RICE UNIVERSITY ECOLOGY AND PALEOECOLOGY OF MARINE INVERTEBRATE COMMUNITIES IN CALCAREOUS SUBSTRATES, NORTHEAST QUINTANA ROO, MEXICO by Allan Anton Ekdale 3 1272 00197 1892 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS Thesis Director's signature: Houston, Texas December 1972 ABSTRACT ECOLOGY AND PALEOECOLOGY OF MARINE INVERTEBRATE COMMUNITIES IN CALCAREOUS SUBSTRATES, NORTHEAST QUINTANA ROO, MEXICO by Allan Anton Ekdale A survey of benthic communities in the vicinity of Isla Cancun and Isla Contoy, Territory of Quintana Roo, Mexico, demonstrates that invertebrate death assemblages (i.e., future fossil assemblages) in this region reflect the in-place accumulation of remains of living benthic communities through long periods of time. Samples were collected with a diver-operated suction dredge, and 290 molluscan species were identified and count¬ ed. Although nearly three-fourths of the species are known only as dead shells in the samples, almost all individuals occurring alive in any sample are represented by dead shells of their species in the same sample. Worn shells are uncommon, and a count of disarticulated bivalve valves revealed a nearly even left-right distribution. A series of twelve Q-mode cluster analyses, utilizing five different similarity coefficients and data based on both the presence or absence and the relative abundances of species, demonstrates that the same associations of samples tend to occur whether living animals only or dead remains only are considered. A Q-mode analysis based on the relative abundances of 180 species in the death assemblages of all 50 samples results in distinct clusters corresponding directly to each of the major environments sampled. An R-mode analysis of 66 common species in the death assemblages yields three distinct groups of organisms from environments of (a) restricted circulation, (b) open marine circulation and (c) swift currents. These results imply that large- scale transport of shell material and mixing of faunas from different environments are negligible in the areas sampled. Rankings of samples in terms of species diversity and total abundance indicate that most samples occupy roughly the same position in the lists whether only dead shells or only living organisms are considered. Associations computed between sample couplets with Chi- square tests and various similarity coefficients demonstrate that the death assemblages in each couplet are similar, whereas the living communities are not. Histograms of growth ring counts and size-frequency distributions of Chione cancellata in lagoon and strait samples resemble a mortality curve expected for a single generation of a living population of that species. The assemblages of dead shells in the sediments of this region represent "time-averaged communities" (Walker and Bambach, 1971), produced by the averaging of benthic commun¬ ities during sedimentation as patchily distributed popula¬ tions migrate across the bottom and leave a record of their mortality behind them in the sediment. TABLE OF CONTENTS INTRODUCTION p. 1 Significance of Modern Benthic Studies p. 1 Previous Work p. 2 Scope and Aim of this Study p. 3 Acknowledgements p. 5 SETTING p. 8 Regional Setting p. 8 Physical Environment p. 10 Sample Localities p. 11 METHODS p. 15 Equipment p. 15 Field Methods p. 17 Laboratory Methods p. 19 SYSTEMATIC DISCUSSION OF FAUNA p. 21 Introduction p. 21 Faunal List p. 22 QUANTITATIVE ASPECTS OF FAUNAL DISTRIBUTION p. 83 Introduction p. 83 Diversity and Abundance p. 85 Cluster Analysis p. 89 Coefficients of Similarity p. 90 Q-mode Clusters p. 93 R-mode Clusters p. 105 Population Distribution p. 107 Population Dynamics p. Ill SUMMARY AND CONCLUSIONS p. 115 REFERENCES p. 119 APPENDICES p. 124 A. Index to Species p. 124 B. Sample Data p. 135 C. Faunal Distribution p. 140 PLATES p. 141 ADDENDUM p. 159 LIST OF FIGURES 1. Map of Northeast Qunitana Roo p. 9 2. Map of Is la Cancun p. 12 3. Map of Isla Contoy p. 14 4. Diagram of suction dredge p. 16 5. Molluscan percentages in samples p. 84 6. Ranking of samples p. 87 7. Q-mode dendrogram with Dice coefficient p. 94 8. Q-mode dendrogram with Jaccard coefficient p. 95 9. Q-mode dendrogram with Match coefficient p. 96 10. Q-mode dendrogram with Dice coefficient p. 97 11. Q-mode dendrogram with Distance coefficient p. 98 12. Q-mode dendrogram with Correlation coefficient .. p. 99 13. Q-mode dendrogram with Correlation coefficient .. p. 104 14. R-mode dendrogram with Dice coefficient p. 106 15. Chi-square association along "B” traverse p. 109 16. Growth ring analysis of Chione cancellata p. 112 17. Size-frequency distribution of C. cancellata .... p. 113 ECOLOGY AND PALEOECOLOGY OF MARINE INVERTEBRATE COMMUNITIES IN CALCAREOUS SUBSTRATES, NORTHEAST QUINTANA ROO, MEXICO INTRODUCTION Significance of Modem Benthic Studies Observations of processes at work in modern environ¬ ments have aided geologists since the time of James Hutton and Charles Lyell in solving riddles of the past. This uniformitarian approach to scientific problems, moving from the familiar towards the unknown, is the virtual foundation of the geological sciences. Since the great majority of fossil assemblages in the geologic record formed as sedimentary deposits in the marine realm, it is important for a paleoecologist to observe the organisms and their relationships with the environment in today's oceans if he ever hopes to understand the ecology of the sea and its inhabitants millions of years ago. Such geologically-oriented studies in modern marine ecology within the last decade or two have proved very beneficial to geologists, particularly in the fields of paleoecology, sedimentology, environmental stratigraphy and evolutionary paleontology. 2 Previous Work Since the classic work of Peterson (1911, 1913, 1914) in marine synecology, descriptive coverage of sea bottom communities has extended from estuarine and intertidal ranges to the abyssal depths, with the greatest bulk of the work centering around coastal lagoon and bay systems and shallow continental shelves. Thorson (1957) reviewed the general geologic importance of such studies which investigate the relation of benthic organisms to their physical environment, especially to the substrate and to the other organisms shar¬ ing the same environment. A broad faunal survey in the Great Bahama Banks by Newell, et al. (1959) established "habitat communities" of benthic invertebrates, so that specific environments could be characterized by distinctive groups of animal species. Turney and Perkins (1972) extensively sampled the molluscs of Florida Bay in a broad study to determine the relation between molluscan distribution and environmental variations and also the contribution of molluscs to the sediment in that region. Quantitative sampling techniques and cluster analysis enabled Warme (1969, 1971) to outline recurrent groups of organisms in Mugu Lagoon in southern California, and he demonstrated that the composition of death assemblages there directly reflects the actual living communities in most of the environments. The work of Ginsburg (1956) on faunal content of carbonate sediments in Florida Bay demon- 3 strated in-place accumulation of bioclastic material with virtually no transport of sand-size or larger particles from one major subenvironment to another. Results from all four of these studies suggest that shell transport after death is of negligible importance in the formation of death assem¬ blages, and this is of great interest to paleoecologists who are constantly concerned about mixing of fossil faunas before burial by waves and currents. Scope and Aim of this Study This study encompasses both the ecology and paleoecology of the shallow sea bottom along the northeastern Yucatan coast. The traditional approach to paleoecology has primarily consisted of attempts to relate organisms to aspects of their physical environments, such as temperature, salinity, oxygen content of the water, etc. (see Hedgepeth and Ladd, eds., 1957). The thesis of this work, however, is that faunal surveys of modern organism communities and popula¬ tions taken independent of the physical environment can yield valuable paleoecological implications. In other words, the organisms themselves, in terms of species presence or absence, abundance, diversity, distributional patterns, population dynamics, and so forth, are regarded as the most significant measures of the environmental factors affecting them. It is very difficult for a paleoecologist to read the 4 salinity or water temperature during formation of fossil deposits. The information which is most readily available to him consists of only the animals and the sediment en¬ closing them. This modern faunal survey was conducted along the northeastern coast of the Yucatan Peninsula (Territory of Quintana Roo, Mexico) to determine the composition and dis¬ tribution of benthic organism communities in this region. Attention was given to both the live and dead constituents in the samples, so that a judgment could be made concerning the relation between the dead remains of animals in the sediment (i.e., the future fossil assemblage) and the fauna actually living in the same sediment (i.e., the true living community). Thus, both the ecology and paleoecology of the shallow marine environment along this portion of the Yucatan coast are considered. A quantitative approach was attempted in all aspects of this investigation, from the initial selecting of sample sites and collecting of the samples themselves