i e s abundance may be independent of resource a v a i l a b i l i t y , i n which case i n d i v i d u a l s 105 are randomly apportioned among the species- (Preston 1948),. Most of the models, however, suggest, a p r i o r i , t h a t the abundance of a s p e c i e s i s l a r g e l y determined by the a v a i l a b i l i t y of r e s o u r c e s which i s , i n t u r n , a d i r e c t f u n c t i o n of the number o f other u t i l i z e r s ; T h i s view i s o v e r - s i m p l i s t i c s i n c e the degree of c o m p e t i t i o n and resource s h a r i n g between c o - o c c u r r i n g s p e c i e s may be a f f e c t e d by f a c t o r s other than the number of s p e c i e s and i n d i v i d u a l s present (Eagle and Hardiman 1977, King 1964). The r e l a t i o n s h i p between the r e l a t i v e abundance o f s p e c i e s and c o m p e t i t i o n f o r r e s o u r c e s i s f u r t h e r complicated i n n o n - e q u i l i b r i u m c o n d i t i o n s where there i s a turnover o f s p e c i e s , or i n environments c o n s i s t i n q of patches of r e s o u r c e s . Under these circumstances, the r a t e s of such processes as c o l o n i z a t i o n * l o c a l e x t i n c t i o n * and m i g r a t i o n to new r e s o u r c e patches are a l l important i n determining the l e v e l of co-e x i s t e n c e among s e v e r a l competing s p e c i e s (Horn and Mac Arthur 1972). There i s * t h e r e f o r e * l i t t l e reason t o assume a f i x e d r e l a t i o n s h i p between the r i c h n e s s and evenness of the gastropod assemblage s i n c e l i t t l e i s known of the i n t e r s p e c i f i c r e l a t i o n s h i p s , or whether e q u i l i b r i u m c o n d i t i o n s e x i s t . In the o p i n i o n of Foreman (1976), the shallow red a l g a l communities can be s u b j e c t to s u c c e s s i o n a l changes, p a r t i c u l a r l y f o l l o w i n g p e r i o d i c i n t e n s i v e g r a z i n g by sea u r c h i n s . L i t t l e attempt was made i n the course of t h i s study t o a s s e s s the e f f e c t of e i t h e r s p a t i a l or temporal changes i n the macrophyte community but i t i s l i k e l y t h a t v a r i a t i o n s i n the a l g a l community s t r u c t u r e 106 c o u l d a f f e c t the r e s i d e n t i n v e r t e b r a t e s i n profound ways, such as a l t e r i n g the h a b i t a t complexity by changing the amount o f a v a i l a b l e s u r f a c e area. Even i f the a l g a l community at S a t u r n i n a I s l a n d was r e l a t i v e l y s t a b l e through time, assemblages of r e s i d e n t i n v e r t e b r a t e s could be expected to show m o d i f i c a t i o n s as o v e r l y abundant s p e c i e s encountered i n c r e a s e d c o m p e t i t i o n from immigrants. I f the share of r e s o u r c e s a v a i l a b l e t o a s p e c i e s then became reduced, through c o m p e t i t i o n * to a l e v e l below t h a t amount r e g u i r e d f o r s u r v i v a l , the s p e c i e s would most l i k e l y become excluded from the assemblage. The two most abundant gastropods . a t Saturnina I s l a n d are apparently dependent upon l a r g e annual r e c r u i t m e n t s i n order t o maintain numerical dominance, and c o u l d be supplanted by other s p e c i e s i f t h e i r h igh r e p r o d u c t i v e output were somehow decreased. Although competition f o r r e s o u r c e s by gastropods was not d i r e c t l y observed i n t h i s study, f e a t u r e s of the d i s p e r s i o n p a t t e r n s do suggest t h a t c o m p e t i t i o n i s being decreased through h a b i t a t p a r t i t i o n i n g through depth and time. Apparently the two dominant s n a i l s , M. c o s t a l i s and L. marmorata, share h a b i t a t s and do not p a r t i t i o n t h e i r f e e d i n g n i c h e s a t S a t u r n i n a I s l a n d , which i s not the case f o r t h e i r congeners, M. o l i v a c e u s and L. c a r i n a t a . Lacuna c a r i n a t a reproduces e a r l i e r i n the year than L. marmorata and, i n the summer, i s d i s t r i b u t e d more s h a l l o w l y . Temporal s e p a r a t i o n of recruitment was not so marked f o r the M a r g a r i t e s s p e c i e s , but M. o l i v a c e u s was c l e a r l y r e s t r i c t e d to a depth zone were M. c o s t a l i s was r e l a t i v e l y low 107 i n abundance. Before any r e a l assessment of c o m p e t i t i o n can be made f o r the S a t u r n i n a I s l a n d gastropod assemblage, experimental i n v e s t i g a t i o n s concerning growth r a t e s , r e p r o d u c t i v e s t r a t e g i e s , p r e d a t o r s , and e n e r g e t i c requirments must be performed. T h i s study has e l u c i d a t e d many of the f e a t u r e s o f the s u b t i d a l ( g a s t r o p o d assemblage a t S a t u r n i n a I s l a n d . I t i s not p o s s i b l e to determine, at t h i s stage of i n v e s t i g a t i o n * which f a c t o r s are most important i n c o n t r o l l i n g gastropod d i s p e r s i o n p a t t e r n s , but i t i s l i k e l y t h a t the importance of any one f a c t o r , i n r e l a t i o n t o a l l o t h e r s , v a r i e s with both depth and season. There i s l i t t l e p u b l i s h e d i n f o r m a t i o n a v a i l a b l e to a s s i s t i n understanding the observed f e a t u r e s of the s u b t i d a l gastropod assemblage at Saturnina I s l a n d . The r e s u l t s of t h i s study, however, provide a b a s i s f o r f u r t h e r , more d e t a i l e d i n q u i r i e s i n t o the s t r u c t u r e of s u b t i d a l communities. 108 REFERENCES CITED Andrews, H. 1925. Animals l i v i n g on k e l p . Publ; Puget Sound B i o l . S ta. 5: 25-27. Abbott, R-T. 1974. American S e a s h e l l s . 2nd ed. Van Nostrand Reinhold Co. , New York. 663 pp. Aleem, A.A. 1949. 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Plocamium cartilagineum ( L . ) D i x o n Laurencia spectabilis P o s t , and Rupr. Constantinea subulifera S e t c h e l l Prionitis lanceolata H a r v . Cryptopleura ruprechtiana ( J . A g . ) K y l i n COMMON ALGAE Pterosiphonia dendroidea ( M o n t . ) F a l k . Ulva fenestrata P o s t , and Rupr. Iridaea cordatum ( T u r n . ) B o r y CallophyHis edentata K y l i n Callophyllis flabellulata H a r v . Gelidium purpurescens G a r d . Gelidium crinale ( T u r n . ) L a m o u r o u x Lithothrix aspergillum J . E . Gray Corallina officinalis v a r . chilensis ( H a r v . ) K u e t z i n g Ralfsia fungiformis ( G u n n . ) S e t c h . and G a r d . Ceramium spp. Polysiphonia spp. Laminaria groenlandica R o s e n v i n g e Gigartina sp. 123 A p p e n d i x B The names, acronyms, and f a m i l i e s o f t w e n t y - f i v e g a s t r o p o d s p e c i e s f o u n d a t S a t u r n i n a I s l a n d . NAME ACRONYM ACMAEIDAE Acmaea mitra R a t h k e , 1833 C o l l i s e l l a pelta ( R a t h k e , 1833) Notoacmea scutum ( R a t h k e , 1833) TROCHIDAE Lirularia l i r u l a t a ( C a r p e n t e r , 1864) Margarites costalis ( G o u l d , 1841) Margarites olivaceus marginatus ( G o u l d , 1841) LACUNIDAE Lacuna carinata G o u l d , 1848 Lacuna marmorata D a l l , 1919 RISSOIDAE Alvania carpenter! ( W e i n k a u f , 1885) Alvania compacta C a r p e n t e r , 1864 CERITHIIDAE Bittium eschrichtii ( M i d d e n d o r f , 1849) Cerithiopsis stejnerger i D a l l , 1884 Cerithiopsis s p . ME LANELLIDAE Balcis micans ( C a r p e n t e r , 1864) CREPIDULIDAE Crepipatella lingulata ( G o u l d , 1846) VELUTINIDAE Velutina laevigata ( L i n n a e u s , 1767) MURICIDAE Oc e n e i i r a inter fossa C a r p e n t e r , 1864 COLUMBELLIDAE Amphissa columbiana D a l l , 1916 Mitrella gouldii ( C a r p e n t e r , 1857) N a s s a r i d a e Nassarius mendicus ( G o u l d , 1849) ACMAMIT COLLPEL NOTOSCU LIRULIR MARGCOS MARGOLI LACUCAR LACUMAR ALVACAR ALVACOM BITTESC CERISTE CERISPP BALSMIC CREPLIN VELULAE OCENINT AMPHCOL MITRGOU NASSMEC A p p e n d i x B. C o n t i n u e d . NAME CANCELLARIDAE Admete circumcincta MARGINELLIDAE Granulina margaritula PYRAMIDELLIDAE Odostomia s p . Turboni11a Vancouver ensis DIAPHANIDAE Diaphana c a l i f o r n i c a 125 APPENDIX C Mean d e n s i t y f o r s i x t e e n gastropod s p e c i e s contoured a g a i n s t s t a t i o n and month of c o l l e c t i o n . STATION STATION 3 2 1 4 3 2 1 DBSNVrdl NO NOUISOd STATION STATION 3 . 2 . 1 4 3 2 1 0 2 OT 0 0 £ 06" 0 1 0 103SNVcJl NO NOUISOd 128 Granulina margaritula Odostomia s p . \ STATION STATION 4 3 2 1 4 3 2 1 0 £ 0 2 OT 0 0 £ 0(3 OT 0 103SNV81 NO NOUISOd 0£ .02 01 0 0£ 0 2 0 1 iOHSNVyi NO NOUISOd 134 A p p e n d i x D. The mean number o f i n d i v i d u a l s p e r q u a d r a t , o r mean d e n s i t y , f o r t e n g a s t r o p o d s p e c i e s o f low abundance. SPECIES MONTH STATION : MEAN DENSITY Collisella pelta May 2 0.2 J u l . 1 1.0 J u l _ . _ 2 1;.2_ Bittium eschrichtii J a n . 1 0.2 Mar. 1 1.4 A p r . 2 1.4 May 1 2-i_ Cerithiopsis stejnegeri Dec. 2 0.2 Dec. 3 0.2 J a n . 1 0.2 Feb. 3 .0.2 J u n . 1 0.2 J u n . 2 0.2 J u n . 3 0.2 J u n . 4 0.2 _Sep. 3 0.2 Ocenebra interfossa Dec. 3 0.2 J u n . 2 0.8 J u l . 4 0.2 Aug. _ 1_ 0^4 Nassarius mendicus J a n . 1 0.6 Feb. 1 0.2 Ju n . 3 0.2 Aug._ _ 3_ 0.2 Velutina laevigata May 2 3.0 J u l . 1 0.2 J u l . 3 0.2 Sep^ 3_ (K2 135 A p p e n d i x D. C o n t i n u e d . SPECIES MONTH STATION • MEAN DENSITY Crepipatella lingulata Mar . 1 0. 2 J u l . 1 0 . 2 J u l . 2 0.4 Aug . 1 0 . 2 Acmaea mitra May. 3 0.2 Ju n . 1 0. 2 Sep . 3 0. 2 u n i d e n t i f i e d r i s s o i d J u n . 2 0.2 J u l . 2 0.2 Turboni1 la vancouverensis Aug. 3 0. 2 

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