Tetrastemma Ehrenberg, 1828, is one of the most speciose (~110 spp.) genera within the phylum Nemertea, comprising marine monostiliferans generally having four eyes. Monophyly of Tetrastemma remains open to question, having been tested to date only with 18S rRNA gene sequences targeting 13 species. Here, we examine the clade Tetrastemma with additional gene markers (18S rRNA, 28S rRNA, histone H3, 16S rRNA, and cytochrome c oxidase subunit I genes) and OTUs (66 specimens representing 53 species in Tetrastemma), along with a taxon sampling covering the entire Eumonostilifera. Our analysis revealed that Tetrastemma (a) as non-monophyletic (as were Tetrastemmatidae) (b) contains a major clade consisting of four geographically cohesive subclades, whereas (c) Tetrastemma arcticum Uschakov, 1926; Tetrastemma bilineatum Coe, 1904; Tetrastemma vittigerum (Bürger, 1904); Tetrastemma wilsoni Coe, 1943; and two specimens tentatively identified as Tetrastemma sp. occur outside this major clade, and iv) the three species of Psammamphiporus Gibson, 1989, and Quasitetrastemma Chernyshev, 2004, nest within in the major Tetrastemma clade. Thus, we (a) synonymize Psammamphiporus and Quasitetrastemma with the senior name Tetrastemma and (b) establish Arctostemma gen. nov. for T. arcticum to yield Arctostemma arcticum (Uschakov, 1926) comb. nov.
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