A COMPARATIVE-STUDY OF NATICID GASTROPOD PREDATION ON VARICORBULA-CALOOSAE AND CHIONE-CANCELLATA, PLIOPLEISTOCENE OF FLORIDA, USA

Naticid gastropod predation on members of the bivalve family Corbulidae has been reported to be unusual in its lack of stereotypy, low rate of success, and lower frequency than predicted by a net energy maximization model. The cause behind these atypical patterns has been attributed to layers of conchiolin within the valve microstructure of corbulids, which are thought to inhibit shell penetration by naticids. The mechanism of shell penetration by drilling gastropods, as well as scattered instances of typical predation rates and patterns on corbulids suggest, however, that factors other than conchiolin layers should be investigated as potential causes of anomalous predation. We examined naticid-prey interactions by comparing naticid predation on Varicorbula caloosae and Chione cancellata from the Plio-Pleistocene of Florida. We tested for borehole site selectivity and prey size selectivity, two typically stereotyped behaviors. We compared the degree of predation success on both species, and compared observed prey-preference rankings with rankings predicted from cost/benefit analyses based on the net energy maximization model of Kitchell et al. (1981). We found that naticid predatory behavior was generally stereotyped in both species. The cases of unstereotyped predatory behavior on V. caloosae may be linked to its small size, rather than the presence of conchiolin layers. We also found that predicted rankings calculated from cost/benefit analyses were generally inconsistent with observed rankings; the size range of C. cancellata predicted to be the most profitable was not preferred. Site and size selectivity, however, are predictable from the net energy maximization model because they are behaviors that maximize net energy gain (Kitchell, 1986). Therefore, our results support the idea of naticids as energy maximizing predators, but we question the assumptions of the Kitchell et al. (1981) model, especially when applied to fossil assemblages.