ADAPTIVE SIGNIFICANCE OF A MORPHOLOGICAL CLINE WITHIN THE PREDATORY WHELK BUSYCON CARICA: WHAT, IF ANYTHING, DO DIFFERENCES IN MORPHOLOGY MEAN?

Busycon carica is a large, predatory marine gastropod with an extensive geographic distribution along the eastern coast of the United States from Massachusetts to Florida and a fossil record extending back to the Pleistocene. Interspecific shape variability is high, with northern populations characterized by a less spinose, thinner, form without a tumid ridge whereas southern morphs possess larger and fewer spines as well as thicker, more robust shells and pronounced tumid ridges. While this cline has been documented in the literature, the possible adaptive significance of the different morphologies has not been tested. This study combines geometric morphometric techniques, used to characterize variability along the cline, with predation experiments, designed to test whether the variable morphologic features serve as anti-predatory adaptations against the stone crab, Menippe mercenaria., which overlaps with the southern part of B. carica’s range.. Preliminary results show that extreme morphotypes of both the southern and northern morphologies are present in a restricted region of the species range between the Carolinas and Georgia. Intermediate forms are abundant in this region and provide a full range of intergradation between the two morphotypes. Predation experiments using M. mercenaria suggest that while the tumid ridge may serve as an anti-predatory adaptation by preventing stone crabs from breaking off the canal in southern forms, spines do not appear to deter these predators. Ongoing work will examine differences in handling time and success rates for subjugation of the two morphologies.