Conservation Genetics of the American Horseshoe Crab (Limulus polyphemus): Allelic Diversity, Zones of Genetic Discontinuity, and Regional Differentiation

Extensive studies of genetic variation of Atlantic horseshoe crab Limulus polyphemus populations have revealed the presence of considerable allelic diversity and population structuring that appear to reflect the actions of various evolutionary processes. We have expanded on our previous efforts to gain a more refined understanding of L. polyphemus population structure by surveying 792 additional animals distributed among 12 additional spawning aggregations. Here we report on variation at 13 microsatellite DNA markers for 1,684 horseshoe crabs sampled from 33 spawning assemblages from northern Maine to the Yucatan Peninsula, Mexico. Average unbiased heterozygosity (uH E ) was high (0.74 ± 0.01), the number of private alleles was low (0.06 ± 0.04), effective population size (Ne) ranged from 22 to 187, inbreeding (F) ranged from −0.07 to 0.07, and tests for genic differentiation among populations indicated shallow but statistically significant differentiation within regions and highly significant differences among regions (P < 0.005). Current findings are consistent with previous research by this group in suggesting a series of genetic discontinuities across the species’ range that could indicate regional adaptive significance or reflect vicariant geographic events. Additional collections allowed improved delineation of structuring (as reflected by two new zones of genetic discontinuity) along the southeast Atlantic coast as well as identification of previously undetected shallow but significant structuring along the Florida Gulf coast. Regional groupings may warrant management unit recognition based on the patterns observed among multiple genetic metrics. The integration of this information with previously identified genetic variation and ecological data is essential to developing an ecologically and evolutionarily sound conservation management strategy.