POST-PLIOCENE REDUCTION IN COMPETITION REVEALED BY FOSSIL RECORD OF GASTROPOD DRILLING PREDATION ON BIVALVE PREY WITHIN SEAGRASS ECOSYSTEMS OF FLORIDA

When predators attack and consume prey, they risk having their prey stolen by competitors or being eaten themselves. Rapid methods of killing and consuming prey reduce these risks and are therefore advantageous when enemies are abundant. Experiments with the seagrass-associated muricid gastropod species, Chicoreus dilectus and Phyllonotus pomum, indicate that in the absence of competitors they use the relatively slow predation method of drilling through the shell wall of their venerid bivalve prey, Chione elevata. In the presence of competitors, however, they alter their attack behavior by employing a method, drilling at the shell margins (edge drilling), that is more than twice as fast as drilling through the shell wall. By speeding up the predation process, edge drilling provides immediate, competitive advantages in reducing potential prey loss to enemies. With this understanding of the predator-prey system today, we analyzed the fossil record of muricid-drilling predation in two Recent, four early Pleistocene, and six late Pleistocene samples of the bivalve Chione elevata, and in four late Pliocene and four middle Pliocene samples of Chione erosa to examine temporal variation in the intensity of competition within seagrass habitats of Florida since the Pliocene. Our results show that Chicoreus and Phyllonotus drilled their Chione prey at the edge relatively often (6%, 32/532) during the Pliocene but not at all (0/792) during the Pleistocene or Recent when they employed only wall drilling. We suggest that the post-Pliocene loss of edge-drilled Chione points toward a substantial reduction in competition in the broad sense (including predation), perhaps mediated by a decrease in the abundance of predatory gastropods, at the end of the Pliocene within seagrass ecosystems of Florida.