A new species of slender flatworm in the genus Eucestoplana and a record of E. cf. cuneata (Platyhelminthes, Polycladida) from the Okinawa Islands, Japan, with an inference of their phylogenetic positions within Cestoplanidae

In this study, we describe a new species of elongated marine flatworm, Eucestoplana ittanmomen sp. nov. , collected from the intertidal zone of the Okinawa Islands, Japan. Eucestoplana ittanmomen sp. nov. is distinguished from other congeners based on the following characteristics: i ) its translucent body lacking coloration, ii ) its dome-shaped penis sheath, iii ) the absence of cilia on the inner wall of the male atrium except outside the penis sheath, and iv ) the presence of an adhesive organ at the posterior end of the body. Additionally, we report the occurrence of E. cf. cuneata (Sopott-Ehlers & Schmidt, 1975) in Japan; E. cuneata has previously been documented in the Galapagos and Fiji Islands. We conducted phylogenetic analyses to infer the positions of the two Eucestoplana species within Cestoplanidae using a concatenated dataset comprising partial 18S and 28S rDNA sequences from E. cf. cuneata and E. ittanmomen sp. nov. from Japan, as well as four known Cestoplana species with sequences available in public databases. Our phylogenetic analyses revealed that Cestoplana and Eucestoplana were reciprocally monophyletic. Furthermore, the genetic distance of the 16S rDNA sequences supported the genetic independence of the two sister species, E. cf. cuneata and E. ittanmomen sp. nov.