CLUMPING BEHAVIOR: AN ANTI-PREDATORY STRATEGY AGAINST DRILLING PREDATION

Predatory drill holes in marine invertebrates serve as one of the rare sources of data for the study of ancient predator-prey interactions since in most other cases the predator does not leave easily interpretable traces of its behavior. For this reason, drill hole data have been very important in the study of predation especially for testing its role in evolution. One commonly used metric of predation intensity has been the frequency of drill holes, while position of drill holes has been used to study the behavior of predators. Because drilling frequency and site stereotypy can be influenced by different defensive strategies, the study of such strategies is an important part of research on predation. In this study, we explore the effect of clumping on drill hole frequency and position.

A previous study showed that Mytilus trossulus used clumping behavior as an anti-predatory mechanism against durophagous predation (Cote & Jelnikar, 1999). In this study, we used laboratory experiments to explore the effect of clumping on drilling predation using the same mussel, Mytilus trossulus, as prey and the drilling gastropod, Nucella lamellosa, as predator. We assigned mussels to two groups: in one, mussels were allowed to clump together with their byssal threads; in the other, they were kept separate. We observed a significant difference in drilling frequency between the two groups, confirming that clumping acts as a successful anti-predatory strategy against drilling predators. However, in spite of potential differences in prey handling and grappling due to clumping, mean hole placement and variation in hole placement (as measured by standard deviation) showed no significant differences between the two groups. These observations suggest that comparison of predation intensities across clumping and non-clumping taxa, must consider the anti-predatory effect of this behavior.