Paper No. 5
Presentation Time: 9:00 AM


WALL, Alex F. and DIETL, Gregory, Paleontological Research Institution, 1259 Trumansburg Road, Ithaca, NY 14850,

Unsuccessful attacks by predators that damage the edge of a bivalved animal's shell are thought to increase susceptibility to other causes of death, especially other predators (Vermeij, 1983; Palaeontology). It is assumed that this is because, in part, many predators of bivalves are chemoreceptive and any exposure of a bivalve’s soft parts allows the leaking of metabolites, effectively advertising the animal’s location. An inability to sustain even slight marginal damage may explain the relative rarity of elaborate or robust armor in the evolutionary history bivalves, especially when compared with that of gastropods.

To test this hypothesis, we conducted a field experiment replicating a kind of sublethal shell damage inflicted by busyconine gastropod predators on their bivalved prey that prevents tight valve closure. Ten 1m2 replicate plots (5 artificially damaged and 5 undamaged), each containing 10 Mercenaria mercenaria (55-65mm shell length), were scattered randomly and independently within and between treatments, across an intertidal flat near Masonboro Island, NC. Deaths within plots were noted every two weeks over a six-month period. Log-rank comparison between treatments showed a significant initial drop in survivorship for the damaged group. These results support Vermeij’s hypothesis. However, an inspection of the residuals showed increasing probability of mortality among undamaged individuals, causing that group’s survivorship to drop below that of the damaged group by the end of the observation period. A possible explanation for this unexpected reversal is increased avoidance behavior, e.g. cessation of feeding in the presence of predators, by damaged clams.