DRILLING UNDER THREAT: AN EXPERIMENTAL ASSESSMENT OF DRILLING BEHAVIOR OF NUCELLA LAMELLOSA IN THE PRESENCE OF A PREDATOR

Predatory drill holes in marine invertebrates represent an important source of information on the nature of biotic interactions and have often been used to explore the ecological and evolutionary roles of such interactions. Typically, drilling frequency is used as a measure of predation intensity (summarized by Kelley et al., 2003) while the frequency of incomplete drill holes is inferred to record failed predation events and therefore to represent a proxy for “prey-effectiveness” (Kelley & Hansen, 2001).

Using the drilling muricid, Nucella lamellosa and its prey, the mussel Mytilus trossulus, the impact of a secondary predator, the crab Cancer gracilis, on drilling was investigated experimentally. In two similar sea tables the snails were kept with the mussels. In one of the sea table, a crab was introduced periodically during our experiment; the other sea table is considered as the control table and served as the base line for our experiment.

The results indicate that the presence of a secondary predator can affect drilling activity, leading to a significant increase in the frequency of incomplete drill holes. The introduction of a secondary predator can also decrease the overall drilling frequency. The size distributions of completely and incompletely drilled mussels indicate that in the presence of the secondary predator the decision by the gastropod to either abandon or continue drilling its prey might be influenced by how much time it has already invested into drilling.

These results are important for the ecological and evolutionary implications of incomplete drill holes frequencies, especially with regard for their use as proxies for evolutionary prey improvement. Based on the above observations, the frequency of incomplete drill holes in fossil assemblages may also be a function of the presence of secondary predators rather than the prey's effectiveness. The threat of a secondary predator may also affect drilling frequency and thus have consequences for interpreting predation intensity. If the presence of secondary predators can be shown to exert a significant influence on drilling and incomplete drill hole frequencies in fossil assemblages, this variable will need to be considered in studies of escalation.