C. A. Reeb á L. Arcangeli á B. A. Block Structure and migration corridors in Paci®c populations of the Sword®sh Xiphius gladius, as inferred through analyses of mitochondrial DNA Received: 10 November 1998 / Accepted: 4 February 2000 Abstract The sword®sh Xiphias gladius is a migratory oceanic species distributed in sub-tropical 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 popula- tion 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. Introduction The sword®sh Xiphias gladius has a life history and va- gility suggestive of high gene ¯ow and little genetic subdivision among populations. Members of this species are found in all tropical, subtropical and temperate seas, and provide lucrative commercial ®sheries around the world. Sword®sh are powerful swimmers, grow to 540 kg, and live for ³9 yr Palko et al. 1981) . These ®sh elevate temperature in the head region with a specialized eye muscle known as the heater organ Carey 1982). This heat-generating tissue allows them to traverse steep thermal gradients ranging from 26 to 6 °C, presumably in search of prey Carey and Robison 1981; Carey 1990). Despite their ability to tolerate a wide range of tem- peratures, sword®sh abundance and distribution is generally associated with surface waters >18 °C and <30 °C Palko et al. 1981). Sword®sh appear to be highly migratory yet solitary animals,rarelypairingexcepttospawn.Sexualmaturityis thought to occur at 5 yr of age, and spawning generally takes place in tropical waters where surface temperatures are >20 °C Palko et al. 1981). Identi®cation of spatially or temporally discrete spawning areas has been dicult because of the long spawning season, the widespread area inwhichlarvaearefound,andthevarietyofmethodsused to survey the ichthyoplankton Grall et al. 1983). None- theless, seasonality in the abundance of larvae and re- productive adults is evident in the northern and southern Paci®c, suggesting the possibility of population subdivi- sion Palko et al. 1981; Grall et al. 1983). Unfortunately, neitherintensiveanalysisofcatchstatisticsnorthelimited recapture of tagged ®sh has provided a clear picture of migratory routes or population structure in the Paci®c. Molecular genetic data can be used to infer popula- tion structure by quantifying the degree of genetic relatedness between geographically sampled local pop- ulations or demes. Demes found to be undierentiated through the analysis of F-statistics Wright 1951; Weir and Cockerham 1984) or lacking geographic partition- ing of discrete phylogenetic clades Avise 1994) are as- sumed to be linked by gene ¯ow and grouped within the same population. Demes showing a phylogeographic partitioning of discrete clades or a signi®cant degree of genetic dierence, as measured by F ST are considered to be separate populations. Marine Biology 2000) 136: 1123±1131 Ó Springer-Verlag 2000 Communicated by M. H. Horn, Fullerton C. A. Reeb &) á L. Arcangeli á B. A. Block Hopkins Marine Station, Department of Biological Sciences, Stanford University, Ocean View Boulevard, Paci®c Grove, California 93950, USA Fax: 001 0)831 375-0793 e-mail: creeb@stanford.edu