Description of a new species of Paraplehnia (Polycladida, Stylochoidea) from Japan, with inference on the phylogenetic position of Plehniidae

Abstract. We describe a new species of polyclad flatworm, Paraplehniaseisuiaesp. nov., from 298–310 m depths in the Sea of Kumano, West Pacific, Japan. Paraplehniaseisuiaesp. nov. is characterized by i) a developed muscular wall proximally occupying about one-third of the prostatic vesicle, ii) no common duct between the spermiducal bulbs and the prostatic vesicle, and iii) a genital pit between the male and female gonopores. We provide a partial sequence (712 bp) of the mitochondrial cytochrome c oxidase subunit I gene as a DNA barcode for the species. Our phylogenetic analyses based on 603-bp 28S rDNA sequences indicate that P.seisuiaesp. nov. is nested in a clade consisting of stylochoid species along with unidentified species of Stylochus. It suggests that Plehniidae belongs to Stylochoidea, although this should be confirmed by future studies that contain Plehniaarctica (Plehn, 1896), the type species of the type genus of the family. The interfamily relationship among the superfamily Stylochoidea remains poorly resolved.