EDGE DRILLING AND ESCALATION

Wall drilling predation has existed since the latest Proterozoic, but edge drilling, a derived style of attack, became locally common only in the latest Neogene. Experimental work has begun to elucidate the driving forces underlying this shift. Some muricid gastropods use edge drilling, which is three times faster than wall drilling, when feeding on bivalve prey in the presence of conspecifics but revert to wall drilling when feeding in isolation. Faster feeding may reduce the risk of kleptoparasitic interactions by food-stealing competitors attracted to the scent of dead or dying prey. However, in theory, natural selection should favor the evolution of edge drilling in any type of interaction where slow feeding puts the driller at risk.

Here, we test whether edge drilling is a general response to enemies through experimental observations of the foraging behaviors of the muricid gastropod Chicoreus dilectus fed Chione elevata, its primary bivalve prey, in isolation from enemies (control group) or exposed to chemical effluents from the stone crab Menippe adina, a natural enemy (experimental treatment). Our study, which included six replicates of each group, shows that both drilling and non-drilling modes of attack were used by the predator. C. dilectus killed and consumed Chione prey 17.5% of the time (N=40 total attacks) when feeding in the presence of crab effluent but 37.5% of the time (N=32 total attacks) when feeding in isolation. Considering only the subsample of shells killed by drilling, edge drilling was used in 39.4% of drilling attacks (N=33) in the presence of crab effluent but just 15% of drilling attacks (N=20) when this chemical cue was absent. Edge drilling by C. dilectus is, thus, a general response to a broad range of enemies and not specific to a particular phylogenetic group or a particular risk. Although this experiment has not yet been replicated for other versatile drillers that can switch between wall and edge drilling behaviors, our results suggest that the geologically recent rise of edge drilling in the fossil record is consistent with escalation theory's prediction of ever-increasing evolutionary influence of enemies.