PATTERNS OF SUBLETHAL ATTACK BY SHELL-BREAKING PREDATORS WERE STABLE THROUGH AN EPISODE OF TROPHIC AND FAUNAL CHANGE, IN THE LATE NEOGENE OF NORTH CAROLINA

A change in the trophic ecology of benthic marine communities appears to have coincided with the well documented late Neogene faunal turnover (~70% species extinction) in mollusk assemblages of the Western Atlantic. We investigate the magnitude of this ecological upheaval, using the frequency of repair scars from sublethal attacks, a proxy for the coupled strength and outcome of interactions with myriad shell-breaking predators (e.g. whelks, crabs, and fishes). If the late Neogene ecological reorganization had pervasive effects on predator-prey interactions, the frequency of repair scars on prey taxa is expected to change, showing a pattern that is congruent with the abundance of predatory gastropods and ecologically similar prey types and the attack (drillhole) frequency of drilling gastropods. To test this hypothesis, five ‘bellwether’ prey taxa (Glycymeris, Anadara, Cyclocardia, Plicatula, and Olivella) were selected to analyze for repair scars (% with scars method) and drillholes. Predation and adundance data were collected from preexisting bulk samples (>5mm fraction), comprising one late Pliocene (Duplin Formation), one early Pleistocene and two middle Pleistocene (Waccamaw Formation) age localities. The results do not support our hypothesis. There was little change in repair and drilling frequency and it was not statistically significant (Chi-squared test of independence). The proportions of anterior, ventral, and posterior repair scars along the commissure were variable in Glycymeris, Anadara, and Cyclocardia, but not significant (Fisher’s Exact Test), suggesting little change in the regime of shell breaking agents. The previously reported decline of predatory gastropod abundance was not recovered in the samples studied (Chi-squared test of goodness-of-fit); instead, there was a large increase in the proportion of predatory gastropods and decrease in genus-richness in the middle Pleistocene samples. These results suggest that predator-prey dynamics showed stability despite the changing ecological context. Stability in predatory interactions could be due to either behavioral or adaptive compensation by species or the absence of strong effects. Furthermore, trophic reorganization may have been more variable within habitats than is observed at a regional scale.