DRILLING GASTROPOD PREDATION ON CREPIDULA FORNICATA ON THE U.S. EAST COAST: LATITUDINAL VARIATION IN RECENT SAMPLES AND COMPARISON TO THE FOSSIL RECORD

Previous work demonstrated that predation by drilling gastropods varies with latitude, with assemblage-level drilling frequencies peaking in midlatitudes on the U.S. east coast. To test if this pattern holds for lower taxa, we examined latitudinal variation in drilling on Crepidula fornicata. We analyzed a total of 1661 Crepidula fornicata specimens from bulk assemblages of Recent molluscs from 24 localities (Maine to Florida). Drilling frequencies were determined for each locality and then summarized for each of four molluscan provinces. Most drilling was by muricid gastropods, with naticid predation common at one locality in New Jersey. In the Nova Scotian Province, none of the specimens was drilled. Drilling frequencies (DF) ranged from 0 – 17% in the Virginian Province with an overall DF of 10%. In the Carolinian Province, drilling ranged from 0 – 33% for a total of 17%. In the Gulf Province, drilling ranged from 0 – 20% with an overall DF of 2%. These patterns of latitudinal variation in drilling corroborate those previously determined at the assemblage level and are similar to those for Crepidula from the fossil record. In previous work by Key and Kelley, Crepidula from the Yorktownian subprovince (Virginia/Carolinas) were drilled more intensely than in Florida for three intervals in the Plio-Pleistocene. However, Pleistocene Crepidula fornicata were drilled at higher frequencies in Florida than in the Yorktownian subprovince. Shell size and thickness also varied latitudinally in our samples; average shell length decreased monotonically from 28.6 mm in the Nova Scotian Province to 20.1 mm in the Gulf Province. Shell thickness was regressed on length and the regression equations used to calculate thickness at a shell length of 25 mm for each province. Shell thickness decreased from the Nova Scotian to the Carolinian Province and then increased slightly in the Gulf Province. Shell thickness was inversely correlated with DF for the four provinces. Prey effectiveness (PE) was measured as the ratio of incomplete drillholes to the total attempted drillholes. PE was 0.49, 0.50, and 0.27 in the Virginian, Carolinian, and Gulf Provinces respectively. These results differ from those for fossil Crepidula, in which PE and DF were inversely correlated. Neither fossil nor Recent data supported prey selectivity by predators.