Cerianthus lloydii (Ceriantharia: Anthozoa: Cnidaria): New Status and New Perspectives †

SIMPLE SUMMARY: Subclass Ceriantharia encompasses marine anemone-like organisms with complete bilateral symmetry. In modern phylogenetic reconstructions, Ceriantharia is considered an ancient group that is sister to all other anthozoans. Only a handful of tube anemones have been reported from the Arctic, including widely distributed North Sea cerianthid Cerianthus lloydii Gosse, 1859, which has also been documented in temperate Atlantic and Pacific waters. The integrity of C. lloydii as a species has been questioned in the literature. To test this hypothesis, we performed a molecular study of C. lloydii from several geographically distant localities using 18S and COI genes. Our data combined with data from public databases show that C. lloydii forms a single genus-level group divided into three distinctive subclades: (1) Northern Europe, the Black and the Barents seas; (2) the Russian and Canadian Arctic and the Labrador Sea; and (3) published sequences of a species determined as Pachycerianthus borealis (Verrill, 1873) from Newfoundland. As all subclades of C. lloydii are only distantly related to the genus Cerianthus Delle Chiaje, 1841, we propose to resurrect the genus Synarachnactis Carlgren, 1924, and describe a new family to accommodate them. Our findings contribute to the understanding of the marine fauna evolution in the Arctic Basin and the Black Sea. ABSTRACT: Subclass Ceriantharia is a well-defined and probably ancient group of marine benthic organisms renowned for their bilateral symmetry, which is reflected in the arrangement of tentacles and mesenteries. Four species of Ceriantharia have been reported in the Arctic, including Cerianthus lloydii Gosse, 1859, also known from the Northern Atlantic and Northern Pacific. The integrity of this species was questioned in the literature, so we performed a molecular study of C. lloydii from several geographically distant locations using 18S and COI genes. The phylogenetic reconstructions show that specimens of C. lloydii form a single group with high support (>0.98), subdivided into distinctive clades: (1) specimens from Northern Europe, the Black and Barents seas, and (2) specimens from the White, Kara, Laptev, and Bering seas and also the Canadian Arctic and the Labrador Sea available via the BOLD database. There are several BOLD COI sequences of Pachycerianthus borealis (Verrill, 1873), which form a third clade of the C. lloydii group, sister to the European and Arctic clades. Based on low similarity (COI 86–87%) between C. lloydii and the type species of the genus Cerianthus Delle Chiaje, 1841—C. membranaceus (Gmelin, 1791), we propose a new status for the genus Synarachnactis Carlgren, 1924, and a new family Synarachnactidae to accommodate C. lloydii.