OBSERVATIONS ON INTERTIDAL ORGANISM ASSOCIATIONS OF ST. CATHERINES ISLAND, GEORGIA I. GENERAL DESCRIPTION AND PALEOECOLOGICAL IMPLICATIONS ROBERT W. MORRIS AND HAROLD B. ROLLINS BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY VOLUME 159: ARTICLE 3 NEW YORK: 1977 __ -- ,. , Z_,_ - -&..-,flll'i. Tr,! :- ". , "' , ,-'._ '. :,"X,',. "., i---".` .1 .W" 'IT , f-."-% - ,e .tci. .: .., I ..' .1 _v ., " :.,.',`..'.4;., , _, 6 " '.-- "- - .'t ,."m.,I t..,%.`,TN' ,,, ,.1. 4's _' ..4` . -- N` ,-.,..ijc. -.:- -,g, 7 -`.." 1.; ._ -, "_6 .-0. :,.,,. -" -.-I4.w,,-., ,i,, -,-, -`:: .-`. .I" ,... .-.. ,-7.Q,,- ,,.."... .. F. ,",-k". i&. ,t," .. ,-..I.%_ .A -,."Iw M ,,, .1111' ', . ,.. x.,. , ",4."N.w ;. I" P,-_.,l * ] ..`,,:_ "I ,l 7,,,`0. z," _'W", .- .,',: _. -.. i'T,. .-, 'k, .4.-. ', .L? . .i_.. I-- 11, ..Nll_*c _',.,i,o.i-, 1; R, .11,I AlA i, ,-111V .,.,14-z" 'A ":_ "I'-p'N -",Ie, ", .., "I;"".' 41T, .,l -- ...;,_, _.. .,..,;-K.JXIK,;.!.t;, . ,,. e 11,1`. 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ROLLINS Research Associate, Department of Fossil and Living Invertebrates The American Museum of Natural History Associate Professor, Department of Earth and Planetary Sciences University of Pittsburgh BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY VOLUME 159: ARTICLE 3 NEW YORK: 1977 BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY Volume 159, article 3, pages 87-128, figures 1-29, tables 1-9 Issued December 22, 1977 Price. $2.80 ISSN 0003-0090 Copyright © The American Museum of Natural History 1977 ABSTRACT Intertidal environments of St. Catherines Association boundaries were sharp only where Island, Georgia are diverse and include exposed spatial heterogeneity was pronounced, such as sand beaches, sandy tidal flats, relict salt marsh within salt marsh complexes. The polychaete deposits (mud and peat) on exposed beaches, and associations along sandy tidal flats were less salt marsh complexes. Nine localities from the clearly bounded and were often marginally inter- northern half of St. Catherines Island were gradational. selected for study because they displayed a wide Many observations on the intertidal associa- variety of intertidal habitats and organism asso- tions of St. Catherines Island were in disagree- ciations. No attempt was made to describe the ment with observations made on similar associa- entire spectrum of intertidal associations on St. tions elsewhere. Contrary to the conclusions of Catherines Island. Woodin (1976), dense infaunal bivalve popula- The bulk of study was made on the relict salt tions (Petricola pholadiformis) did coexist with marsh deposits. Besides representing unique dense epifaunal bivalve populations (Brachi- modern habitats for infaunal and epifaunal bi- dontes recurvus). Also, repeated observations of valve-dominated associations, the salt marsh de- dense tube-building polychaete populations posits gave us valuable paleoecological insights. failed to show epifaunal bivalves as predominant The organism groupings recognized in the present co-occurring forms. study are associations in the sense of Kauffman Polychaete associations along a narrow tidal and Scott (1976), but communities in the usage flat displayed onshore-offshore distributional of most workers. One relict mud occurrence con- trends that may have paleoecological utility in tained an assemblage of six distinct associations. the resolution of transgression-regression se- The fidelity of replication of these associations in quences. the fossil record depends, in part, upon the steep- Tidal creek populations of Ilynassa obsoleta ness of the intertidal environmental stress gradi- have a narrower aperture relative to total height ents and the resultant spatial and temporal en- than similar populations from more open sand vironmental heterogeneity. The infaunal bivalves flats. It is suggested that this is correlated with of the relict muds showed expected trends of size the greater duration of intertidal exposure in the distribution with depth, the larger individuals oc- tidal creek habitat. Study of another intertidal cupying the greater depths. It is suggested that snail, Littorina irrorata, demonstrated consider- the lateral transportation of dislodged mud clasts able lateral as well as vertical motility in that has an analog in the geological past in the occur- species. rence of "exotic" fossiliferous pods and stein- kerns. INTRODUCTION It is often said that one approach toward bet- constantly changing. However, both ecologists ter understanding of the fossil record is through and paleoecologists often study figure minus the study of living organisms and organism com- ground. munities in their normal habitats (the "transfer- This study should not be viewed as an attempt red ecology" approach, Lawrence, 1971). The to completely describe the intertidal communi- converse should be equally apparent, because, as ties of St. Catherines Island, Georgia. Rather, em- expressed in terms of the Gestalt metaphor of phasis is on selected aspects of the intertidal asso- figure-ground (McLuhan and Nevitt, 1972), every ciations; in particular, those aspects that have situation is a figure for a ground of events and re- paleoecological impact. Intertidal associations lationships. A figure can only be understood in were studied in the following environments: ex- terms of its ground (context) and the ground is posed sand beach, protected sand beach, sandy 89 90 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY VOL. 159 tidal flat, relict salt marsh (mud and peat) de- community to be simply "a group of organisms posits on exposed beach, and salt marsh complex. occurring in a particular environment, presum- To date, the nearshore marine organisms of ably interacting with each other and with the en- St. Catherines Island have never been the subject vironment, and separable by means of ecological of any published study. To our knowledge, the survey from other groups." The intertidal organ- only previous work on the subject is an unpub- ism groups recognized in this study are best de- lished report (Feinberg and Old, 1972), that scribed as associations, following the approach of listed the marine organisms, mainly molluscs, col- Kauffman and Scott (1976), since no attempt lected during a short visit to the island. However, was made to deal with all the component species several excellent studies have been published con- of a particular environmental setting. An associa- cerning the intertidal and subtidal faunas and tion is a group of organisms derived from a single sediments of nearby Sapelo Island (Frey and community (Kauffman and Scott, 1976, p. 19). Howard, 1969; Howard and Dorjes, 1972; How- ard and Reineck, 1972; Howard and Frey, 1973; ACKNOWLEDGMENTS Dorjes, 1972; Hertweck, 1972; Wiedemann, 1972). In addition, studies of nearshore marine The present paper gives some of the results of molluscan communities (Bird, 1970) and bio- investigations conducted on St. Catherines Island genic sedimentary structures (lebensspuren) during a two-week period (June 23-July 7) in made by benthic organisms (Frey, 1970) in the 1975 and a three-week period (June 7-June 28) Beaufort, North Carolina, area have proved quite in 1976. We are very grateful to the Edward John helpful in this investigation. Noble Foundation for providing the funds and Because the term "community" is constantly opportunity that made this study possible, as used by both ecologists and paleoecologists in a well as for permission to visit the island and use variety of ways, any ecological study should, at its facilities. We are also indebted to the Ameri- the onset, attempt to clearly state the manner of can Museum of Natural History for administering use of the