Marine Biology �2000) 136: 1123±1131 Ó Springer-Verlag 2000

Structure and migration corridors in Paci®c populations of the Sword®sh Xiphius gladius, as inferred through analyses of mitochondrial DNA Abstract The sword®sh Xiphias gladius is a migratory oceanic species distributed in subtropical and temperate waters worldwide. Studies utilizing mitochondrial 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 Paci®c. We sequenced 629 base pairs of the control region for 281 sword®sh collected in the Paci®c. A rate heterogeneity parameter, alpha, was found to be 0.201, indicating substantial variation in mutation rate within the control region of sword®sh. Hierarchical analysis of molecular variance supported signi®cant genetic structuring among Paci®c populations. Northern and southern populations in the western Paci®c were signi®cantly divergent, while populations in the east appeared to be genetically continuous. Regression analysis supported a correlation of genetic di€erentiation with geographic distance along a U-shaped corridor of gene ¯ow. These results reveal a pelagic biogeographic pattern heretofore unrecognized in the Paci®c, and reject the null hypothesis that Paci®c populations of sword®sh are unstructured and comprise only a single homogeneous stock.