EDGE DRILLING IN RECENT BIVALVES FROM RED SEA: ECOLOGICAL VS. ENVIRONMENTAL ATTRIBUTES

Edge drilling is a rare predation pattern where the driller creates a hole on the commissure between the valves of a bivalve. It is speculated that edge drilling is faster than wall drilling and may therefore be advantageous in situations where predators face high risks while feeding. However, the incidence of edge drilling is extremely rare, both in the fossil and Recent record. We have investigated the high incidence of edge-drills in a diverse assemblage of Recent shallow water (0-52m; majority from above 20m) marine bivalves from the Northern Red Sea. The effect of ecological and environmental attributes on edge drilling is studied using the data of over 15,000 specimens.

Overall drilling frequency (DF) is calculated as percentage of drilled individuals over the number of all the valves. The edge drilling frequency (EDF) is calculated as a ratio of edge-drilled valves over all drilled valves. DF has little effect on EDF, but size has a positive relationship with EDF of a species. Among the studied bivalve families, Limopsidae shows the highest EDF.

Among ecological attributes, feeding habit and life modes are important predictors of EDF. Suspension feeders show the maximum incidence of edge drilling. Semi-infaunal/endobyssate is higher EDF than both epifaunal and infauna. The EDF of epifaunal groups is also significantly higher than those of infauna. Among the epifaunal groups, byssally attached nestlers show a significantly higher EDF than encrusters and recliners. Contrary to previous speculations, environmental attributes such as depth do not have significant contribution in determining EDF. However, the EDF varies with the change in substrate showingthe highest incidence in fine grained sediments.

The prey handling by the carnivorous gastropod influences the position of the drill hole. Therefore it was speculated that stereotypy would be influenced by the ecological attributes of the prey. This claim is supported by this study. Moreover, it also demonstrates that the environmental factors could affect traits like site stereotypy of drilling. Such traits were conventionally considered to be controlled only by physiological and ecological attributes. Therefore, details of environment and ecology of the prey must be considered to fully understand the nature of predation in modern and fossil habitats.