"EDGE DRILLING": SNAIL PREDATORY BEHAVIOR OR A PHANTOM OF TYPOLOGICAL THINKING?

Edge-drill holes—predatory borings by shell-drilling gastropods that intersect the shell margin of bivalves—are viewed by several recent studies as resulting from behavior distinct from that which produces the more common wall-drill holes that perforate a single valve. Edge drilling is then interpreted as having an adaptive value due to a tradeoff between decreased handling time and increased risk of injury to the predator if the prey closes its valves on the snail’s proboscis. We argue that, in the case of naticid snails, the contradistinction between edge- and wall-drill holes is artificial and does not indicate behavioral plasticity. If edge and wall drilling constitute different behaviors by the predator, we expect them to produce a disjunctive or bimodal distribution of holes on the shells of the prey population. We tested this hypothesis using fossil collections of the bivalves Astarte and Cyclocardia, two Neogene genera from the Atlantic Coastal Plain that are frequently preyed on by naticids and known to bear edge drill holes. The naticids Lunatia and Neverita are the predators most likely responsible for prey mortality. We measured the umbo-to-edge placement of the drill holes. In contrast to our predictions, drill holes form a unimodal distribution, tailing away from the edge, on the shells of both genera. Therefore, the typological distinction between “edge” and “wall” appears to obscure variation in a single behavioral trait. Incomplete drill holes were concentrated toward the umbo in Astarte, suggesting selection on drill hole placement. Our results accord well with previous studies showing that naticids have a limited capacity to learn and that prey handling precludes sudden valve movement. Thus, for naticids, edge drill holes do not represent an evolutionary tradeoff. Next, because prey morphology can influence placement of drill holes, we sought to determine if the concentration of drill holes near the shell margin is due to the manipulation limit of the predator. Outer borehole diameter (a proxy of predator size) and drill hole placement were not significantly related. Nevertheless, the extent to which drill hole placement represents a “selective” behavior of the predator, as opposed to an interaction that is also influenced by prey morphology, deserves further scrutiny.