Ophiactis plana Lyman, 1869

Fig. 26k–m

Ophiactis plana Lyman, 1869: 330–331 . — Clark, H.L., 1915: 265, pl. 10(1–2). — Clark, H.L., 1918: 301–302. — Clark, H.L., 1923: 333. — Mortensen, 1933b: 345–346, fig. 57. — Clark, H.L., 1939: 76–77. — Clark, A.M., 1974: 464–465. — Clark, A.M. & Courtman-Stock, 1976: 163–164, figs 157, 162. — Alva & Vadon, 1989: 839. — Glück et al. 2012: 10, fig. 4a–b. — Olbers et al. 2019: 254–255, fig. 258–259. — O’Hara, 2024b: 77, fig. 75.

Ophiactis flexuosa . — Lyman, 1882: 116–117, pl. 20(1–3) (in part). — Studer, 1882: 17 [Non Ophiactis flexuosa Lyman T, 1879; see Clark, H.L., 1923].

Ophiactis profundi Lütken & Mortensen, 1899: 140–142, pl. 6(4–6). — Koehler, 1922a: 192–193, pl. 63(8). — Nethupul et al. 2022a: 68, fig. 38–39.

Ophiactis pteropoma Clark, H.L., 1911: 134–135, fig. 50. — Matsumoto, 1917: 154, pl. 3(9) [according to Koehler, 1922a].

Ophiactis profundi var. novaezelandiae Mortensen, 1924: 128–131, fig. 13.

STUDY MATERIAL. — JC066: stn 4-12, Coral seamount, 41° 22.333´S, 42° 54.066´E to 41° 23´S, 42° 54.1´E, 730 m, 16/11/2011: 1 (NHMUK 2025.58) (DNA code= OPR14); 1 (NHMUK 2025.57) .

COMPARATIVE MATERIAL EXAMINED. Ophiactis plana Lyman, 1869: EXBODI/CP3852, Banc Sud Durand, 22° 16.6´S, 168° 43.2´E, 582 m, 14/9/2011 , MNHN IE.2007.6735 (DNA code= IE.2007.6735). IN2022 _ V09 /126, Site:039, 23° 17.75´S, 113° 5.1391´E to 23° 17.992´S, 113° 4.9411´E, 425– 375 m, 10/12/2022, WAM (DNA code= IN2022 _ V09 _126). MIRIKY/CP3178, entre Nosy-bé et Banc du Leven, 12° 58.88´S, 48° 9.09´E to 12° 59.01´S, 48° 9.0402´E, 378–380 m, 25/6/2009 , MNHN IE.2023.4013 (DNA code=CP3178a). SAYA / DW5407, W Saya de Malha, 10° 59.357´S, 60° 18.914´E to 10° 59.649´S, 60° 19.103´E, 193–198 m, 6/11/2022, MNHN IE.2023.4239 (DNA code= IE.2023.4239). SS02/2007/77, Cascade 1200m 5, 43° 55.406´S, 150° 27.889´E to 43° 55.778´S, 150° 28.352´E, 590–660 m, 10/4/2007, MV F144837 (DNA code=F144837). TARASOC/DW3481, Moorea Island, 17° 29´S, 149° 45´W, 610 m, 22/10/2009 , UF 13016 (DNA code= UF13016). TARASOC/DW3502, Tahiti Island, 17° 35´S, 149° 17´W, 430–580 m, 25/10/2009 , UF 13019 (DNA code= UF13019) .

Distribution. NE Atlantic (650–914 m), NW Pacific (88–1702 m), W Atlantic (48–800 m), E Atlantic (41– 41 m), W Indian Ocean (22–1441 m), E Indo-W Pacific (55–1618 m), E Pacific (850–1644 m), S America (748– 748 m), S Africa (0–730 m), S Australia (25–1443 m), New Zealand (55–1583 m).

Remarks. The taxonomy of many Ophiactis species is very confused. The incidence of specimens with 6 arms is high and many of these can divide by fission (A.M. Clark 1967). However, multi-armed and fissiparous forms do not always form separate clades on a phylogeny (see tree in Christodoulou et al. 2019) and can be similar genetically to 5 armed forms. Morphology is often variable, especially in fissiparous forms. Location is not necessarily a good guide to identity as some species are evidently very widespread, occurring throughout the Indo-Pacific and sometimes into the Atlantic as well. In some regions, notably the Western Atlantic, there are numerous small ill-defined fissiparous species that need to be delimited.

A multi-armed bathyal species called O. plana (type locality Florida), O. profundi (type locality eastern Pacific) or O. pteropoma (type locality: Japan) is evidently one of the widespread species. It has a single oral papilla on the side of each jaw, fan-shaped dorsal arm plates with a straight to slightly convex distal border, large radial shields (typically 1/5 dd), 3–4 arm spines, and rarely has disc spines. It typically has 6 arms (rarely 5 or 7) and small specimens are regularly found with 3 large and 3 small arms indicating that they are regenerating after having previously divided by fission. However, specimens identified as this species prove to be polyphyletic on a phylogeny. The majority, including the 2 specimens from Coral Seamount (2.5–3 mm dd), do form a monophyletic clade that occurs from the SW Indian Ocean to Moorea, South China Sea to New Zealand. But other 6-armed specimens cluster with either 5-armed species such as O. definita and O. perplexa or shallow water fissiparous species such as O. macrolepidota and are probably mis-identified atypical members of those species.

Unfortunately, we do not have genetic sequences from the Atlantic or eastern Pacific that may guide the nomenclature of this species. In the interim, we follow Mortensen (1933b) in regarding O. plana and O. profundi as conspecific.