We establish a new interstitial polyclad species, Theama japonica sp. nov., based on specimens collected from coarse-sandy habitats in three Japanese main islands (Hokkaido, Honshu, and Shikoku) along the coasts of the Pacific Ocean and the Sea of Japan. Theama japonica is characterized by i) two pairs of cerebral eyespots and four to six precerebral eyespots; ii) eosinophilic secretion glands distributed in the distal half of the inner ventral part of the prostatic vesicle; iii) a conical penis papilla, bent up dorsally, with a sclerotized inner wall; iv) the prostatic sheath with an inner angular fold on the dorso-distal side; and v) the external cilia longer dorsally than ventrally. Partial sequences of the cytochrome c oxidase subunit I (COI) gene from 20 specimens collected at eight localities along Japanese coasts represented 19 haplotypes. The uncorrected p-distances among these COI haplotypes fell within intraspecific variations observed in other polyclads. A network analysis based on these COI haplotypes suggested a geographically non-cohesive genetic structure of the species, possibly indicating the species' high dispersibility. Molecular phylogenetic analyses based on a concatenated dataset of 18S and 28S rDNA sequences showed T. japonica formed a clade with other Theama species. The resulting tree also indicates that our new species is more closely related to Theama sp. from Colombia than species from Panama and Croatia.
A new fish leech, Branchellion brevicaudatae sp. n., is described based on specimens parasitizing the gills of the short-tail stingray, Bathytoshia brevicaudata (Hutton, 1875), collected from Japanese waters. The new species can be distinguished from other congeners by having: i) pulsating vesicles emerging from posterior base of branchiae, one pair per somite; ii) dorsal white spots, not arranged in longitudinal row; and iii) blackish body. A phylogenetic tree based on partial sequences of the mitochondrial cytochrome c oxidase subunit I gene from the new species and other piscicolid worms showed that the new species is sister to Branchellion torpedinis Savigny, 1822. This is the first record of Branchellion Savigny, 1822 from Japanese waters.