Ophiacantha swio, O’Hara & Thuy, 2025

Ophiacantha swio sp. nov.

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Fig. 16k–n View FIGURE 16 , 18 View FIGURE 18

TYPE LOCALITY. Coral seamount, 41° 21.7673´S, 42° 54.9067´E GoogleMaps to 41° 22.4´S, 42° 54.6´E, 740 m

TYPE MATERIAL. — JC066 : stn 4-37, Coral seamount, 41° 21.7673´S, 42° 54.9067´E to 41° 22.4´S, 42° 54.6´E, 740 m, 20/11/2011, holotype: 1 ( NHMUK 2025.34 About NHMUK ); paratype: 1 ( MV F321042 ) ( DNA code= JC066-1248 ); paratype: 1 ( NHMUK 2025.35 About NHMUK ) GoogleMaps .

OTHER STUDY MATERIAL. — JC066 : stn 8-5, Atlantis Bank, 32° 42.862´S, 57° 14.666´E to 32° 43.3´S, 57° 15.2´E, 828–994 m, 10/12/2011: 1 ( NHMUK 2025.36 About NHMUK ) GoogleMaps ( DNA code= JC066-3720 ) .

COMPARATIVE MATERIAL EXAMINED. Ophiacantha fuscina O’Hara & Stöhr, 2006 : in2022_v08/126, Cocos (Keeling), 11° 47.492´S, 96° 50.4342´E to 11° 47.504´S, 96° 51.5463´E, 820–822 m, 14/10/2022, identified by O’Hara (2024b), MV F308140 ( DNA code=F308140). KANADEEP2/CP5074, Mont J, 750–901 m, 25/9/2019, MNHN IE.2019.3239 ( DNA code=IE.2019.3239). Ophiacantha longidens Lyman, 1878 : EXBODI/DW3938, Récif Pétrie, 18° 36.2´S, 164° 24´E, 505–761 m, 27/9/2011, MNHN IE.2007.6817 ( DNA code=IE.2007.6817). SAYA/DW5429, SE Seamount, 11° 52.331´S, 62° 22.833´E, 554 m, 14/11/2022, MNHN IE.2023.4297 ( DNA code=IE.2023.4297). Ophiacantha swio Chargos /25, Great Chargos Bank, Eagle Island, 6° 10.45369´S, 71° 18.9175´E, 483 m, 22/10/2022 ( DNA code=Chargos.646). PAMELA-MOZ01/DW1, Glorieuses, 11° 22.756´S, 47° 16.4097´E to 11°22.749´S, 47°17.2302´E, 753–824m, 28/9/2014, MNHN IE.2013.8424( DNA code=IE.2013.8424). PAMELA-MOZ01/DW2, Betsiboka, 15° 21.786´S, 45° 57.4619´E to 15° 20.907´S, 45° 56.5654´E, 727–1179 m, 5/10/2014, MNHN IE.2023.4012.

COMPARATIVE MATERIAL NOT EXAMINED. Ophiacantha linea Shin & Rho, 1986 : Sogwip’o, Cheju , 34° 14´N, 126° 34´E, 50–60 m ( DNA code= KC990833 View Materials ) GoogleMaps .

Diagnosis. Disc covered in spinelets with a rounded pedicel, waisted stalk, and 3–7 apical thorns diverging at 2/3 spinelet height. Oral shield wide than long, smaller than adoral shields, 3–4 oral papillae. Five arms, bell-shaped DAPs, glassy VAPs, striated LAPs, up to 12 hollow arm spines, upper smooth to 3 segments in length, lower ones with some thorns, one flat rounded tentacle scale to half a segment in length.

Description. Holotype 9.5 mm dd, disc covered in separate to overlapping thin circular perforated scales that bear a multifid spinelet, with an expanded base, a long smooth pedicel and 3–7 divergent apical thorns that arise about 1/3 from the top of the spinelet; radial shields mostly hidden by plates and spinelets, only the distal tip exposed; disc spinelets continue onto lateral disc surface but sparse ventrally, reduced to a conical-shape without thorns near oral shield; oral shield lozenge-shaped, 2x as wide as long with obtuse proximal and distal angles and acute lateral ones; adoral shields smaller than oral shields, sausage-shaped with concave proximal and convex distal sides, not meeting interradially and separated radially by the 1st VAP, both oral and adoral shields covered in speckled epidermis; exposed section of the oral plate is small triangular, contiguous, bearing 3 sometimes 4 spiniform to leaf-shaped lateral oral papillae, 2–3x as long as wide, 5 leaf-shaped teeth, larger than the oral papillae with a rounded apex; 2nd oral tentacle pore opens within jaw slit.

Arms five, to 38 mm, not moniliform, DAPs kite to bell-shaped with a convex margin and convergent lateroproximal sides, separate, distally the DAPs are more triangular, wider than long, with a slightly convex distal edge; LAPs striated with a raised distal edge proximal to the arm spines; First VAP lozenge-shaped, 2x as wide as long, 2nd–3rd VAPs pentagonal with an obtuse proximal angle, convergent laterodistal sides and a truncate distal edge, wider than long, contiguous, succeeding VAPs separate, as wide as long, with an obtuse proximal angle, concave lateral sides and convex distal margin, glassy; up to 12 arm spines, meeting dorsally on basal segments, upper ones smooth hollow, slender, tapering to a sharp point, to 3 segments in length, middle and lower spines develop small sparse thorns, particularly near the spine base, lowest ones slightly bent, bluntly pointed, as long as a segment, distally 6 arm spines, the lowermost 2 are only 1/2 as long as the segment, becoming hook-like with ventrally directed thorns only at the arm tip; one flat rounded tentacle scale, basal ones oval to slightly bent, 2x as long as wide, 1/2 as long as the VAP, becoming leaf-shaped with a pointed thorny tip, 2/3 as long as the VAP. Colour (preserved) disc grey with light brown arms and oral frame, arm and disc spines white, glassy.

Paratype and other material variations. The largest paratype ( NHMUK 2025.35 About NHMUK , 7.5 mm dd) has 10 arm spines and little spherical nodules at the end of some of the thorns on the disc spines. A second paratype ( MV F321042 , 6 mm dd) and the ( 5–6 mm) Madagascar specimens are similar. On smaller specimens (Atlantis specimen, 4.5 mm dd, and Chargos 3.5 mm), the pedicel and thorns of the disc spinelets are very slender, often trifid, or with these primary thorns bifurcated, the oral papillae number 3 and there are only 8 arm spines basally .

Paratype ossicles (MV F321042). The DAPs, VAPs and LAPs ( Fig 18a–d View FIGURE 18 ) can have some distal transverse striations. The ear-like arm spine articulations have a larger ventral and smaller dorsal lobe joined by a central flange. The muscle and neural pores are round and easily distinguished from the surrounding stereom ( Fig. 18e View FIGURE 18 ). The dental plate ( Fig. 18g View FIGURE 18 ) is 2.5x as long as wide, with 5 obvious articulations for teeth, the largest ventrally; the teeth are minutely thorny ( Fig. 18h View FIGURE 18 ); vertebrae zygospondylous, basal vertebrae are as wide as tall ( Fig. 18i View FIGURE 18 ) with distally orientated ventral muscle attachment surfaces and the distal articulation surface with a triangular shaped epanapophysis sitting dorsal to the wing-like zygapophysis; middle arm vertebrae almost semi-circular with ventrally orientated ventral muscle attachment surfaces and a cruciform distal articulation surface ( Fig. 18l View FIGURE 18 ), and a proximal articulation surface that looks like ‘scales of justice’ with a long zygosphene and lateral zygocondyles ( Fig. 18k View FIGURE 18 ); tentacle scales are minute thorny at the tip and can be slightly bent on basal segments ( Fig. 18o View FIGURE 18 ); lower arm spines are also terminally thorny, middle arm spines can have sparse thorns along their margins ( Fig. 18p View FIGURE 18 ).

Distribution. Coral ( 740 m) and Atlantis ( 828–994 m) Seamounts, Chargos ( 483 m), Madagascar ( 727–1179 m).

Remarks. The new species is morphologically and genetically similar to Ophiacantha longidens , O. fuscina , and O. linea . They all have thin glassy arm plates, multifid disc spinelets, bell-shaped DAPs, and numerous arm spines. Ophiacantha longidens differs in having shorter simpler disc spinelets, typically with 3 short terminal thorns. Ophiacantha fuscina has 2–4 long slender upwardly curved thorns on the disc spinelets, often webbed at their base. Ophiacantha linea is the most similar to O. swio but differs in having shorter arms, only 3x dd, and strong striations on the LAPs and basal VAPs. It also occurs in much shallower water ( 50–60 m) off Korea.

Etymology. Named after the acronym of the South-West Indian Ocean (SWIO).