COMPARING PREDATION FREQUENCY BETWEEN TWO MORPHOLOGIES OF SPISULA SP. (BIVALVIA)

There have been extensive studies examining predatory drillholes on bivalves and the implications of predation as a potential agent of microevolution. Previous studies have focused on the variation in predation frequency among various prey taxa, but have not studied the degree to which predation varies relative to the morphological variability of a single species, if at all. To address this question, we examined two end-member shell morphologies of Spisula sp. from the St. Mary’s Formation (Miocene) of Maryland. The Spisula shells, 2.82.-12.60 mm long, had proportionally small predatory drillholes. We infer that the holes found on Spisula were the result of successful attacks from two genera of small naticids, Neverita and Euspira, also found within the St. Mary’s.

From four localities, we collected, measured, and examined Spisula for drilling predation. Based on morphometric analysis, we discriminated two morphological end-members (t-test; p<<0.01). One has a shorter hinge and less convex shell, whereas the other morph has a longer hinge and more pronounced convexity. We hypothesized that there might be a difference in predation frequency between the morphologies of Spisula.

We found a statistically significant difference in predation frequency between the two morphs of Spisula (chi-square test; p<<<0.01), indicating there was a preference by naticids for the shorter hinged morph. Naticids use their muscular foot to grapple and orient their prey to maximize their chances for a successful attack. We speculate the St. Mary’s naticids may have been able to grapple the shorter hinged form more easily. Regardless of the reason for this preference for the shorter hinged morph, it probably constituted evolutionary selection on Spisula. Therefore, variation in shell shape and predation frequency on Spisula may be used to examine the selective pressures of predation in the context of microevolution.