DRILLING PREDATION ON A PLEISTOCENE MOLLUSCAN ASSEMBLAGE FROM LA BELLE, FLORIDA, AND RELATION TO ENCOUNTER RATES

We analyzed drilling predation on Pleistocene bulk samples from spoil banks 3.5 km west of La Belle, Florida, on loan from the American Museum of Natural History. Samples were collected by Squires and Heaslip in 1955 and assigned to the Caloosahatchee Formation, but some faunal characteristics indicate the assemblage at least in part represents the Bermont Formation, which had not been distinguished from the underlying Caloosahatchee at the time of collection.

Gastropods larger than 5mm and > 80% complete with intact apices were analyzed. Specimens were sorted to genus level, with some being identified to species level. In total, 1979 complete specimens, representing 27 genera, were analyzed. Abundance, number of specimens with complete drill holes, and number of incomplete drill holes were tallied for all taxa. Most drill holes are suspected to have been caused by predatory naticid gastropods.

Predation was assessed using drilling frequency (DF) and prey effectiveness (PE). DF was determined by the number of specimens with complete drill holes divided by the number of complete specimens. PE was calculated as the number of incomplete drill holes divided by the number of attempted drill holes.

For the gastropod assemblage as a whole, DF was 0.06 and PE was 0.18. Cerithium was the dominant gastropod (865 specimens) with a DF of 0.08. About half the genera were too rare for valid calculation of genus-level DF. Of the genera with >10 specimens, Cantharus was the most commonly drilled (DF = 0.12). All of the 10 most common genera had some evidence of drilling predation (complete and/or incomplete drill holes), but relative abundance was not correlated with DF (Spearman’s ρ = 0.103, p = 0.7792).

Previous analysis of 21,322 whole bivalve specimens from these samples yielded the same DF (0.06) as the gastropod assemblage but much lower PE (0.08). The dominant bivalve Transenella (14,109 specimens) had a DF of 0.04. Of 44 genera identified, 21 were represented by ≥ 20 valves, and DF ranged from 0.007 to 0.22. Relative abundance was inversely correlated with DF for these 21 genera (ρ = -0.6823, p = 0.0007). If relative abundance is a proxy for encounter rate, drilling predators did not select prey based on encounter rate but were selective of both gastropod and bivalve prey, preferentially attacking rarer rather than dominant taxa.