Structure and migration corridors in Pacific populations of the Swordfish Xiphius gladius, as inferred through analyses of mitochondrial DNA

The swordfish Xiphias gladius is a migratory oceanic species distributed in sub-tropical and temperate waters worldwide. Studies utilizing mitochondri al DNA (mtDNA) have demonstrated genetic subdivision between ocean basins, as well as within the Atlantic basin. However, there has been no support of population subdivision within the Pacific. We sequenced 629 base pairs of the control region for 281 swordfish collected in the Pacific. A rate heter ogeneity parameter, alpha, was found to be 0.201, indicating substantial va riation in mutation rate within the control region of swordfish. Hierarchic al analysis of molecular variance supported significant genetic structuring among Pacific populations. Northern and southern populations in the wester n Pacific were significantly divergent, while populations in the east appea red to be genetically continuous. Regression analysis supported a correlati on of genetic differentiation with geographic distance along a U-shaped cor ridor of gene flow. These results reveal a pelagic biogeographic pattern he retofore unrecognized in the Pacific, and reject the null hypothesis that P acific populations of swordfish are unstructured and comprise only a single homogeneous stock.