Ophiocomina arnaudi O'Hara & Thuy 2022, sp. nov.

Ophiocomina arnaudi sp. nov.

Fig. 11A–F, 12A–S

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Type material. MD50 DC108, south-east of Île Saint-Paul, 38° 48.83´S, 77° 35.68´E, 460–510m, Charcot dredge, 18/7/1986, MNHN IE.202019.4747 (holotype), MNHN IE.2009.1599 (52 paratypes).

Other material examined. MD 50 DC71, MNHN IE.2009.1594 (2). MD 50 DC79, MNHN IE.2009.1595 (2). MD 50 CP80, MNHN IE.2009.1596 (6). MD 50 DC82, MNHN IE.2009.1597 (3). MD 50 DC99, MNHN IE.2009.1598 (2). MD 50 DC114, MNHN IE.2009.1600 (1). MD 50 CP115, MNHN IE.2009.1601 (1). MD 50 CP116, MNHN IE.2009.1602 (9). MD 50 DC127, MNHN IE.2009.1603 (1). MD 50 DC146, MNHN IE.2009.1604 (1).

Comparative material. Ophiocomina nigra (Abildgaard, 1789): Roscoff, Brittany, 48° 43´N, 3° 59´W, 1997, MV F92216 (5). Strangford Lough, County Down, The Limestone, 54° 30´N, 5° 40´W, 20 m, 5/8/1969, MV F91624 (5). Clarkcoma sp. ICEFISH/85-BT45, Tristan da Cunha, 37° 20.18´S, 12° 30´W, 210–223 m, 7/7/2004, MV F167538 (DNA code=TOH_0050 & OTC1a).

Holotype description. Disc is 8.5 mm dd, pentagonal, covered in dense granules 0.07 mm dia, as high as wide, cylindrical with a granular surface, ~132 per mm 2 in the centre of the disc (Fig. 11E), slightly sparser near the radial margin (Fig. 11D), obscuring the thin underlying scales and radial shields (Fig. 11B). Ventral disc surfaces (Fig. 11C) are covered in small glassy imbricating scales, with a wedge of granules extending from the margin to about 1/3 to 1/2 of the distance to the oral shield. There are no genital granules or spines.

Oral shields are spear-head shaped, 1.2 times as wide as long, with a hemispherical proximal edge and slightly lobed distal margin, slightly depressed near distal margin. Madreporite is as wide as long, with a convex rather than lobed distal margin. Adoral shields are long and narrow, fully contiguous proximally, sometimes extending around the oral shield to abut the first LAP, other adoral shields appear to finish at the lateral angle of the oral shield, perhaps with the distant extension lying underneath the oral shield and allowing the first LAP to be contiguous with the oral shield. Jaws are long and narrow, oral plates slightly protuberant near dental plate, with 6 rounded teeth (Fig. 11F) and 4–5 conical tooth papillae in 1–2 rows, teeth not projecting further than the tooth papillae, lacking hylanated cap; 5–6 oral papillae around the jaw side, including one tall conical, proximately-directed infradental papilla, 1–2 slender lateral oral papillae, 1–2 flattened oval adoral shield spines, and a small extension of the ventral compartment ossicle that is situated near to the first VAP.

DAPs are approximately 1.05 mm wide and 0.63 mm long at arm base, fan-shaped, with a straight proximal edge, tapered proximo-lateral sides and a convex distal margin, the distal margin is sometimes truncate in the middle where it meets the succeeding plates, contiguous for 1/5 of their width, becoming more kite-shaped distally, with longer proximal taper but still just contiguous, arms slightly noded and DAPs just separate at arm tip. LAPs with a few thorns on the proximal side of the pronounced arm spine ridge. VAPs on the first segments are heart-shaped, a little wider than long, with a rounded distal margin and a notched proximal edge marking the end of the jaw slit; 2nd VAPs are axe-head-shaped, 1.6 times as wide as long, with a straight to slightly concave ventral margin, concave proximolateral sides around the tentacle pore, and a wide convex uplifted distal edge; succeeding plates become larger and more quadrilateral, with a slightly convex proximal margin, and a less pronounced distal margin, plates touch but don’t overlap.

Arm spines are long (Fig. 11A), flattened, oval in cross section, inner lumen less than half diameter, stereom very finely granulated, with a blunt or truncate tip with tiny thorns, 3 spines on first segment, 4 on next three segments under the disc, and 5–6 arm spines once free of the disc, with the upper one often being very long (2.8 mm) on one side and short and lanceolate on the other (but not alternating in number), other spines in descending order are ~2.5, 1.8, 1.5 and 1.3 mm, ventralmost spines can be a little curved; in the distal half of the arm the number of arm spines falls to 4–5; with 3 needle-like spines at the arm trip. Two oval tentacle scales occur on the LAPs, half the length of the VAP, subequal at first, but the inner one becoming smaller by about the 6th segment.

Paratype variations. Paratype disc size varies from 4.8–10.7 mm dd. Largest with up to 8 arm spines. 1–2 small spines can be present on the side of the oral plate within the jaw slit. There are up to 6 flat blade-like teeth (Fig. 11F), the most ventral are broadly triangular with a rounded proximal tip, the dorsal teeth and more quadrangular with rounded lateral corners, and up to 10 conical tooth papillae, a little larger near the teeth. Underlying disc scales are rounded, thin, glassy, 0.7–1.0 mm dia, often fragmented. Radial shields are small, with a beaded surface, approximately 4 times the size of the disc scales, separated by 3–4 scales, and covered in granules. Disc granules can occur on the first DAP. Peristomial plates are obscured by dried internal tissue. Colour is generally grey-yellow, with a hint of darker patches, arm tips pinkish, banded with 2–3 segments darker and 3–4 segments lighter.

Paratype ossicle description. Proximal LAPs (Fig. 12A–B) are two times higher than long, strongly arched, relatively thin, with strongly protruding ventral portion; dorsal edge slightly convex; outer surface with finely meshed stereom and with a fine, regular vertical striation close to the ventral and dorsal edges and close to the raised distal portion; outer proximal edge with a band of more finely meshed stereom devoid of spurs. Distal plates becoming longer than high (Fig. 12C). Spine articulations (Fig. 12D) are freestanding in a continuous vertical row on a strongly raised distal portion of the LAP, ear-shaped, very large, composed of proximally merged dorsal and ventral lobes enclosing a large muscle opening and much smaller nerve opening and with a well-developed sigmoidal fold; dorsal and ventral lobes expanded dorsally and ventrally, respectively, in the three dorsalmost spine articulations. Inner side of LAPs have a large, prominent, well-defined, slender vertebral articular ridge reminiscent of a reversed sickle; no spurs on the inner distal edge; tentacle notch is relatively large, oblique, shallow. Small perforations occur in a vertical row, with a slender, shallow furrow dorsally.

Vertebrae (Fig. 12F–J) are short, with very wide, rounded triangular dorsal fossae and with relatively small ventral muscle fossae on both distal and proximal faces; articulations small, typically zygospondylous, with a relatively large zygosphene on the distal face; very slender L-shaped lateral articulation structure corresponding to articular structure on the inner side of the LAP; dorso-distal muscle fossae are deeply carved into the lateral side of the vertebra; large ventral furrow, with moderately small, well-defined podial basins.

Oral plates (Fig. 12M) are slightly wider than long, with adradial muscle fossa lining the lateral edge of articulation area. Dental plates (Fig. 12P) are undivided, rounded, with single row of large, shallow tooth sockets surrounded by separate, weakly protruding ridges; ventral tip of dental plate widest, with a cluster of smaller, shallow tooth sockets each surrounded by a continuous ring. Teeth (Fig. 12N, O) are dome-shaped to rounded rectangular, devoid of enamelled tip. Abradial genital plates are shorter than the adradial ones, slender, bar-like without ridges or groves and with a concave adradio-distal tip. Radial shields (Fig. 12S) have an oblique isosceles-triangular outline, with a more thickly-meshed stereom in the middle part of the outer surface. A proximal dorsal arm spine (Fig. 12E), dorsal arm plate (Fig. 12K), and ventral arm plate (Fig. 12L) are also figured.

Distribution. SPA (135–525 m).

Remarks. This species is referred to the genus Ophiocomina on the basis of the granulated disc, presence of tooth papillae on the ventral dental plate, a series of oral papillae, adoral shields that meet interradially, long hollow arm spines, and two oval glassy tentacle scales. The only other Ophiocomina species, O. nigra, differs in having smaller more separated disc granules that often persist to the oral shields, more fragmented disc scales, and wider more oval DAPs with convex laterodistal and lateroproximal margins. The genus Ophiocomina is now placed in the family Ophiotomidae on the basis of DNA evidence (O’Hara et al. 2017). The species Ophiocomina australis H.L. Clark, 1928 is now considered a Clarkcoma species (Naughton et al. 2014; O’Hara et al. 2017).

Ophiocomina differs from Clarkcoma species (except C. australis) in lacking the thick dense stereom cap on the teeth that makes them protrude further than the tooth papillae. This cap is present even on 8 mm juveniles of C. canaliculata (Lütken 1869), C. pulchra (Clark 1928), C. bollonsi (Farquhar 1908) and several undescribed Australian species in the genus (Naughton et al. 2014). These species have numerous tooth papillae (up to 30 in C. bollonsi) and swollen fleshy tips (sacculae) that occur on many arm spines. The diminutive C. australis differs from all other Ophiocomina and Clarkcoma species in having only one tentacle scale as an adult.

Naughton et al. (2014) and O’Hara et al. (2017) also sequenced specimens of an undescribed species in the Clarkcomidae from Tristan da Cunha in the South Atlantic collected by the ICEFISH expedition. This is possibly the same species as specimens from Gough Island previously reported (Koehler 1908; Mortensen 1936) as Ophiotreta valenciennesi (Lyman, 1879). The ICEFISH specimens, up to 18 mm dd, differ from O. arnaudi sp. nov. in having dark markings on the circular disc, small spines on the basal DAPs, and shorter, more curved arm spines. Ophiotreta valenciennesi differs in having hemispherical disc granules and only 1–2 tooth papillae.

Etymology. Named after Patrick M. Arnaud who led the 1986 MD50 expedition to the Île Amsterdam and Saint-Paul.