CLIMATE AND GASTROPOD PREDATION: VARIATION IN DRILLING AMONG RECENT MOLLUSCAN PROVINCES ALONG THE U.S. EAST COAST

Interpretation of temporal patterns in predation by drilling gastropods is hampered by incomplete knowledge of spatial patterns in drilling at one time. Drilling varies spatially, but some studies have observed an increase, and others a decrease, in drilling with latitude. Because drilling is a slow process that puts the predator at risk, it has been hypothesized that frequency of successful drilling should increase, and that of failed drilling should decrease, at higher latitudes where enemies pose fewer risks. To test this hypothesis, we collected bulk samples of modern molluscs from beach assemblages from Maine to the Florida Keys, spanning 24 degrees of latitude. The frequency of complete (successful) and incomplete and multiple (failed) drill holes was tabulated for higher and lower taxa from 24 samples (>10,000 specimens).

We explored latitudinal variation in drilling by comparing patterns among the Nova Scotian, Virginian, Carolinian, and Gulf molluscan provinces. Results were more complex than predicted. In general, drilling frequencies were greatest in the Carolinian Province and declined to the north and south. For instance, arcid bivalves had drilling frequencies of 20%, 35%, and 15% in the Virginian, Carolinian, and Gulf provinces respectively. Similar patterns were exhibited by individual genera for which data were available and for the bivalve fauna as a whole, as well as for the entire molluscan fauna (bivalves + gastropods; provincial frequencies were as follows: 8% - Nova Scotian, 13% - Virginian, 28% - Carolinian, and 14% - Gulf province). In contrast, failed drilling, though infrequent, was typically more common in the Gulf province than the Carolinian province (4% vs. 1% for most groups). Possible explanations for the patterns include: 1) taxa from more tropical localities may be better defended against predators, producing lower drilling frequencies and higher incidence of failed drilling; 2) presence of multiple predators in warmer waters may decrease drilling on bivalve prey if predators interfere with or prey upon one another; 3) at cooler latitudes alternative predation modes such as smothering may be more common, resulting in lower drilling frequencies. Work in progress will determine drilling patterns for additional lower taxa and assess alternative explanations for these patterns.