Occurrence of the Paracalanus parvus species complex in offshore waters south of Japan and their genetic and morphological identification to species

The Paracalanus parvus species complex is one of the most dominant and ecologically important copepod groups in the waters around Japan. However, the overall taxonomic classification of the species complex has not been sufficiently defined to date. In this study, both genetic and morphological analyses were performed on specimens of the P parvus species complex collected along the "O-line", a monitoring line located at 138 degrees E, south of Japan. Among the individuals collected, P. nanus specimens were clearly distinguishable morphologically, whereas other specimens could be classified into three reproductively isolated species. These distinctions are based on genetic analyses of the mitochondrial cytochrome oxidase subunit I (mtCOI) and nuclear large subunit (nLSU) ribosomal RNA. By comparing the genetic and morphological data with findings from previous studies, these three species were identified as P tropicus, P. indicus, and an as-yet undescribed species (Paracalanus sp. (NWP)). The latter possesses a humped forehead and was found to be a dominant species within the surveyed continental slope area. As a distinguishing characteristic, the humped forehead was not consistently useful as it was prominent in only about half of the individuals collected. Morphological discrimination between this species and P. indicus was only made possible by comparing the relative length of exopod segment 3 on swimming leg 4. Despite morphological similarities among member species, ecological studies on the P. parvus species complex conducted around Japan may mostly concern the undescribed species, Paracalanus sp. (NWP), considering that P indicus was found only in warmer waters in this study.