DRILLING PREDATION INTENSITY AND FEEDING PREFERENCES BY NUCELLA (MURICIDAE) ON LIMPETS INFERRED FROM A DEAD-SHELL ASSEMBLAGE

Abstract

Although limpets are common in rocky intertidal shores, little is known about drilling predation on them. Drilling intensity and preferences by Nucella (Muricidae) on three Lottiidae species (Lottia pelta, L. digitalis, and Tectura scutum) were explored in a modern limpet death assemblage from False Bay (San Juan Island, Washington, USA). Of the 1,531 shells, only 61 (4%) were drilled, with drilling frequencies of 5.9% (L. digitalis), 2.4% (L. pelta) and 0.5% (T. scutum). The higher drilling frequency observed for L. digitalis may reflect spatial differences in prey distribution within the intertidal zone. Hole diameter correlated positively with limpet size, suggesting that larger predators drill larger prey. No differences in drilling frequency were observed due to prey ornamentation or size; however, drill holes were never observed on the largest and thickest L. pelta shells, suggesting a possible size refugium. The majority of holes occurred near the apex, indicating stereotypic attack behavior. Uniform frequency distributions across taphonomic grades and similar central tendencies between drilled and undrilled shells suggest that holes were not affected by taphonomic bias. The preservation of drilled and undrilled shells differed significantly, however; thus, drill holes may have negatively affected the preservation potential of shells, possibly by weakening the shell. Poor shell preservation indicates that biostratinomic effects may play a larger role in preservational biases and underestimation of predation frequencies than previously thought. Studies using drilling frequencies demand careful identification of predatory traces when shells are poorly preserved. In addition, careful evaluation of predation frequency is needed when predatory strategies that may not leave visible traces are possible.