REVENGE OF THE SNAILS: SHIFTS IN MOLLUSK DOMINANCE BETWEEN THE MIDDLE PLEISTOCENE AND RECENT SAN SALVADOR, BAHAMAS

Reef ecosystems worldwide have been extensively altered by humans. Few studies have detailed the impact on the mollusk fauna and fewer still have compared modern to pre-human reef systems. We analyze the shelly fauna preserved in sands on the Cockburn Town fossil reef (132–120 Ka) and compare it to death assemblages sampled from similar facies on four near-shore living reefs on the west (leeward) side of San Salvador island, Bahamas. Abundance and identity of mollusk fossils were documented in 20 cm x 20 cm quadrats (N = 21) on the fossil reef. Recent samples were surface collected from similar quadrats on the sandy bottom in or immediately adjacent to living reefs. All modern samples had higher Shannon-Weiner Diversity Index values (average = 2.44) than the fossil reef (1.85). This is due primarily to higher gastropod species richness (average = 16 species per site) compared to 10 species in the fossil reef. We observe that bivalve species richness remains approximately the same into the Holocene (s = 13 on the fossil reef; s averaged 10.8 species on modern reefs). The fossil bivalve assemblage was dominated by Chione cancellata and Codakia orbicularis; modern bivalve assemblages were dominated by Barbatia cancellaria, Chione cancellaria and Ctenocardia guppyi. In the fossil reef, gastropod species were of even abundance and represented by fewer than three individuals each. In modern reefs, herbivourous Bulla occidentalis, and omniverous Cerithium litteratum and Cerithium eburneum dominated some sites. Predatory gastropods were also more numerous and speciose in modern reefs.

To look more closely at the ecological impact of the enhanced gastropod diversity, we examined the common bivalve Chione cancellata, which was abundant in both fossil and modern reefs. The average width of Chione valves differed between fossil ( 2.1 ± 0.4 cm) and modern (1.5 ± 0.5 cm) reefs (p < 0.001). This suggests an age structure dominated by younger individuals in the modern. Only 5.9% of fossil and 25.5% of modern C. cancellata valves displayed gastropod predation drill holes, a statistically significant difference (p = 0.002). These results are consistent with greater predation pressure by gastropods in the modern reef environments and the greater number of predatory species.