DOES BODY SIZE INFLUENCE THE OUTCOME OF THE INTERACTION BETWEEN PREDATORY WHELKS AND THEIR BIVALVE PREY?

The large body size of many shelled invertebrates is often assumed to be an important defense against predators, because shell strength typically increases as body size increases, as does the time required by predators to kill their prey.

As such, shell size is thought to have been an important factor limiting successful predation throughout the Plio-Pleistocene history of the interaction involving shell-chipping busyconine whelks and their hard-shelled bivalve prey, Mercenaria. Increases in Mercenaria body size since the Pliocene are interpretable as evolutionary responses to whelk predators, but no tests have explicitly examined whether prey are expected to respond to their whelk predators by increasing body size.

Does size matter? To test the adaptive significance of shell size in Mercenaria mercenaria, we offered progressively larger prey to individual whelks, Sinistrofulgur sinistrum, until a threshold was reached where prey size resulted in an unsuccessful whelk attack. Our data suggest that body size is an important defense, although even the largest (106mm) Mercenaria that were available were occasionally vulnerable to successful whelk predation. The largest (240mm) whelks never failed in their attacks on Mercenaria, while smaller whelks had an upper size limit of successful predation set by their own body size. There was considerable inter-individual variation among predators, however, in their prey size limits.

These results suggest that body size is a primary factor governing the outcome of the whelk-Mercenaria interaction, and that size is likely a target of selection for Mercenaria in response to whelk predators. Whelks should exert selection pressure for body size increase in Mercenaria, with the characteristics of a large shell translating into an increased probability of failure of an attack by a whelk predator.