MARKED MITOCHONDRIAL-DNA DIFFERENCES BETWEEN MEDITERRANEAN AND ATLANTIC POPULATIONS OF THE SWORDFISH, XIPHIAS-GLADIUS

Restriction analysis of mitochondrial DNA (mtDNA) from 204 individuals of swordfish (Xiphias gladius) revealed no differentiation among samp les from three sites in the Mediterranean Sea (Greece, Italy, Spain), but a high degree of differentiation between Mediterranean samples and a sample from the Gulf of Guinea. A fifth sample from the Atlantic si de of the Straits of Gibraltar (Tarifa) consisted mostly of mitotypes that are common in the Mediterranean, but contained several of mtDNA t ypes of the Guinea sample not found in the Mediterranean. We conclude that, in spite of free migration of swordfish across the Straits of Gi braltar, little genetic exchange occurs between the populations inhabi ting the Mediterranean Sea and the tropical Atlantic ocean. This is th e first evidence of genetic differentiation among geographic populatio ns of this highly mobile species that supports a world-wide fishery.