Kulikovia alborostrata and Kulikovia fulva comb. nov. (Nemertea: Heteronemertea) are Sister Species with Prezygotic Isolating Barriers

The heteronemertean Kulikovia alborostrata (Takakura, 1898) was originally described as Lineus alborostratus based on material from Misaki, Japan. Although this species was regarded as consisting of two color variants, purple and brown-yellow, the identity of these variants has never been examined based on topotypes. In this study, we performed a multi-locus phylogeny reconstruction, species delimitation analyses, and cross-fertilization experiments to examine the species status of Takakura's original taxon concept consisting of these color variants. Our results suggest that the purple type is identical to Lineus alborostratus Takakura, 1898 auct. (currently Kulikovia alborostrata), whereas the brown-yellow type is conspecific with Lineus fulvus Iwata, 1954, originally established from Hokkaido. These two species appear to have a sister-taxon relationship and are reproductively isolated from each other by prezygotic mechanisms involving gamete incompatibility, minimally separated with 2.8% (16S rRNA) and 14.4% (COI) uncorrected p-distances. We propose that the purple type be considered as representing the true identity of the nominal species Lineus alborostratus (currently assigned to the genus Kulikovia) to maintain the common usage of the name. Although Takakura's type material is not extant, we consider that neotypification is unnecessary in this case because no taxonomic/nomenclatural confusion persists. We also propose to transfer Lineus fulvus to yield Kulikovia fulva comb. nov.