| The circumtropical blacktip shark, Carcharhinus limbatus , utilizes coastal nurseries throughout the world and is targeted by shark fisheries in the western Atlantic, Gulf of Mexico, and Caribbean Sea. Although many coastal elasmobranch species have widespread distributions and are important components of coastal ecosystems, few studies have examined the genetic relatedness of their populations. Previous investigations have typically been limited to sharks of multiple age groups collected within a single geographic region and have failed to detect genetic structure among sampled locales. I examined the fine-scale structure of blacktip shark continental nurseries in the western Atlantic Ocean, Gulf of Mexico, and Caribbean Sea using mitochondrial DNA (mtDNA) control region sequences and eight nuclear microsatellite loci scored in neonate and young-of-the-year blacktip sharks. In addition, mtDNA control region sequences were used to investigate the genetic relationships of widespread blacktip shark populations from several locations in the eastern Atlantic and Indo-Pacific. Data support female philopatry to natal nursery regions throughout the western Atlantic, Gulf of Mexico, and Caribbean Sea with higher levels of male-mediated gene flow. Nurseries along the Florida Gulf coast are genetically similar, suggesting straying among geographically proximate nurseries. Two geographically distinct maternal clades (a western Atlantic, Gulf of Mexico, and Caribbean Sea Glade and an eastern Atlantic and Indo-Pacific Glade) were identified within the blacktip shark's range. Limited sharing of haplotypes among populations within Glades may reflect genetic structure throughout each geographic region. Straying among proximate nurseries could serve as a dispersal mechanism and may have been interrupted by a vicariant event, such as the rise of the Isthmus of Panama, resulting in the two Glades. This scenario implies historical dispersal across the Pacific Ocean (supported by the recovery of identical haplotypes from the Philippines, Hawaii, and the Gulf of California) and an oceanic barrier to migration across the Atlantic. |
