Molecular phylogeny and evolution of the pelagic copepod genus Neocalanus (Crustacea: Copepoda)

Portions of the mitochondrial genome (ca. 4 kb), encoding three protein-coding (COI, ND4L, ND6) and two ribosomal RNA (srRNA, lrRNA) genes, were sequenced for all six currently recognized species, plus one form, of the pelagic calanoid copepod genus Neocalanus. In Neocalanus gracilis, the ND6 gene was not found in the sequenced portion of the mitochondrial genome. Unambiguously aligned sequences were subjected to Bayesian, maximum-likelihood, maximum-parsimony, and neighbor-joining analyses using Eucalanus bungii as an outgroup. The resultant tree topologies from these four methods were congruent, robust, and all nodes were supported by high bootstrap values and posterior probabilities of 92–100%. Two tropical and subtropical species (N. gracilis and N. robustior) occupied the most basal position, and a subantarctic (N. tonsus) and three subarctic Pacific species (N. cristatus, N. plumchrus, and N. flemingeri) diverged subsequently. Transequatorial dispersal of the ancestral population during glaciations is suggested for this pattern of speciation, in which sister clades exhibited antitropical distributions. Although the area of ocean is much broader in the subantarctic than the subarctic Pacific, a higher number of species occur in the subarctic Pacific (three) than the subantarctic (one). The possibility that marginal seas, such as Japan Sea and Okhotsk Sea, function as natal areas for the divergence of species is discussed.