Stegopoma plicatile: Cornelius, 1995a: 114–117, fig. 25; Schuchert, 2000: 421; Schuchert, 2001: 51–53, fig. 31A–E; Calder, 2012: 20–21, fig.20.
Material examined. MSM 16 /3, stn GeoB 14908–1, ROV, 17°40.213’– 17°40.191’N, 16°40.829’– 16°40.289’W, 463–574 m, 16-XI-2010: one colony 57 mm high, attached to Lophelia pertusa, no gonothecae.
Biology. Colonies have been found growing on hard substrata such as rocks and stones, other hydroids and on old rope (Cornelius 1995a); they have also been found on bivalves, crustaceans, cirripeds, ascidians and artificial substrata (Gil & Ramil 2017a). Fertile material has been found in January, May, July, September, November and December (Christiansen 1972; Gili et al. 1989; Schuchert 2000, 2001; Calder 2012; Gil & Ramil 2017a).
The colony studied by us was growing on L. pertusa.
Distribution.Stegopoma plicatile is mainly distributed in the Arctic and boreal regions of the Atlantic and Pacific Oceans, but it was also reported in the East and West Pacific, Tasman Sea, Philippines, Strait of Magellan and Antarctic (Vervoort 1972; Schuchert 2001; Calder 2012). In the East Atlantic, it was reported from Svalbard (Ronowicz 2007) to off Brest (Cornelius 1995a), Mauritania (Gil & Ramil 2017a) and Namibia (Gili et al. 1989) in West Africa; however, there are no records from Brest to Mauritania. Its bathymetric distribution ranges from 15 to 1940 m but usually from 75 to 500 m (Schuchert 2001).
Our material was collected from depths of 463 to 574 m.
Remarks. This species is characterised by its large, erect and branched polysiphonic colonies and pedicellate and non-pedicellate hydrothecae, the latter partially adnate to the stem or hydrocladia and curving outwards in the distal part.
Schuchert et al. (2017) used DNA barcoding and found that colonies of S. plicatile collected near Bergen (Norway) were identical to the medusa Ptychogena crocea Kramp & Dumas, 1925 collected from the same area. The restricted distribution of P. crocea, a deep-sea medusa endemic to the Norwegian coast, versus the worldwide distribution of S. plicatile, strongly suggest that the current concept of S. plicatile actually represents a species complex and not a single species, and, in consequence, its identity was considered ambiguous by Schuchert et al. (2017). Therefore, in this paper, we use the name Stegopoma plicatile (Sars, 1863) for the colonies collected from Mauritanian carbonate mounds, the morphology of which falls within the traditional concept of S. plicatile.