A New Heteronemertean Genus with Variable Potential of Anterior Regeneration for Species Formerly Belonging to Lineus (Nemertea: Pilidiophora)

The heteronemertean genus <i>Lineus</i> Sowerby, 1805, currently accommodating about 100 species, is known to be non-monophyletic and is in a desperate need of taxonomic revision. In this article, we focus on a group of species including <i>L. caputornatus</i> Takakura, 1898, <i>L. grubei</i> (Hubrecht, 1879) auct., <i>L. pictifrons</i> Coe, 1904, and <i>L. subcingulatus</i> Takakura, 1898. These four morphologically similar species formed a clade to the exclusion of the type species, <i>L. longissimus</i> (Gunnerus, 1770), on a molecular phylogeny based on partial sequences of the 16S rRNA, 18S rRNA, 28S rRNA, cytochrome <i>c</i> oxidase subunit I, and histone H3 genes. For this clade we propose the name <i>Bilucernus</i> gen. nov. We redescribe <i>L. grubei</i> auct. from Japan as <i>Bilucernus takakurai</i> sp. nov. <i>Bilucernus</i> clade was nested within a larger clade that also contained members of <i>Kulikovia</i> Chernyshev, Polyakova, Turanov, and Kajihara, 2017, <i>Lineopselloides</i> Gibson, 1990, <i>Siphonenteron</i> Renier in Meneghini, 1847, and <i>Tenuilineus</i> Riser, 1993. We observed that body fragments from the intestinal region regenerated a head in <i>B. caputornatus</i> comb. nov. and <i>B. subcingulatus</i> comb. nov., but did not in <i>B. takakurai</i> sp. nov. The type specimen of <i>Lineus bipunctatus</i> Takakura, 1898 was most likely a regenerating individual of <i>B. caputornatus</i> comb. nov. We synonymize <i>L. bipunctatus</i> with <i>L. caputornatus</i>, giving nomenclatural priority to the latter. In addition, <i>Lineus albifrons</i> Coe, 1934 is herein also synonymized with <i>L. caputornatus</i>. We speculate that the unusually wide geographic distribution of <i>B. caputornatus</i> comb. nov. is related to its ability to regenerate head from fragments of body.