A molecular phylogeny of Tetrastemma and its allies (Nemertea, Monostilifera)

Abstract

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.

Cutting the ribbon: bathyal Nemertea from seeps along the Costa Rica margin, with descriptions of 2 new genera and 9 new species

“…In contrast to all abyssal and hadal Amphiporina described so far, the newly described species of Tetrastemma do not belong to the same subclade including the genera Nemertovema Chernyshev & Polyakova, 2018and Proamphiporus Chernyshev & Polyakova, 2019(Chernyshev & Polyakova 2019. Instead, they are part of a subclade comprising the majority of species of Tetrastemma (Chernyshev et al 2021). The Costa Rican specimens are closely related to the two species Tetrastemma nigrifrons and T. stimpsoni that previously belonged to the now-rejected genus Quasitetrastemma (Chernyshev et al 2021).…”

“…Instead, they are part of a subclade comprising the majority of species of Tetrastemma (Chernyshev et al 2021). The Costa Rican specimens are closely related to the two species Tetrastemma nigrifrons and T. stimpsoni that previously belonged to the now-rejected genus Quasitetrastemma (Chernyshev et al 2021).…”

“…Like the aforementioned heteronemertean "hold-all" genera, the genus Tetrastemma has been regarded as paraphyletic and lacks distinct morphological synapomorphies (Chernyshev 2004;Strand & Sundberg 2005a, 2005b. A recent phylogenetic analysis revealed that the major group of Tetrastemma is indeed monophyletic, although several species previously assigned to this genus (e.g., Tetrastemma vittigerum) actually represent different genera (Chernyshev et al 2021). Therefore, based on some morphological differences between Vieitezia luzmurubeae and Chernyshevia escarpiaphila gen. et sp.…”

First eumonostiliferous nemertean from the Nishi-Shichito Ridge, Genrokunemertes obesa gen. et sp. nov. (Eumonostilifera, Nemertea)

“…Within a clade sister-related to Antarctonemertes with 69% of BS, phylogenetic relationships were not well resolved except for relationships between Galathenemertes giribeti ( Chernyshev & Polyakova, 2019 ), Gononemertes parasita ( Bergendal, 1900 ), Tetrastemma vittigerum ( Bürger, 1904 ), Vieitezia luzmurubeae ( Junoy, Andrade & Giribet, 2011 ), and Monostilifera sp. 9 Iturup ( Chernyshev et al, 2021 ) as well as a monophyletic clade of Kurilonemertes . Genrokunemertes obesa was sister-related with Monostilifera sp.…”

Diversity of interstitial nemerteans of the genus Ototyphlonemertes (Nemertea: Monostilifera: Ototyphlonemertidae) in the South China Sea, with a comment on the distribution pattern of the genus

“…Other examples can be seen in the highdiversity and relatively well-studied genera, Tetrastemma Ehrenberg, 1828 (Hoplonemertea) and Cephalothrix Örsted, 1843 (Palaeonemertea). In the former genus, Chernyshev et al (2021) recognized four geographically cohesive subclades (North Pacific, American Atlantic, North Atlantic, and Asian-Australian subclades). In the latter genus, the clade of Cephalothrix simula/hongkongiensis species complex (containing at least four species/networks) is confined to the Northeast Asia (occurrences in other areas are thought to be recent anthropogenic introduction), the clade of Networks 1, 10, 16, and 24 is only recorded from European waters (see Chen et al, 2010;Sagorny et al, 2019).…”