Deep-sea benthic hydroids (Cnidaria, Hydrozoa) from Antarctic submarine ridges off the Ross Sea (Antarctica)

Abstract Knowledge of benthic hydroids inhabiting the Antarctic continental shelf waters, particularly of relatively well-studied areas, has increased in recent years. This has allowed us to recognise them as one of the main and most characteristic zoological groups of the Antarctic benthos. However, little is known about the hydroids dwelling on the continental slope or in deeper waters, let alone about those inhabiting the seafloor far away from the Antarctic continent, despite the Southern Ocean extending significantly northwards. This study contributes to reducing that knowledge gap by studying material collected from a series of deep-sea ridges north of the Ross Sea, from which hydrozoans have never been reported. Twelve species, including Halecium divergens sp. nov., have been found and studied. Except for Turritopsis sp., belonging to the Anthoathecata family Oceaniidae, all species belong to Leptothecata, in particular to the families Campanulariidae, Haleciidae, Lafoeidae, Phylactothecidae, Sertularellidae, and Symplectoscyphidae. Lafoeidae is the most represented family with four species. Sertularella pseudovervoorti and Filellum liberum are found for the second time. The discovery of Symplectoscyphus frondosus , a species previously considered endemic to the shelf and slope of the eastern Ross Sea, significantly extends its known northern distribution limit. Tulpa diverticulata and the genus Tulpa are reported in Antarctic waters for the first time. The lower limit of the bathymetric range for several species has been extended. Despite being well within Antarctic waters, the studied area hosts a very distinctive fauna, markedly different from the typical Antarctic benthic hydroid fauna. Its endemisms, the presence of species unknown in the Antarctic region and the absence of representatives of the most characteristic Antarctic genera account for its originality.