Population divergence in the sinistral whelks of North America, with special reference to the east Florida ecotone

yVbstract This study evalualed models of species relationships among sinistral whelks in the genus Bu.sycon in the Atlantic and Gulf of Mexico. Gene frequencies at eight polymorphic allozyme loci, shell morphology, anatomy, and partial DNA .sequences for the cytochrome c oxida.se 1 (COI) milochondrial gene were examined in eight populations, ranging from New Jersey to the Yucatan peninsula, and from the dextraliy coiled si.ster laxon Busycon carica (Gmelin. 1791). Whelks were collected in 1997 and 1998. The maximum COI sequence divergence recorded among 32 sinistral individuals was 1.96%, which together with the absence of any gross or qualitative morphological differences, suggested all eight populations should be considered conspecific. High levels of divergence between the allopatric western Atlantic and Gulf of Mexico populations, as revealed by iixed or nearly fixed differences at several allozymeencoding loci were interpreted as e\'idence that the east FHorida ecotone constitutes a significant barrier to gene flow. Size trimming also revealed several significant quantitative dilTerences in shell and radular morphology between the three pooled Atlantic populations and five pooled Gulf populations. The Yucatan sample was the most distinctive conchologically, with heavy spines and tumid ridges, possibly related to stone crab prédation. Based on the evidence all left-handed whelks of North