WHO'S MAKING ALL THOSE SCARS? FREQUENCY AND IDENTITY OF CRAB PREDATION ON FOSSIL AND LIVING TURRITELLINE GASTROPODS FROM FLORIDA AND PANAMA

Turritelline gastropods (Caenogastropoda, Turritellidae) show frequent breakage and repair at their apertural lip throughout their long and abundant history (Early Cretaceous-Recent). Almost every shell shows at least one such scar, with more than 5 on larger shells not unusual. Most of these scars are assumed to have been caused by unsuccessful predation by decapod crustaceans, and a number of previous studies have used scar frequency to infer ancient predation activity on these snails. Yet there is no published account of a crab actually attacking a turritelline, nor is anything known about which decapod taxa are involved. One obstacle to such study is that very few data are available on the shape of breakage scars made by different living decapod species.

Our working hypothesis is that different species of crabs will make distinguishable breakage scars when attacking the same gastropod species, and that if this is true in the Recent, we may be able to recognize and quantify predation by different crab taxa in the fossil record. To test this hypothesis, we first carried out a pilot study of fossil turritellines from the Pliocene of Florida, and found that at least four categories of breakage scars can be distinguished based on shape, with at least two of these categories assignable to decapods based on comparison with modern data from the literature. We have now documented, apparently for the first time, attacks by crabs (Panopeus sp. and Xanthodius sternberghii) on living turritellines (Turritella banksi from the Pacific coast of Panama) in lab aquaria. Preliminary results of these observations indicate that Panopeus sp. produces scalloped, distinctly toothed breaks similar to some of those attributed to decapods in the Florida fossils, while X. sternberghii creates smooth, straight-edged breaks that were heretofore unexplained in the fossil material. Concurrent surveys of dead T. banksi shells and fossil turritellines from the nearby Upper Miocene Gatun Formation are consistent with the earlier Florida data in showing distinguishable shape categories of breakage scars, including the scallopped breaks witnessed on the captive snails. Consistent with the presence of numerous unsuccessful scars on most shells, very few of the attacks we have observed (<10%) have been fatal.