Analysis of morphometry and mitochondrial DNA sequences from two Trichiurus species in waters of the western North Pacific: taxonomic assessment and population structure

The species status of cutlassfishes ( Trichiurus spp.) in waters of the western North Pacific (five hydrographic areas) was examined using the full-length (consisting of 1141 base pairs) DNA sequence of the mitochondrial cytochrome b gene (mtCyt b ) and truss network-based morphometrics. Two historically confusing Trichiurus morphs are suggested to be two separate species because a decisive difference in the genetic and morphometric data between Trichiurus japonicus and Trichiurus lepturus , using a restricted congener, Trichiurus brevis as the out-group was found. Of all 241 polymorphic sites in mtCyt b at the P _0·95 level, 192 sites fixed within species and variable between species was found. Two-level AMOVA (species and hydrographic areas) results showed significant species differences, while no hydrographic structuring was found within species. Both maximum parsimony and neighbour-joining distance trees supported clear branches for T. japonicus and T. lepturus at 100% bootstrap support. Discriminant function analysis with adjusted body size performed on the morphometric data of T. japonicus and T. lepturus revealed a decisive specific gap of non-overlapping scattering. Combining mtCyt b findings with documented geological events in the western North Pacific, the authors postulate that T. japonicus was evolved peripherally from T. lepturus in middle Miocene, and these two partially sympatric species were achieved secondarily after the Pliocene when intermediate temperatures were located in the subtropical transitions.