One of These Beaches Is Not Like the Other: Drilling Frequencies in Modern Mollusc Shells, San Salvador Island, Bahamas

Studies of drilling predation on molluscs are frequently conducted in the paleontological record, but research in modern settings can provide insights into patterns of predation in the geologic past. The research presented here is a comparison of drilling frequencies between two beaches (Haitian Boat Beach and Sandy Point Beach) on San Salvador Island, Bahamas. Bivalve and gastropod shells were collected from shell accumulations on both beaches. From each collection, 600 bivalve shells and gastropods > 2mm were randomly selected and counted. All drilled shells were noted to determine if a difference existed in drilling frequency between the two beaches. Sandy Point Beach (southern tip of San Salvador) had 6% drilled shells and Haitian Boat Beach (northwest of San Salvador) had 17% drilled shells. A chi-squared test demonstrated that these beaches showed statistically different frequencies of drilling.

At both beaches, valves belonging to seven genera of bivalves were the most abundant shells counted, but abundances do not appear to be linked to drilling intensity. For instance, at Haitian Boat Beach, Glycymeris accounts for 14% of total shells counted and only 18% of Glycymeris valves are drilled. At Sandy Point Beach, Glycymeris accounts for 7% of shells counted, yet 44% of these shells are drilled. Interestingly, gastropods, which make up a small proportion of the population at both beaches, exhibit high drilling frequencies. Based on the shape of the drill holes, naticid gastropods are the likely predators. Future work will focus on the possible environmental and/or ecological differences between the two beaches that may provide insights into what controls drilling frequencies in modern carbonate settings.