No honor among snails: Conspecific competition leads to incomplete drill holes by a naticid gastropod

Incomplete drill holes in shelled invertebrate prey produced by predatory gastropods are often interpreted as failed attacks, and the frequency of incomplete drill holes in a sample is thought to provide insight into the effectiveness of prey defensive adaptations. Here, we conduct an experimental test of the hypothesis that differences in the frequencies of incomplete drill holes do not necessarily indicate anything about the prey and can instead be triggered by disturbance competition among conspecific drilling gastropods. Our test employed the western Atlantic naticid gastropod Neverita delessertiana and its venerid bivalve prey Chione elevata. The experiment consisted of an isolation treatment, where predators fed in the absence of conspecifics, and a competition treatment, where predators fed in groups of three. Competitive interactions, which included grappling, prey theft, and cannibalism, were found to be important in determining the presence of incomplete drill holes, as this trace was absent in the isolation treatment but present in the competition treatment, with the frequency of incomplete drill holes increasing in interactions among larger gastropods. These results suggest that interpretations of both field and fossil data must consider the role of competitive disruption as an additional mechanism underlying long-term patterns in the frequency of incomplete drill holes in the fossil record.