IS EDGE DRILLING A PERVASIVE FACULTATIVE RESPONSE TO ENEMIES?

It has been hypothesized that, when threatened by their own enemies, muricid snails shift from drilling bivalve prey at the thick valve wall to at the thinner valve edges (edge drilling) to reduce prey handling times. Faster feeding increases fitness in selective environments by reducing exposure to competitors and predators or by allowing the driller to consume greater quantities of prey. If confirmed, edge drilling frequencies in the fossil record may indicate past intensities of enemy-driven selection. We conducted a lab-based experiment of predatory interactions between a small drilling muricid gastropod Favartia cellulosa and its mussel prey Brachidontes exustus. Under competitive treatment (snails fed mussels in the presence of other conspecific competitors), Favartia drills their prey relatively quickly (p=0.041) without drilling at valve edges, although valve edges of this prey are significantly thinner than the shell wall (p=0.029). We propose that the facultative edge drilling response to enemies is a species-specific adaptation with limited phylogenetic distribution rather than a plesiomorphic and pervasive feature of muricid predator-prey interactions. For this reason, edge drilling frequency cannot be used as a generic indicator of a competitive environment wherever muricids are found.