A new ribbon worm, Tetrastemma luteum sp. nov., reported from Japanese waters, is described using morphological and DNA sequence data based on 15 specimens—10 from Oshoro, 2 from Otsuchi, and 3 from Sugashima—all associated with brown algae in the family Sargassaceae. The new species is characterized by its entirely yellowish body; a rounded, notched, and knobbed head that is similarly coloured to (or lighter than) the body; brown-orange, more indented anterior and bright-orange posterior ocelli; a central stylet shorter than the stylet basis; ventrolateral, non-connected anterior cephalic furrows that are not visible in dorsal view; and a posterior cephalic furrow encircling the head. Tetrastemma luteum sp. nov. exhibits variation in the degree of its body colouration, body and head pigmentation patterns, head shape, stylet basis shape, and number of accessory stylets. Additionally, we report a new locality, Oshoro, for Tetrastemma phaeobasisae Kulikova, 1987, based on a single specimen, representing the first record of this species in Japan. This species was previously known to be distributed only in Vostok Bay and Peter the Great Bay, Russia. Its occurrence in Oshoro is confirmed both by morphology and DNA sequence. Furthermore, we tested the phylogenetic positions of T. luteum sp. nov. and T. phaeobasisae within the genus Tetrastemma Ehrenberg, 1828 using newly determined 16S, 18S, and 28S ribosomal RNA, cytochrome c oxidase subunit I, and histone H3 gene sequences. Our phylogenetic analyses confirmed the genus placement of T. luteum sp. nov. being nested within the Asian-Australia Pacific subclade of the genus together with T. phaeobasisae. The new species is a sister taxon to the clade consisting of Tetrastemma pseudocoronatum Chernyshev, 1998 and T. phaeobasisae.
Amphiporina, Biodiversity, Cox1, Hoplonemertea, Monostilifera, Systematics
