PREY EFFECTIVENESS? CONSPECIFIC COMPETITION LEADS TO AN INCREASE IN INCOMPLETE DRILL HOLES

Gastropod drill holes in bivalve prey are widely used to study predator-prey interactions in the fossil record. Previously, an incomplete drill hole has been assumed to indicate effectiveness of the prey phenotype (i.e., a thick shell or defensive ornamentation) in deterring attacks, and changes in the frequency of incomplete drill holes have been used as evidence of the adaptive struggle between predator and prey. In real-world scenarios, however, predators feed in the presence of their own enemies who are capable of disrupting attacks-in-progress. Here, we test the hypothesis that the frequency of incomplete drill holes varies predictably with changes in the local abundance of conspecific predators, even when the prey phenotype is held constant.

Specimens of the naticid gastropod Neverita delessertiana collected from the Gulf of Mexico were fed the bivalve Chione elevata in two treatments: (1) a control group with one naticid gastropod per aquarium (12 replicates), and (2) an experimental group with three naticid gastropods per aquarium (4 replicates). Aquaria were filled with glass beads instead of sediment so that feeding attempts and competition could be observed directly. In the control treatment, there were no incomplete drill holes in 67 feeding attempts, whereas in the experimental treatment, 21.7% of 47 attacks produced incomplete drill holes. Incomplete holes were the direct result of intense competition. Naticids grappled for prey up to several times a day and often fought one another by radular rasping. Successful competitors occasionally stole prey and then continued the bore hole started by the first predator. Scavenging and suffocation were also common in the competition treatment. Multiple drill holes in a single prey shell occurred in both treatments. These results indicate that competition alone (i.e., not the prey phenotype) is capable of explaining much of the variation in the frequency of incomplete drill holes observed in the fossil record.