Latitudinal Variation in Shell Thickness and Drilling Predation on Recent Hemimactra of the Eastern US

Interpreting temporal changes in molluscan drilling predation in the fossil record requires understanding of spatial variation in drilling. Available data on spatial variation for fossil and Recent mollusks are contradictory, with some analyses supporting an increase, and others a decrease, in drilling with latitude. Previous work by Kelley and Hansen using modern mollusks along the U.S. east coast revealed an unexpected peak in drilling frequency (DF) in the Carolinas and declines both north and south. Spatial variation in drilling may relate to prey defenses (i.e., ease of CaCO₃ precipitation), predator abundance, diversity, or metabolic rate, alternative predation modes (e.g., smothering), or abundance of enemies of drilling predators. This study explores latitudinal variation in DF and shell thickness of Hemimactra (formerly Spisula), a common prey of drilling gastropods found previously to exhibit a mid-latitude drilling peak.

Shell length and thickness at the site of drilling were measured for Hemimactra from at least two localities per biogeographic province. Thickness was regressed on length for each locality and by province. Using regression equations, thickness was predicted at a standard length of 46.43 mm (average shell length for all provinces). Thickness at the standard length was greatest in the Nova Scotian Province; no Hemimactra were drilled. The Virginian and Carolinian Provinces had intermediate thicknesses relative to length and comparable DF (~25%). No drilling was noted in the Gulf samples and relative thickness was less than for more northerly provinces. The pattern in DF is consistent with previous work by Kelley and Hansen showing a peak at mid-latitudes. Thickness of Hemimactra may be a deterrent to drilling in the Nova Scotian Province, explaining the absence of drillholes. However, Hemimactra was least thick in the Gulf Province, requiring another explanation for the paucity of drillholes in warmer latitudes. Ongoing research investigates possible causes for this pattern.