Regional differentiation and sex-biased dispersal among populations of the horseshoe crab Limulus polyphemus

Fourteen microsatellite DNA loci were surveyed in 892 horseshoe crabs Limulus polyphemus collected from 21 sites ranging from Franklin, Maine, to the Yucatán Peninsula, Mexico. This rangewide survey revealed a high degree of genetic diversity (up to 50 alleles per locus) and heterozygosity (up to 97.2%). There appears to be substantial gene flow between each population and its nearest neighbors. The correlation of genetic distance and geographic distance supports isolation by distance as a mechanism underlying the structure of L. polyphemus along the Atlantic coast of North America. Two statistical descriptors, mean assignment percentage and assignment variance, suggested the presence of male-biased dispersal throughout the study area. Within the continuum of isolation by distance and in the presence of male-biased dispersal, the clustering of specific collections on a tree of genetic distances and hierarchical gene diversity analyses demonstrate discrete regional units. These results suggest defining four management units within the United States: the Gulf of Maine, mid-Atlantic, Atlantic Florida, and Gulf Florida. Further sampling is needed, particularly from North Carolina to the southernmost tip of Atlantic Florida, to test this hypothesis. Multilocus assignment tests indicate a high probability of correctly assigning individuals to the proposed management unit from which they were collected and could serve as a useful tool for identifying the source of animals that are caught at sea.