Patterns in Diversity and Distribution of Benthic Molluscs Along a Depth Gradient in the Bahamas

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Dowgiallo, Michael Joseph
Reaka-Kudla, Marjorie L.
Species richness and abundance of benthic bivalve and gastropod molluscs was determined over a depth gradient of 5 - 244 m at Lee Stocking Island, Bahamas by deploying replicate benthic collectors at five sites at 5 m, 14 m, 46 m, 153 m, and 244 m for six months beginning in December 1993. A total of 773 individual molluscs comprising at least 72 taxa were retrieved from the collectors. Analysis of the molluscan fauna that colonized the collectors showed overwhelmingly higher abundance and diversity at the 5 m, 14 m, and 46 m sites as compared to the deeper sites at 153 m and 244 m. Irradiance, temperature, and habitat heterogeneity all declined with depth, coincident with declines in the abundance and diversity of the molluscs. Herbivorous modes of feeding predominated (52%) and carnivorous modes of feeding were common (44%) over the range of depths studied at Lee Stocking Island, but mode of feeding did not change significantly over depth. One bivalve and one gastropod species showed a significant decline in body size with increasing depth. Analysis of data for 960 species of gastropod molluscs from the Western Atlantic Gastropod Database of the Academy of Natural Sciences (ANS) that have ranges including the Bahamas showed a positive correlation between body size of species of gastropods and their geographic ranges. There was also a positive correlation between depth range and the size of the geographic range. Nearly 80% of the species of gastropods in the ANS data set are less than 30 mm in body size, indicating that most gastropods in the Bahamas are small. A relatively high number of species of gastropods in the ANS data set that occur in the Bahamas had geographic ranges that extended into the Eastern Pacific (37%) and into the Brazilian (50%) provinces, though ranges of species tended to show highest densities centered in and near the Caribbean province. One of the more obvious faunal boundaries for the gastropods in the ANS data set was their northernmost limit, around Cape Hatteras, where colder northern water masses converge with the warmer waters of the Gulf Stream.