A giant new species of (Polycladida, Prosthiostomidae) from southwestern Japan.

We describe a new species of polyclad flatworm, , from the rocky subtidal zone in the East China Sea along the coasts of the Kyushu and Okinawa Islands, Japan. is characterized by i) the entire periphery of the dorsal surface narrowly fringed with orange, ii) a marginal-eyespot band extending to the position of the mouth (about anterior one-eighth of body), and iii) two prostatic vesicles covered by a common muscle sheath, which is penetrated by the ejaculatory duct. We performed a molecular phylogenetic analysis based on 945-bp 28S rDNA sequences of 16 species of Prosthiostomidae currently available in public databases in addition to those of and Tsuyuki et al., 2019. In the resulting tree, our new species was nested in a clade composed of species. The tree topology was in favor of a taxonomic view that should be defined by having i) a common muscle sheath that encloses two prostatic vesicles and ii) marginal eyespots that may or may not surround the periphery of the dorsal surface.