How effective are ecological traits against drilling predation? Insights from recent bivalve assemblages of the northern Red Sea

Biotic interactions, especially predation, have drawn considerable attention in recent times as an important agent of natural selection. Drilling predation is often used as a model system by paleobiologists to evaluate evolutionary and ecological effects of such predatory interaction on the composition of molluscan communities. Using drilling frequencies, it is possible to test quantitatively specific predictions that postulate the effectiveness of ecological traits against predation in present-day environments. The high frequency of drilling (DF) in a diverse recent bivalve assemblage from shallow water environments of the northern Red Sea enables us to test such evolutionary hypotheses, predicting low DF in large-sized bivalves, in infaunal bivalves, in siphonate and mobile bivalves, lower DF in bysally attached than in other epifaunal bivalves, and high DF in bivalves occurring in shallow habitats. We evaluate these predictions on the basis of more than 15,000 bivalve specimens collected at stations with meter-scale spatial resolution using three different methods to calculate DF, namely 1) per-species per-station DF, 2) per-species DF by pooling all stations, and 3) per-station DF by pooling all species. The results are not always consistent among these three methods. Among morphological attributes, we found size and shape of a species to be a good predictor of drilling frequency. However, life habit shows a pattern contrary to our prediction because infaunal groups show the highest DF. Although we did not find any significant correlation between predator abundance and DF at small spatial (station) scales, the dominance of naticid gastropods and low abundance of muricid gastropods at regional scales can explain the much higher DF observed in infauna than in epifauna. Characteristics of the siphon or mobility did not always play a major role in dictating the DF. Attachment type of epifauna does not always show a predictable pattern in deterring drilling predation; however, bysally attached bivalves sometimes show a slightly lower DF compared to cemented bivalves. Although water depth has no significant effect on DF, it might be influenced by the relatively narrow depth range of the stations. We did not find significant variation in DF between groups adapted to different habitats (i.e, substrate types); however, DF seems to differ between groups with different feeding mode. Our study demonstrates that some ecological traits (such as infaunalization or predator avoidance by choosing deeper habitat) that are claimed to be anti-predatory, do not offer effective defense against drilling predation in present-day environments-a result consistent with the idea of ever escalating predator-prey dynamics.