Comparison of predation signatures left by Atlantic oyster drills ( Urosalpinx cinerea Say, Muricidae) and veined rapa whelks ( Rapana venosa Valenciennes, Muricidae) in bivalve prey

Abstract

Predation signatures left by Atlantic oyster drills ( Urosalpinx cinerea, 8.8 to 29.8 mm shell length, SL) and veined rapa whelks ( Rapana venosa, 3.0 to 79.9 mm SL) in oyster and mussel prey were quantitatively described and compared for a similar size range of predators (8–33 mm SL). Rapa whelks and Atlantic oyster drills leave boreholes in the valves of mussels and oysters when predator shell lengths are less than 34 mm. Rapa whelks with shell lengths greater than 34 mm consumed their oyster and mussel prey without drilling or edge boring. The absence of a predation signature of any kind on prey was first observed at rapa whelk shell lengths greater than 10 mm. In rapa whelks, the shell length range of 10 to 34 mm appears to parallel a transition in predation strategy between evident signatures (drilling) and no signatures (non-drilling). The zones over adductor muscle scars were the most frequent site for drilling by both predators. Borehole dimensions scale with size of the predator for Urosalpinx and Rapana. Rapa whelk outer borehole diameters were significantly larger than outer borehole diameters made by similarly sized Atlantic oyster drills. Rapa whelks leave a borehole that has a more steep-sided vertical profile than Atlantic oyster drills. Observed differences in borehole morphology between species may provide a valuable tool to identify predators in field collected valves of prey species. The ability to identify the presence of rapa whelks < 34 mm shell length in a habitat would offer an earlier warning for the presence of this invader than is currently offered by collections of rapa whelks as bycatch in commercial fishing gear at shell lengths greater than 70 mm. Differences in predation strategies between native and introduced gastropods may result in functional trophic differences that have broad scale ecological consequences.