Effect of Durophagy on Drilling Predation

Predation is recognized as an important agent of natural selection, and studying predator-prey interactions in the fossil record allows evaluation of its evolutionary impact. Predator-prey systems involving drilling predators are especially relevant because the fossil evidence of predation can be analyzed quantitatively. For example, frequency of complete and incomplete bivalve and gastropod drillholes has been used to evaluate success of predators relative to prey. A recent experiment performed with live specimens demonstrated that drilling frequencies can be affected by the presence of secondary predators (crabs). In the experiment, the drilling muricid snail, Nucella lamellosa, and its prey, the mussel Mytilus trossulus, were placed in two similar sea tables. A crab, Cancer gracilis, was introduced periodically into one of the tables during the experiment; the crab-less sea table served as the control. We found that in the presence of the secondary predator (crab), the success rate of the drilling predator (snail) decreased (frequencies of incomplete holes increased; drilling frequencies decreased).

To test whether the presence of secondary predators had the same consequences in the past, we turned to the fossil record of Cenozoic mollusks. Using the frequency of repair marks on fossil bivalves and snails as a proxy for secondary predators and drillhole frequencies as a measure of success of drilling predators, we found patterns consistent with our predictions. With increasing frequency of crab predation marks, frequency of complete drillholes decreased and frequency of incomplete drillholes increased. These results suggest that the presence of secondary predators has to be considered in interpreting frequencies of incomplete drill holes in fossil assemblages.