Latitudinal Patterns in Naticid Gastropod Predation Along the East Coast of the United States: A Modern Baseline for Interpreting Temporal Patterns in the Fossil Record

Interpretation of the well-studied temporal record of predation by drilling gastropods requires understanding spatial patterns in drilling, in order to dissect any environmental trends from evolutionary patterns. However, current knowledge of spatial patterns in drilling at one time is incomplete; 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 (43° N latitude) to the Florida Keys (25° N). The frequency of complete (successful) and incomplete and multiple (failed) drillholes was tabulated for higher and lower taxa from 24 samples (> 11,000 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. Drilling frequencies were greatest in the Carolinian Province and declined to the north and south for arcid bivalves, infaunal bivalves, and the total fauna (bivalves + gastropods; provincial frequencies for the total fauna were as follows: Nova Scotian, 8%; Virginian, 13%; Carolinian, 28%; and Gulf Province, 15%), and for the genus Spisula. In contrast, failed drilling, though infrequent, was typically more common in the Gulf Province than in 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, slower metabolisms may produce lower drilling frequencies; (4) alternative predation modes such as smothering may be more common at higher latitudes, resulting in lower drilling frequencies. Despite the apparent patterns, spatial variability reinforces the need to examine multiple samples in order to characterize the drilling frequency at a single location (or time interval).