Variation in the American Horseshoe Crab: A Spiny Variant from the Southeast

American horseshoe crabs, Limulus polyphemus, show both genetic and phenotypic variations across their broad range. Analysis of microsatellite (nuclear DNA) variation shows two genetically discrete populations along the southeast coast of the USA: Southeast Atlantic and Florida East. The Florida East horseshoe crabs are among the smallest along the Atlantic coast and are found in the Indian River Lagoon system of central Florida. The Southeast Atlantic horseshoe crabs occur in coastal North Florida, Georgia, and South Carolina where tides are exceptionally high and environmental conditions relatively stable. Individuals in this population are the largest along the Atlantic coast, and some have a unique “extra” pair of spines on the anterior margin of the opisthosoma (opercular pleurite). When first observed in 1815, Thomas Say proposed that these horseshoe crabs should be considered a separate species, Limulus australis. We show that the “extra” spines are a polymorphic trait within the Southeast Atlantic population. There is a region of overlap and admixture in northeast Florida between Southeast Atlantic and Florida East horseshoe crabs with interbreeding and random mating between individuals from the two populations. The frequency of “extra” spines and Southeast Atlantic alleles gradually declines from southern Georgia to central Florida. The function of the “extra” spines is not known, but we consider several possibilities including predator defense and increasing the ability of juveniles to hide, right, or bury. The maintenance of stable, genetically, and phenotypically distinct populations indicates strong adaptation to local environmental conditions and suggests that management efforts should be targeted within regions at a local scale.