First record of Bulaceros porcellanus Newman & Cannon, 1996 (Platyhelminthes, Polycladida, Cotylea) from Japanese waters, with a revision of the generic diagnosis based on morphology and molecular phylogeny

The cotylean polyclad flatworm Bulaceros porcellanus Newman & Cannon, 1996 has so far only been recorded in a few tropical locations in the Indo-Pacific Ocean, including the Lakshadweep Islands (India), the Great Barrier Reef (Australia), and the Marshall Islands (Republic of the Marshall Islands). In this study, we report B. porcellanus for the first time from Japanese waters based on specimens collected in Misaki (Kanagawa) along the Pacific coast of Honshu. This new record expands the species' known geographic distribution northward to include the warm temperate region of Japan. Our molecular phylogenetic analysis revealed that B. porcellanus formed a clade with Pseudoceros harrisi Bolanos, Quiroga & Litvaitis, 2007, and was separated from the monophyletic group composed of 19 other species of Pseudoceros Lang, 1884, including its type species Proceros velutinus Blanchard, 1847 (now Pseudoceros velutinus). Based on the morphological and phylogenetic closeness between P. harrisi and B. porcellanus (type species of Bulaceros Newman & Cannon, 1996), we transfer P. harrisi into Bulaceros with a revision of the diagnosis of Bulaceros.