Molecular insights into the population structures of cosmopolitan marine fishes

Many marine fishes are cosmopolitan, occurring in continuous (e.g., circumtropical) or discontinuous (e.g., antitropical) distributions. Little is known of the genetic basis of population structure of these species, even though several support extensive fisheries. To develop a database that would facilitate comparison of the population structures among cosmopolitan fishes we consistently included restriction fragment length polymorphism (RFLP) analysis of mitochondrial DNA (mtDNA) as a common approach to our investigations of these species. This article presents a review of those analyses. Considerable intra`specific genetic variation was revealed within all cosmopolitan marine species. Continuously distributed species displayed population structures ranging from a lack of significant heterogeneity between ocean samples to shallow but significant structuring within an ocean basin. In general, greater intraspecific genetic divergence was revealed within discontinuously distributed fishes. Levels of population structuring ranged from species comprising conspecific populations with no mtDNA haplotypes in common to those comprising populations with homogeneous distributions of mtDNA haplotypes across ocean basins. The close affinity of haplotypes among conspecific populations of all discontinuously distributed species was consistent with contact since the Pleistocene. Although general patterns of genetic population structure were similar among continuously and discontinuously distributed cosmopolitan marine species there were some striking differences. These differences underscore the need for a thorough understanding of the genetic basis of population structure of each species for proper management.