Tetrastemma luteum sp. nov. and Tetrastemma phaeobasisae Kulikova, 1987 from Japan
Our study described a new species and documented a new distribution record within the speciose genus Tetrastemma Ehrenberg, 1828. Integrative taxonomy confirmed that the yellowish T. luteum sp. nov. displays notable variation in body coloration and stylet-apparatus morphology. We also reported the first record of T. phaeobasisae in Japan, supported by the close match between its COI gene sequence and that of a Russian specimen. With these findings, the known diversity of Tetrastemma in Japanese waters was increased from 17 to 19 species, and the distribution of T. phaeobasisae is now extended from Vostok Bay and Peter the Great Bay in Russia to Oshoro Bay in Japan. Our phylogenetic analysis showed that geographic distribution does not always correspond to cladogenesis in Tetrastemma. Using our specimens, we demonstrated that DNA barcoding is essential for reliably identifying unicolored or polymorphic species, highlighting the limitations of morphology-based nemertean taxonomy.
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