Description of a New Species of the Marine Flatworm Prosthiostomum (Platyhelminthes: Polycladida) and its Three Known Congeners from Misaki, Japan, with Inference of Their Phylogenetic Positions within Prosthiostomidae.

The present study provides morphological descriptions of four species of Prosthiostomum (Polycladida, Prosthiostomidae)-P. auratum Kato, 1937; P. hibana sp. n.; P. cf. ostreae Kato, 1937; and P. vulgare Kato, 1938-based on specimens collected among branching coralline algae and kelp holdfasts in Misaki, Japan. The new species P. hibana sp. n. is characterized by i) the dorsal surface of the body covered with numerous orange maculae, some of which coalesce together to form larger ones; ii) a pair of linear cerebral-eyespot clusters, each consisting of relatively few (7-9) cerebral eyespots; iii) 3-4 pairs of ventral eyespots embedded in parenchyma: iv) the inner wall of the male atrium deeply ruffled; v) the lumen of the seminal vesicle being narrow and elongated in shape; and vi) a large sucker situated in the center of the body. We remark on some morphological characters that were not mentioned in the original description of P. auratum. We infer the phylogenetic positions of these four species within Prosthiostomidae using the maximum-likelihood analysis based on partial 28S rRNA and COI gene sequences determined de novo, in addition to those that are currently available in public databases. In the resulting tree, the four species-P. auratum, P. hibana sp. n., P. cf. ostreae, and P. vulgare-were nested in a clade that was composed of all the other Prosthiostomum species included in the analysis.