Ophioplinthaca brachispina, Nethupul & Stöhr & Zhang, 2022

Nethupul, Hasitha, Stoehr, Sabine & Zhang, Haibin, 2022, Review of Ophioplinthaca Verrill, 1899 (Echinodermata, Ophiuroidea, Ophiacanthidae), description of new species in Ophioplinthaca and Ophiophthalmus, and new records from the Northwest Pacific and the South China Sea, ZooKeys 1099, pp. 155-202 : 155

Ophioplinthaca brachispina sp. nov.

Figs 3 , 4 , 5

Material examined.

Holotype. Northwest Pacific • 1 specimen; near Mariana Trench, Southwest of Guam Island, seamount; 11°49.09'N, 140°6.93'E; depth 2713 m; 23 October 2019; Collecting event: stn. SC039; Shenhaiyongshi msv leg; preserved in -80 °C; GenBank: OK043829 ; IDSSE-EEB-SW0106 . Paratype. Northwest Pacific • 1 specimen; same data as for holotype; GenBank: OK043830 ; IDSSE-EEB-SW0107 .

Diagnosis.

Disc sub-circular and deeply incised interradially to nearly 1/4 disc radius (Fig. 3A ). Disc scales irregular, variable in size, bearing disc spines in center of disc (Fig. 3C ). Radial shields completely separated by large single disc scale (Fig. 3G ). Oral shield as wide as long, pentagonal with pointed proximal end, curved lateral margins along adoral shields, truncated distal edge with straight to slightly angular lateral margins (Fig. 3H ). Surface of arm plates along entire arm rough with small spines (Fig. 3K-M ).

Holotype description.

Disc diameter 12 mm, arm base width 3 mm (Fig. 3 ).

Disc. Disc sub-circular and deeply incised interradially to more than 1/3 disc diameter, creating five wedge-shaped lobes over each arm base in contrast to sunken center and interradii of disc (Fig. 3A, B ). Disc scales irregular, variable in size, compact, and overlapping in center of disc (Fig. 3C ). Most central disc scales bear disc spines/stumps (Fig. D-F). Disc scales increasingly enlarged from disc center to periphery, interradially and between radial shields (Fig. 3D-F ). Disc spines in disc center 0.2 to 0.3 mm high, cylindrical to conical, pointed thorny or bifurcated tip. Disc spines at distal end of wedge-shaped lobes 0.1-0.2 mm high, conical, thorny, with pointed tip (Fig. 3D-F ). Radial shields large, naked, roughly triangular, ~ 1/3 disc diameter in length, twice as long as wide, triangular proximal end, and smooth, truncated or slightly convex distal end. Radial shields on three of five lobes proximally separated, but distal ends connected. Radial shields on other two lobes completely separated by large single disc scale (Fig. 3A, G ). Ventral disc covered by smaller scales than those on radial shields, and overlapped without bearing spines (Fig. 3B ). Genital slits conspicuous and extending from oral shield to periphery of disc (Fig. 3H ). Oral shield as wide as long, pentagonal with pointed proximal end, curved lateral margins along adoral shields, truncated distal edge with straight to slightly angular lateral margins (Fig. 3H ). Madreporite similar to other oral shields, but with hydropore at lateral edge (Fig. 3B ). Adoral shield 2.5 × as long as wide, with straight or slightly curved lateral margin, but near first ventral arm plate straight, and pair of shields proximally connected (Fig. 3H ). Adoral shields enclose proximal edges of oral shield, and slightly separate oral shield from arm by connecting to lateral arm plate of first arm segment (Fig. 3H, I ). Jaw triangular, large, and longer than wide, bearing one slightly blunt, wide, and large ventralmost tooth and three or four spiniform lateral oral papillae (Fig. 3H ). Proximalmost one or two lateral oral papillae spine-like pointed, rugose, and distalmost lateral oral papillae with shorter and rounded base with more or less pointed tip (Fig. 3H ). One adoral shield spine, situated at lateral margin of adoral shield in mouth angle, slightly similar to distalmost lateral oral papilla, but with blunt tip (Fig. 3I ). Cluster of small granules visible between distal end of jaw and proximal end of first ventral arm plate (Fig. 3I ). Usually, cluster of granules covered by adoral shield spine (Fig. 3H ).

Arms. Five moniliform arms with rough plates. Dorsal arm plates longer than wide, slightly separated, straight to slightly convex distal end, triangular proximal end, with curved lateral margins on first few proximal arm segments, but as long as wide, fan-shaped, and widely separated on middle to distal half of arm (Fig. 3J, K ). Dorsal arm plate with dense rough surface and short spines (Fig. 3K, M ). First ventral arm plate rectangular to slightly trapezoid, as wide as long, straight proximal end, and distal end without rough surface (Fig. 3I ). Second ventral arm plate trapezoid to slightly pentagonal, as wide as long, triangular proximal end, straight distal end, concave and diverging lateral edges, and contiguous with first ventral arm plate (Fig. 3I ). The following ventral arm plates two or three times as wide as long, with obtuse proximal end, slightly wavy proximolateral margins, curved lateral angles, straight distal end, and widely separated (Fig. 3L ). All ventral arm plates except first one with dense rough surface (Fig. 3I, L ). Lateral arm plates meeting above and below, with dense rough surface and short spines (Fig. 3K-M ). Up to five arm spines (Fig. 3M ). Three or two dorsal arm spines, three arm segments in length, thorny, lateral margins with row of tall sharp thorns, apex truncated or bluntly rounded (Fig. 3K-M ). Two ventral arm spines, one to two arm segments in length, pointed, thorny, rugose. Proximal arm segments bear five arm spines, distalwards decreasing to four beyond middle section of the arm (Fig. 3K-M ). First tentacle pore covered by two oval, rough tentacle scales (Fig. 3I ). The following tentacle pore covered by scale half as long as ventral arm plate, blunt to pointed tip with thorny surface (Fig. 3L ). Tentacle scales on middle to distal half of arm decreasing in size, small, more pointed, leaf-like, with thorns.

Color. In live specimen, orange-brown disc, and arm spines, but arms pale brown (Fig. 3 ).

Ossicle morphology of paratype.

Arm spine articulations well developed and placed at slight angle to distal edge of lateral arm plate (Fig. 4A ). Volute-shaped perforated lobe forms dorsal and distal part of articulation, but reduced in dorsalmost one (Fig. 4A ). Arm spine articulating structures with large muscle opening and small nerve opening in second articulation, decreasing significantly in size ventralwards (Fig. 3A ). Ventral half of lateral arm plate surface covered by conspicuous thorns, inner side with depression, a continuous ridge, and a prominent knob close to ventral edge forming vertebral articulations, shaped like a broad, nose-shaped beak (Fig. 4A, B ). Dorsal arm spine laterally compressed, thorny, and several longitudinal rows of perforations with widely spaced tall thorns (Fig. 4C ). Entire ventral arm spine surface covered with slightly longer thorns, with blunt apex (Fig. 4D ). Disc spines 0.2-0.3 mm high, cylindrical, pointed thorny or bifurcated tip (Fig. 4E ). Dorsal arm plate triangular, as long as wide, with rugose surface (Fig. 4F ). Vertebrae with streptospondylous articulating structures, short, broad podial basin at proximal end and narrow small distal end (Fig. 4G-K ). Dorsal end of vertebrae distally triangular and proximally flattened with longitudinal groove along midline (Fig. 4J ). Ventral side of vertebrae with broad ambulacral groove (Fig. 4I-K ).

Paratype variations.

One specimen from same location as holotype, but badly damaged due to rough handling. Therefore, only small disc part with arms present. Possibly smaller than holotype according to size of arms (arm base width 1.5-2 mm). Arm characters similar to holotype, but spines slightly thinner, and denser compared to holotype (Fig. 5A-D ).

Distribution.

2713 m depth, Northwest Pacific, near Mariana trench, Southwest of Guam Island.

Etymology.

Species name derived from a combination of two Latin words, brachium (arm), spina (spine) referring to the unique rough arm surface with spines.

Remarks.

Deep interradial incisions into the disc, which are lined distally by enlarged disc scales are the main delimiting character of the genus Ophioplinthaca from other genera within the family Ophiacanthidae . Ophioplinthaca brachispina sp. nov. showed similar morphological characters to many other Ophioplinthaca species. However, O. brachispina sp. nov. can easily be distinguished from congeners by the rough thorny surface on the arm plates and additionally by the number of arm spines, disc spines, and tentacle scale (Table 3 ). Ophioplinthaca brachispina sp. nov. is the only Ophioplinthaca species with a rough surface with thorns on the whole arm.

Some species share morphological characters with the new species. Ophioplinthaca globata Koehler, 1922 is similar to O. brachispina sp. nov. by having similar disc spine shape, arm spine shape, radial shields separated proximally and connected distally, number of lateral oral papillae, and separated ventral and dorsal arm plates, but differs by number of arm spines (up to six), the disc spines being scattered across the disc, radial shields separated by disc scales, characters of the oral shield, and a smooth surface on the arm plates along the entire arm. Ophioplinthaca hastata Koehler, 1922 is similar to O. brachispina sp. nov. by having a slightly similar shape of the disc spines, separated dorsal and ventral arm plates, and similar tentacle scales on the distal end of the arm, but differs by number of arm spines (up to seven) and shape of dorsal arm spines, size of radial shields, characters of oral parts, and smooth arm surface. Ophioplinthaca athena A.H. Clark, 1949 is similar to O. brachispina sp. nov. by having similar disc spines with thorny tip, similar number of arm spines, separated radial shields, number of lateral oral papillae, but differs by large radial shields, thorny and leaf-like tentacle scales, separated dorsal and ventral arm plates. Ophioplinthaca amezianeae O’Hara & Stöhr, 2006 is similar to O. brachispina sp. nov. by having similar thorny tentacle scales, separated radial shields, separated dorsal and ventral arm plates, number of lateral oral papillae, but differs by number of arm spines, tall and thorny disc spines, and spiniform lateral oral papillae. Ophioplinthaca bythiaspis (H. L. Clark, 1911) is similar to O. brachispina sp. nov. by having separated radial shields and number of lateral oral papillae, but differs by oval tentacle scales, conical disc spines, number of arm spines and contiguous dorsal arm plates. Ophioplinthaca grenadensis John & A. M. Clark, 1954 is similar to O. brachispina sp. nov. by having similar number of arm spines, separated radial shields, number of lateral oral papillae, and separated arm plates but differs by leaf-like thornless tentacle scales, long and thick disc spines. Ophioplinthaca plicata (Lyman, 1878) is similar to O. brachispina sp. nov. by having similar disc and arm spines, and number of lateral oral papillae, but differs by continues dorsal arm plates, pointed tentacle scale with rounded base, and contiguous radial shields. Ophioplinthaca rudis (Koehler, 1897) is similar to O. brachispina sp. nov. by having similar thorny leaf-like tentacle scales, similar number of arm spines, separated radial shields, separated dorsal and ventral arm plates, but differs by number of lateral oral papillae, tall and thorny disc spines, and spiniform lateral oral papillae.

One of the most distinguishing characters to delimit the new species from almost all species in the genus Ophioplinthaca is the presence of spines with rough surface on lateral, ventral, and dorsal arm plates. The paratype (relatively smaller than the holotype) has thinner and denser spines on the arm. Although, some Ophioplinthaca species have a rough surface on dorsal arm plates or the distal margin covered with minute spines ( Ophioplinthaca plicata and Ophioplinthaca incisa ; ( O’Hara 2010), this is the first record of a species with spines on the entire arm in the genus Ophioplinthaca .

Figures

Figure 3. Ophioplinthaca brachispina sp. nov., holotype (IDSSE-EEB-SW 0106) A dorsal disc B ventral disc C center of the disc D-F disc spines G radial shield H oral frame I ventral side of the arm base J dorsal side of the arm base K dorsal arm L ventral arm M lateral arm. Abbreviations: ars arm spine, as adoral shield, ass adoral shield spine, dap dorsal arm plate, das dorsal arm spine, dp disc plate, gs genital slit, j jaw, lap lateral arm plate, m madreporite, os oral shield, rs radial shield, ts tentacle scale, vap ventral arm plate, vas ventral arm spine. Scale bars: 2 mm (A, B); 1 mm (C, G-M); 200 µm (D-F).

Figure 4. Ophioplinthaca brachispina sp. nov., paratype (IDSSE-EEB-SW 0107) A, B lateral arm plate C dorsal arm spine D ventral arm spine E disc spine F dorsal arm plate G-K vertebrae G proximal view H distal view I ventral view, J dorsal view, K lateral view. Abbreviations: d dorsal, de depression, dist distal, dl dorsal lobe, lac lateral ambulacral canals, mo muscle opening, no nerve opening, pb podial basin, prox proximal, ri ridge, th thorns, v ventral, vl ventral lobe. Scale bars: 800 µm (A, C, F-G, I-K); 500 µm (B, E, H); 100 µm (D).

Figure 5. Ophioplinthaca brachispina sp. nov., paratype (IDSSE-EEB-SW 0107) A dorsal arm B ventral arm C lateral arm D lateral arm plate (small thorns on the lateral arm plate surface shown in the orange rectangle). Abbreviations: dap dorsal arm plate, das dorsal arm spine, lap lateral arm plate, ts tentacle scale, vap ventral arm plate, vas ventral arm spine. Scale bars: 1 mm (A-C); 200 µm (D).