VARIATION IN MOLLUSCAN ASSEMBLAGE STRUCTURE, BODY SIZE, AND PREDATION ALONG A SALINITY GRADIENT ON SAN SALVADOR ISLAND, BAHAMAS

Many ecological parameters vary along gradients in environmental conditions. For example, some predatory gastropods have evolved a specialized organ that excretes a mixture of acids and enzymes to weaken shells of their prey, which may be less effective in hypersaline settings than in normal marine conditions. We observed molluscan community composition, body size, and predation frequency along a natural salinity gradient on San Salvador Island, Bahamas. Salinity measurements and five bulk samples of death assemblages were taken at 13 sample locations including Pigeon Creek, Snow Bay, Storr’s Lake, Watling’s Blue Hole, and 6 hypersaline and marine ponds from the northeast corner of the island. The locations varied in salinity and ocean connectivity (openly connected, tidally influenced, or fully isolated), which could influence the ability of predators to access each location. 2,600 individuals belonging to 103 species were measured and identified. Salinity ranged from 26 to 76 ppm, with most locations around 40 ppm. Non-metric multidimensional scaling (nMDS) analyses were performed to ordinate samples in terms of fauna composition. Most locations clustered into groups representing similar salinity regimes. Predators and drill holes were absent from all samples except Snow Bay and Pigeon Creek. There was a slight gradient in drilling frequency following the salinity gradient in the four Pigeon Creek sampling locations. Molluscan diversity was lowest at Storr’s Lake (2 gastropods), while the rest of the lakes and ponds contained at least 10 species, with similar taxa. Diversity was highest at the mouth of Pigeon Creek and at Snow Bay (44 species). At these sites, 3 gastropod and 3 bivalve species experienced drilling, and these species were also present in the lakes and ponds. Clearly, the lack of preferred prey species was not the cause for the absence of drilling predation in these samples. At least two gastropod species were present at most locations, yet body size did not correlate with salinity. Researching the effects of modern environmental gradients on molluscan assemblages, predation, and body size can improve interpretations of paleocommunities from the fossil record.