Genetic analysis on Pseudodiaptomus inopinus (Copepoda, Calanoida) species complex in Japan: revival of the species name of P. japonicus Kikuchi, 1928

Pseudodiaptomus inopinus, a common estuarine calanoid copepod in the Northwest Pacific, has been suggested to be a species complex. The population of the mainland of Japan consists of two allopatric forms that are distinguishable by the length of the posterior processes of the genital operculum; the forms with short and long processes are tentatively called SP- and LP-forms, respectively. We analyzed the mitochondrial cytochrome c oxidase subunit I (COI) gene and the nuclear ribosomal internal transcribed spacer 1 (ITS1) gene of 34 individuals (12 SP- and 22 LP-forms) collected from various localities. The genetic differences between the two forms were 12-15% for COI and 7-10% for ITS1. They were clearly separated into different clades in both COI and ITS1 trees. These results confirmed that the two forms belong to different species. The SP-form is a distinct species because the shape of the genital operculum of P. inopinus s. str. has long posterior processes as in the LP-form. Pseudodiaptomus japonicus Kikuchi, 1928, which is the SP-form and was once synonymized with P. inopinus, is revived. The genetic comparison using available sequence data indicates that Pseudodiaptomus koreanus Soh et al., 2012 is the same species as P. japonicus.