Description of a new species of <i>Prostheceraeus</i> Schmarda, 1859 and redescription of a poorly known species, <i>Phrikoceros sagamianus</i> (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea)

The superfamily Pseudocerotoidea Lang encompasses Euryleptidae Stimpson, Pseudocerotidae Lang, and Stylostomidae Dittman. These families include conspicuous, flamboyant marine flatworms with a worldwide distribution. Here, we describe a new euryleptid species belonging to Prostheceraeus Schmarda, 1859, based on specimens collected by dredging at depths of 100–180 m off Misaki, Japan. The new species, Prostheceraeus fragum sp. n., is distinguished by its unique dorsal colouration in that oval red maculae are radially distributed along the translucent cream body periphery. Additionally, based on newly collected topotypic specimens, we transfer the poorly known pseudocerotid species Pseudoceros sagamianus Kato, 1937, to the genus Phrikoceros Newman and Cannon, 1996. These specimens possess (1) a pharynx with shallow, simple folds, (2) a deeply ruffled body margin, and (3) ear-like pseudotentacles with folds. Our molecular phylogeny, based on partial 18S and 28S rDNA sequences, yielded two key findings: (1) Pr. fragum sp. n. nested within the clade containing all other Prostheceraeus species, and (2) Phrikoceros sagamianus comb. n. formed a clade with all other Phrikoceros and Monobiceros species, distinct from the Pseudoceros clade. These results support the generic affiliations established through our morphological examinations. Furthermore, based on the resulting phylogenetic trees, we discuss the systematics of Euryleptidae in relation to Stylostomidae. http://www.zoobank.org/urn:lsid:zoobank.org:pub:F4EA3E5D-F3E6-440E-AAD7-D75E17FA11AE