AbstractThe mangrove crab Ucides cordatus (Linnaeus, 1763) is a keystone species in mangroves, and recent changes in population sizes have been attributed primarily to overharvest, selective capture, and loss of habitat due to destruction of mangroves. We analyzed genetic variability and phylogeographic pattern among populations of U. cordatus along the western Atlantic coast in order to document levels of genetic differentiation, aiming to provide data to be used in future management and conservation strategies. MtDNA cytochrome oxidase I (COI) was analyzed and compared among populations. We computed a haplotype network, haplotype and nucleotide diversities, index of fixation, Mantel test, analysis of molecular variance (AMOVA), neutrality tests, and pairwise mismatch distributions under the sudden expansion model of demographic expansion; Maximum likelihood phylogenetic reconstruction was also undertaken for the haplotypes found. We found no geographic structure and no significant indication for restricted gene flow between populations from sites along a distance of 12,000 km, indicating that sampled individuals currently represent a single panmictic population. Demographic results revealed demographic expansion along the evolutionary history of U. cordatus, probably related to climatic biogeographic events. Although there is no current evidence of any substantial genetic break or of demographic reduction between the western Atlantic populations that we investigated, the genetic signal of regional population depletion warrants future monitoring in the course of resource management.