Ophioplinthaca athena A. H Clark, 1949

Figs 10, 11

Ophioplinthaca athena A. H Clark, 1949: 23-24, fig. 9; Chen et al. 2021b: 60-61, fig. 3.

Material examined.

Northwest Pacific • 1 specimen; near Mariana Trench, Southwest of Guam Island, seamount; 12°8.83'N, 139°0.37'E; depth 1987 m; 27 November 2020; Collecting event: stn. SC041; Shenhaiyongshi msv leg; preserved in -80 °C; GenBank: OK043833; IDSSE-EEB-SW0110 .

Description.

Disc diameter 12.5 mm, arm base width 1.5 mm (Fig. 9).

Disc. Sub-circular and incised interradially, creating five wedge-shaped lobes over each arm base in contrast to sunken center and interradii of disc (Fig. 10A, B). Disc scales small, irregular, overlapping, and some scales bear more than one stump (Fig. 10C-F). Scales increase in size distalwards from disc center to periphery interradially (Fig. 10D-F). Disc stumps in disc center with cylindrical base and few radiating spinules at truncated tip (Fig. 10D-F). Spines at disc periphery and around radial shields, slightly smaller, less cylindrical, more conical, smooth, with pointed tip (Fig. 10D, E). Radial shields large, 3 × as long as wide, acute proximal end, much wider and slightly convex distal end, pairs separated along proximal half, and barely connected distally (Fig. 10F). Ventral disc covered by small, overlapping disc scales without or rarely bearing conical granules (Fig. 10B, G). Genital slits conspicuous and extending from oral shield to periphery of disc (Fig. 10G). Madreporite arrowhead-shaped, as wide as long, triangular with pointed proximal end, lobed distal edge with thickened lateral margins. Other oral shields widely triangular, twice as wide as long, wide proximal angle, distal edge folded ventralwards with minute central lobe, and lateral angle connected to first lateral arm plate (Fig. 10H). Adoral shield 2 × as long as wide, with concave proximolateral margin, pair of shields proximally connected, and connected to first lateral and ventral arm plates (Fig. 10H). Jaw longer than wide, bearing one slightly blunt, flat, elongated, and large ventralmost tooth and four elongated, spiniform lateral oral papillae (Fig. 10H). Lateral oral papillae, finely rugose, equal in height to ventralmost tooth, with pointed tip (Fig. 10H). One small scale-like rounded adoral shield spine at lateral margin of adoral shield at edge of second tentacle pore (Fig. 10I).

Arms. Five slightly moniliform arms, with smooth plates. Dorsal arm plates twice as long as wide, with truncated proximal end in first dorsal arm plate (Fig. 10J), but following plates with triangular proximal end, slightly curved proximolateral margins, and convex to slightly wavy distal margins covered with minute spines (Fig. 10J, K). Dorsal arm plates at proximal to middle arm segments barely separated, but distally widely separated (Fig. 10J, K). First ventral arm plate trapezoid, as wide as long, with sunken proximal end, and distal end connected to second ventral arm plate (Fig. 10I). Following ventral arm plates twice as wide as long, with obtuse proximally, straight proximolateral margins, curved lateral angles, straight to slightly wavy distal end, distal margins covered with minute spines, and widely separated (Fig. 10L). Lateral arm plates meeting above and below (Fig. 10K-M). Up to five arm spines. Proximal arm segment bearing two dorsal and three ventral arm spines (Fig. 10M). Dorsalmost arm spines at proximal end two to two and a half arm segments in length, smooth or with few thorns at lateral edge (Fig. 10M). Next dorsal arm spine much longer, nearly four arm segments in length, smooth or with thorns at lateral margin (Fig. 10M). Ventral arm spines short, less conspicuous thorns, more rugose surface (Fig. 10L, M). First tentacle pore covered with two leaf-like tentacle scales with pointed tip (Fig. 10H, I). Following pores covered with leaf-like pointed tentacle scale with rounded base and tip covered in micro spinules (Fig. 10L).

Color. In live specimen, orange-brown dorsal disc, light color in arms and ventral disc, arm spines orange, disc spines and papillae red (Fig. 10).

Ossicle morphology. Arm spine articulations well developed, four in number, and placed at slight angle to distal edge of lateral arm plate. Volute-shaped perforated lobe forms dorsal and distal part of articulation, with large muscle opening and small nerve opening (Fig. 11A). Distal half of inner side of lateral arm plate with group of small, irregular perforations parallel to row of spine articulations; a continuous ridge and a prominent knob close to ventral edge form vertebral articulation, shaped like a broad, nose-shaped beak (Fig. 11B). Dorsal arm spine thorny, with several longitudinal rows of perforations and widely spaced tall thorns (Fig. 11C). Dorsal arm plate triangular with smooth surface (Fig. 11D). Vertebrae with streptospondylous articulation, short, broad podial basin at proximal end and narrow small distal end (Fig. 11E-I). Dorsal end of vertebrae distally triangular and proximally flattened with longitudinal groove along midline (Fig. 11E, F). Ventral end of vertebrae with broad ambulacral groove and lateral ambulacral canals (Fig. 11G-I).

Distribution.

1866-2157 m depth, Southwest of Guam Island, Northwest Pacific, Kupuai, Hawaii Islands.

Remarks.

Ophioplinthaca athena was described by A. H Clark (1949), and recorded from deep waters in the Hawaiian Islands. Ophioplinthaca athena resembles O. papillosa, O. globata, O. hastata, O. plicata, O. carduus, O. semele, O. clothilde, and O. dipsacos in disc spine characters, but differs in arm spine, oral frame, and radial shield characters (Table 3).

Ophioplinthaca athena from the present study is similar to the holotype description, but it differs slightly by separated dorsal arm plates and the shape of the dorsal arm spines, although the latter varies within our individual. Therefore, the shape of the arm spines is not a suitable morphological character to delimit O. athena . The description of the holotype mentioned that dorsal arm plates were contiguous, but in our specimen, they are just separated along the arm, and there are no paratypes of O. athena . Therefore, this difference may be related to the size of the specimen (holotype 14.5 mm disc diameter; A. H. Clark 1949), and these small morphological differences can be considered as intraspecific variation within O. athena .