CHANGES IN INTENSITY OF MICROBORING IN BIVALVES ACROSS DEPTH (15-264M) AND OVER TIME (2, 6, AND 13 YEARS) FROM LEE STOCKING ISLAND, BAHAMAS

The abundance, size and depth of penetration of microborings on bivalves at Lee Stocking Island, Bahamas were analyzed using the scanning electron microscope (SEM). The bivalves, Codakia orbiculata and Arctica islandica were systematically immersed at various depths in the Caribbean: shallow platform top (15-33m), vertical platform edge (70-88m), talus slope (192m), semi-consolidated hardground (213-222m), trough and crest of deep relict dunes (250-264m). The bivalves were collected at 2, 6, and 13 years and microborings on the inner shell surface were analyzed and counted using the SEM. Past studies have examined the mechanisms used by euendoliths to penetrate carbonate substrates along with some work on species distribution of euendoliths at different depths. However, other studies are short-term (up to 2 years) and use substrates such as coral blocks and iceland spar. The results reported here are from natural shell material, which provide a better proxy for translating microboring patterns by depth and environment into the fossil record.

Several shells, each of Arctica and Codakia, were analyzed per site for each year. Both water depth and the time of exposure on the seafloor play a major role in microboring patterns. The results show that regardless of shell type, shells at shallow depths had a greater density of microborings that decrease with increasing water depth. Similarly, shells from shallow sites also had the deepest penetration of borings into the shell. Over time, the shells from deeper sites showed increased density of borings at the shell surface but these borings occupied less depth within the shells than shells from shallow sites. The general trend is that within two years, both shell species experienced extensive boring that is more intense at shallow, euphotic sites. This study supports the findings of others, showing a decrease in boring with increasing depth of burial and an increase in boring density over-time. Changing patterns of microboring with environment over time may prove useful in identifying paleobathymetric information from fossilized shells.