Descriptions of two new deep-sea nemertean species from the genera Alvinonemertes and Sagornya gen. nov. (Hoplonemertea, Oerstediina) with discussion on close genetic relationship between deep-sea species

Abstract

In a recent study by Sagorny et al. (2022), nine new species of deep-sea hoplonemerteans, including five from the new genus Alvinonemertes Sagorny, von Döhren, Rouse & Tilic, 2022, have been described from off the Pacific coast of Costa Rica and the Oregon margin. The new species Alvinonemertes tilici sp. nov., found during the research cruise #94 aboard the R/V Akademik M.A. Lavrentyev in the Emperor Seamounts, has proven to be very close morphologically and genetically to A. dariae Sagorny et al., 2022 from the methane Parrita Seep, off Costa Rica. As a phylogenetic analysis based on five gene markers (cytochrome c oxidase subunit I, 16S rRNA, 18S rRNA, 28S rRNA, and histone H3) has shown, the genus Alvinonemertes is not monophyletic. Four species of Alvinonemertes are here segregated into a new genus, Sagornya gen. nov. The new species Sagornya iturupica sp. nov. is described based a specimen collected from the bathyal zone in the Sea of Okhotsk. This species is close genetically to S. tatjanae (Sagorny et al., 2022) comb. n. from seeps along the Oregon margin. The family Oerstediidae is divided into two sister groups: the subfamilies Oerstediinae and Tetranemertinae subfam. nov. To date, the new clade Gononemertes is the largest clade in the family Oerstediinae in terms of number of deep-sea and symbiotic species. Examples of close genetic similarity between deep-sea species from the genera Nemertovema, Tetrastemma, Alvinonemertes, Sagornya, and Carinina are considered. The study has shown that the genetic differences between deep-sea hoplonemerteans from the western and eastern Pacific Ocean are lower than those between sibling hoplonemertean species from Pacific shallow waters off North America and Asia. A hypothesis is advanced here that species with limited geographical ranges are frequently found among deep-sea nemerteans. The wide distribution of deep-sea hoplonemerteans can be provided by long-swimming lecithotrophic larvae, as in Cratenemertidae sp. MCZ IZ 45644.