Molecular Phylogeny of the Genus <i>Paracanthonchus</i> (Nematoda: Chromadorida) with Description of <i>P. yeongjongensis</i> sp. nov. from Korea

During an investigation of the intertidal zone at Yeongjongdo Island, Incheon, Korea, a new species belonging to the genus Paracanthonchus was found and is reported. Paracanthonchus Mikoletzky, 1924 is the largest genus within the family Cyatholaimidae, and the species identification of this genus has been difficult mainly due to overlapping characteristics and a lack of genus/species-defining apomorphic characters. The new species is characterized by the buccal cavity, armed with one large dorsal tooth and two subventral teeth, the presence of lateral differentiation, seventy-six tubular precloacal supplements, and proximally paired gubernaculum. Alongside the description, we are updating the review of the genus by providing an up-to-date list of valid species, as well as a comprehensive tabular key to the genus with measurements of species-discerning characteristics. We also provide partial sequences of mtCOI, 18S and 28S rRNA to verify the new species belongs to the genus Paracanthonchus and to discuss the phylogeny of the family Cyatholaimidae as well as the genus Paracanthonchus. Our phylogeny agrees with previous findings that, while Cyatholaimidae forms a monophyletic clade, many genera within, including Paracanthonchus, are paraphyletic. For now, it appears that subfamilies are not well-represented by molecular means, and much more molecular data along with species defining morphological traits will have to be accumulated to meaningfully organize this family and the genus within