Abstract

Of More than 4000 specimens of the naticid gastropods Euspira heros (Say) and Neverita duplicata (Say) from southern New Jersey, the lower Pleistocene of North Carolina, upper Miocene of Maryland, and the lower Miocene of Delaware, subsamples with complete and incomplete boreholes (n = 613) were compared for borehole-site stereotypy, prey size-selectivity, prey profitability, and prey effectiveness. In confamilial encounters, adaptation of naticid predators is evidenced by a shift in borehole-site stereotypy on the body whorl toward the umbilicus during the last 18 my, particularly for N. duplicata. Inferentially, an umbilical drilling position enabled the base of the predator's foot to occlude the prey aperture and prevent egress of the dangerous prey's foot, thereby reducing the prey's escape potential. The tradeoff was that the umbilical site required the predator to drill through a thicker shell location.

Prey effectiveness, the ratio of incomplete boreholes to total attempts, was initially low (0.03) for both species in the lower Miocene, but increased appreciably from the Pleistocene to the Recent for N. duplicata (0.32). Such increase in successful prey escape indirectly may reflect prey adaptation since the Miocene. Cost/benefit curves, i.e., log of the ratio of apertural lip thickness /internal shell volume regressed on whorl diameter (WD), have significantly greater negative slopes for Miocene versus Recent conspecifics. Lower cost/benefit ratios for successive size classes of modern naticids suggest that confamilial prey have become increasingly profitable molluscan options as they increased in size, despite increased risk of fatality to the predator. Decreasing naticid prey size-selectivity, as evidenced by lower regression correlation coefficients since the Miocene, reflects increasing mismatches between predator and prey size. Outcomes of size mismatches in predatory encounters between E. heros and N. duplicata were not predictable necessarily given potential differences in species agressiveness and foot size. This unpredictability fueled coevolution between these cannibalistic moonsnails and their dangerous intraspecific and interspecific prey.

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