The Abyssal Parasitic Flatworm : New Records, Anatomy, and Molecular Phylogeny, with a Discussion on Its Systematic Position.

AbstractThe order Fecampiida, a group of parasitic turbellarians, has been poorly studied in terms of its species diversity, morphology, and ecology. Fecampiida is positioned within the monophyletic clade Adiaphanida, along with Tricladida and Prolecithophora, but their phylogenetic relationships are not well understood. Although the nervous and muscular systems of only two species in Fecampiida have been studied, recent research inferred morphological similarities between Fecampiida and Prolecithophora. In this study, we collected fecampiid cocoons and juveniles at depths of 1861-4438 m in Japanese waters. We identified the species on the basis of swimming juvenile specimens and by using histological and molecular methods, while we also examined its musculature and nervous system. Our study revealed a more complex nervous system than previously reported, with dorsal, lateral, and ventral pairs of longitudinal nerve cords connected through an anterior neuropile and posterior transverse commissures. While the nervous and muscular morphology suggested similarities with Prolecithophora, our phylogenetic analysis did not support a close relationship between Fecampiida and Prolecithophora.