Species Diversity of <i>Tetranemertes</i> (Nemertea: Monostilifera) in Japan

The eumonostiliferous hoplonemertean genus Tetranemertes Chernyshev, 1992 consists of 10 named species worldwide, whereas only one has been reported from Japanese waters. In this paper, we i) describe Tetranemertes aurantia sp. nov., ii) transfer Emplectonema kandai Kato, 1939—the only nemertean species known to be bioluminescent—to this genus, iii) briefly mention T. unistriata Cherneva, Ellison, Zattara, Norenburg, Schwartz, Junoy, and Maslakova, 2023, the only species previously known in Japan, and iv) report two species left undescribed due to lack of information on the morphology of the stylet apparatus. Our study raised the number of known Japanese congeners to five. Tetranemertes aurantia sp. nov. is established based on a single specimen collected among bycaught samples of a fishery gill net from a depth of 10 m off Sugashima island, Mie, Japan. Each of the two species left undescribed was also represented by a single specimen, from i) a depth of 288 m off Kakeromajima island in the Amami Islands, and from ii) a tidal flat in Misaki, respectively. Our molecular phylogenetic analysis suggests that Tetranemertes is the sister taxon to Amphiporus cf. cruentatus Verrill, 1879, albeit not fully supported. It is suggested that Ommatoplea Ehrenberg, 1828 may be a senior synonym of Tetranemertes.