Inferring the signature of past intra-predatory competition from drilling predation patterns: insights from Red Sea and Adriatic Sea death assemblages

The fossil record of drill holes on molluscan shells left by carnivorous predators has often been used to test the evolutionary impact of ecological interactions. Ecological experiments document a significant change in the predatory behavior of drillers due to competition and predict a substantial influence of such interaction on predator-prey dynamics. However, it is not always reliable to extrapolate results from experiments to the fossil record. The well-studied bivalve death assemblages from the Bay of Safaga (northern Red Sea) and the Bay of Panzano in the Gulf of Trieste (northern Adriatic Sea) provide a unique opportunity to test if the experimentally observed impact of competition on predatory drilling behavior can be detected in time-averaged molluscan assemblages. The death assemblages of the two regions exhibit unequal molluscan predator-prey abundance ratios (PPAR). Considering that competition among the predators increases when the number of prey individuals per predator decreases, we used PPAR as a new indirect proxy for intra-predatory competition and evaluated its relationship with various measures of drilling predation intensity and predatory behavior. In contrast to the expectations from the behavioral experiments of previous studies, our results show that higher PPAR in the Adriatic assemblage indicating more competitive seascape is associated with higher drilling frequency (DF), and lower edge-drilling frequency (EDF) compared to the Red Sea assemblage. On the other hand, the Adriatic assemblage has higher incidence of incomplete drill holes, which were mostly produced by larger predators. However, a number of taphonomic, ecological and environmental factors may obscure the relationship between PPAR and drilling predation measures observed in the studied death assemblages. Because it is nearly impossible to account for all of these factors in fossil assemblages, the PPAR-based approach of tracking intra-predatory competition may be difficult to apply in deep time, demonstrating the limits of using indirect proxies and experimental results to trace competition in the fossil record.