QUANTIFYING ADAPTIVE COSTS: STABLE ISOTOPE GEOCHEMISTRY AND ANTI-PREDATORY MORPHOLOGIES IN BUSYCONINE WHELKS

We present growth rate data, derived from stable isotope geochemistry, for four specimens of Busycon carica (an episodic, indeterminately growing, predatory marine gastropod) from Wassaw Sound, Georgia. Modern specimens of B. carica display a variety of different morphologies in the southern part of their range, with weakly and well ornamented shells offering varying degrees of protection against durophagous predators. Well ornamented shells (thicker, longer spines, possession of prominent shell sculpture) have been hypothesized to have reduced growth rates relative to weakly ornamented shells (thinner, less spinose, less sculptured). The two well ornamented morphologies assessed in this study took 5 and 7 years to reach a length of 100 mm, as compared to the two weakly ornamented whelks, which reached lengths of 100 mm in 3.5 and 4 years, suggesting that possessing well ornamented shells substantially reduces growth rates in this species. The anti-predatory benefits associated with possessing a well ornamented shell likely offset the associated lower growth rate for these individuals.

The development of heavily ornamented shells only occurs in modern populations south of Cape Hatteras, North Carolina, whereas during the Pliocene, heavily ornamented shells occurred in populations from Florida northwards to Virginia. The geographic distribution of morphologies is consistent with the hypothesis that the western Atlantic during the Pliocene was warm and highly productive. During the Pliocene, the western Atlantic was likely overall more environmentally permissive than in the Recent, allowing the spread of “costly” adaptations throughout a large portion of Busycon’s range. The diminished expression of adaptations after the Plio-Pleistocene boundary may represent a response to both lowered permissivity, and diminished predation pressure.