Mitochondrial DNA variation and population genetic structure of the blue crab Callinectes sapidus in the eastern United States

The blue crab (Callinectes sapidus) is an ecologically important component of estuarine ecosystems throughout the eastern United States and supports both commercial and recreational fisheries of high economic importance. We investigated the population genetic structure of blue crabs in the waters of the eastern United States using restriction fragment length polymorphism analysis of the mitochondrial DNA (mtDNA) molecule. Genetic diversity was notably high; nucleotide diversities were in the range 1.0%-2.6%, haplotype diversities were in the range 0.7-1.0, and 79% of the haplotypes occurred in single individuals. A mismatch analysis of pairwise haplotype differences indicated that C. sapidus experienced a historic period of rapid population expansion. The relationship among mtDNA haplotypes was complex and may represent pre- and post-expansion lineage sorting. Genetic differentiation of the collections was suggested by a significant analysis of molecular variance (AMOVA), but no geographic patterns to the differences nor geographic partitioning of haplotypes was evident. Nevertheless, blue crabs showed a significant cline in haplotype diversity along Atlantic Ocean coastal waters-haplotype diversity decreased with increasing latitude-and a distinctly low haplotype diversity in the northernmost New York collection. The putative population expansion and development of the cline in genetic diversity are consistent with a historic range expansion northward along the Atlantic Ocean coast, perhaps as a result of Pleistocene-era climatic fluctuations. The cline in diversity parallels a cline in allele frequencies at an esterase locus previously observed through allozyme electrophoresis. The combination of a distinct cline and the geographically widespread distribution of common and closely related haplotypes suggests that short-term gene flow in blue crabs is regional whereas long-term gene flow is long-distance. Management of this species should focus on maintaining local blue crab populations and habitats to ensure the long-term sustainability of the species and its fishery.