Ophiactis nidarosiensis Mortensen, 1920: 60–63, fig. 5.— Mortensen 1933a: 346–347, fig. 58a.— Mortensen 1936: 264.— Alva & Vadon 1989: 839, fig. 5c–d.— Olbers et al. 2019: 250–252, fig. 254–255.
Material examined. MD 50 DC108, MNHN IE.2009.1590 (2). MD 50 DC114, MNHN IE.2009.1591 (3). MD 50 CP149, MNHN IE.2009.1592 (3). MD 50 DC157, MNHN IE.2009.1593 (4).
Comparative Material. Ophiactis hirta Lyman, 1879: Eltanin 15/1343, Southeast Pacific Basin, North Of Amundsen Sea, 54° 50´S, 129° 50´W, 567–604 m, 7/11/1964, USNM E52227 (2). EXBODI/CP3893, Mont Vauban Partie Sud-Est, 22° 24.1´S, 171° 46.9´E, 786–814 m, 19/9/2011, MNHN IE. 2007.6981 (2). SLOPE/32, South of Point Hicks, 38° 21.9´S, 149° 20´E, 1000 m, 23/7/1986, MV F90159 (17). TAN1402/57, Censeam Guyot, Stratum 4, 36° 54.47´S, 169° 50.78´W, 1013– 1010 m, 15/2/2014, NIWA 94253 (12).
Distribution. Arctic (150–560 m), NE Atlantic (810–1095 m), E Atlantic (175–423 m), S Africa (102–200 m). SPA (135–510 m).
Remarks. The MD 50 specimens are small (<1.5 mm dd) and have six arms, two distal oral papillae, fanshaped DAPs that are longer than wide with a convex distal edge and tapered proximal sides, 3 arm spines and an occasional disc spine. The smallest specimens have only 3 arms or show other signs of fissiparity.
Clark (1976) tentatively referred some small six-armed SPA specimens from 40–80 m to Ophiactis savignyi Müller & Troschel, 1842 on the basis that they had two distal oral papillae, truncate lower arm spines and fanshaped DAPs with a flattened distal edge. However, although the lower arm spines on the MD 50 specimens are widened compared to the upper one, they differ from O. savignyi in being terminated in a pointed thorny apex, and the DAPS are more droplet-shaped and show no sign of a truncate distal edge.
Instead, they are similar to equal-sized specimens of Ophiactis nidarosiensis from the Atlantic and Ophiactis hirta Lyman, 1879 from SE Australia/ New Zealand (e.g., MV F90159) which are also fissiparous. The similarity between these two species has been noted by Mortensen (1924). Both have six or more arms, two distal oral papillae, and DAPs that are longer or as long as wide with rounded distal margins. Mortensen (1924) stated that O. nidarosiensis differed in being fissiparous, oral shields with a distal lobe and relatively narrow VAPs. But all these characters can occur in SE Australian specimens that we have identified. Similar specimens that we have provisionally identified as O. hirta occur off New Caledonia, Louisville Ridge and the Heezen Fracture Zone of the south-central Pacific. Without genetic data it is unclear whether there is one or more cryptic species present across this wide distribution. In the interim, the SPA animals are treated as belonging to the species reported from South Africa.