GASTROPOD ASSOCIATIONS AS A PROXY FOR SEAGRASS VEGETATION IN A TROPICAL, CARBONATE SETTING (SAN SALVADOR, BAHAMAS)

Seagrass meadows are productive marine ecosystems which stabilize sediments, and provide food and shelter for a diverse community of associated organisms. The recognition of these important habitats in the geological record is problematic since marine angiosperms rarely fossilize. Thus, the presence of paleo-seagrass vegetation often has to be inferred from the occurrence of associated organisms with a higher potential for fossilization, such as mollusks. Since most mollusk taxa are not restricted to seagrass meadows, the species composition and feeding ecology (e.g., the high abundance of small grazers) of fossil mollusk faunas needs to be considered when distinguishing paleo-seagrass meadows from other marine habitats. In this study the utility of faunal composition and feeding ecology of gastropods as an indicator of seagrass vegetation was tested using present-day ecosystems. Gastropod death assemblages from shallow water seagrass meadows, sand flats, and adjacent unvegetated areas from San Salvador Island, Bahamas were bulk sampled in July 2012. Vegetation varied across localities in terms of number of occurring sea grass species and vegetation density. Twenty one standardized (n=200) samples of gastropod shells were compared in terms of species composition and relative abundance of feeding guilds (herbivores, predatory and browsing carnivores, and suspension feeders). Multivariate analyses indicate that feeding guilds and species composition are effective tools for distinguishing between gastropod assemblages from vegetated versus unvegetated areas. No significant difference is seen across seagrass localities, nor between seagrass localities and the adjacent sand flats. This suggests that vegetation density and species composition of seagrass meadows do not induce notable variation on the composition of gastropod assemblages. Assemblages from unvegetated areas which are adjacent to seagrass meadows are mixed with shells transported from the vegetated zone, but differ in relative abundance and taphonomic signature when comparing with assemblages from the vegetated zone. The results suggest that gastropod assemblages may be a useful proxy of seagrass meadows in the fossil record.