Ophiacantha bathybia H.L. Clark, 1911

Figs 27–28

Ophiacantha bathybia H.L. Clark, 1911: 233–234 .

Material examined

CHINA • 1 spec.; South China Sea, E of Hainan Island, seamount; 18°11.96′ N, 114°21.01′ E; depth 3536 m; 11 Sep. 2017; collection event: stn SC016; MSV Shenhaiyongshi leg.; GenBank: MZ 203273; IDSSE EEB-SW0024 • 1spec.; South China Sea,E of Hainan Island, seamount; 18°11.93′N, 114°21.01′ E; depth 1550 m; 9 Sep. 2017; collection event: stn SC016; MSV Shenhaiyongshi leg.; GenBank: MZ 203274; IDSSE EEB-SW0025 • 1 spec.; South China Sea, E of Hainan Island; 18°12.03′ N, 114°21.25′ E; depth 3500 m; 6 Sep. 2017; collection event: stn SC016; MSV Shenhaiyongshi leg.; preserved in -80°C; IDSSE EEB-SW 0044.

Description (IDSSE EEB-SW 0025)

MEASUREMENTS. Disc diameter 17.5 mm.

DiSC. Flat, pentagonal, covered by thin skin, underlying scales with short disc spines (Fig. 27A–B), 0.3–0.4 mm high, rough, with two to three sharp terminal thorns, similar on both dorsal and ventral disc (Fig. 27C–E). Radial shields long, narrow, well separated, parallel to each other, concealed by thin skin, their form and position clearly visible through skin, with thorny stumps (Fig. 27A, E). Ventral disc near oral shields with scales without thorny stumps (Fig. 27B). Oral shield much wider than long, broadly triangular, with wide, pointed proximal angle and a straight or convex distal edge (Fig. 27F). Madreporite larger, with central depression, pentagonal. Adoral shields large, three times as long as wide, narrow, not separated (Fig. 27B, F). Adoral shields enclose proximal oral shield edges and extend around it, separating it from first lateral arm plate (Fig. 27C, F). Jaws wider than long, ventralmost tooth with large, blunt, pointed or flat edge, and three pointed, spiniform lateral oral papillae with thick and rounded base (Fig. 27F). Genital slits conspicuous and extending from oral shield to periphery of disc (Fig. 27I).

ARMS. Dorsal arm plate on first few arm segments triangular, somewhat fan-shaped and distal margin more convex, but along arm becoming straighter, towards rhombic, somewhat swollen, two times as wide as long, barely separated (Fig. 27G). Ventral arm plate widely triangular, two times as wide as long, distal margin convex, proximally with low, wide angle, lateral side weakly concave and widely separated, but along arm ventral arm plate decreasing in size, as wide as long, with convex distal margin (Fig. 27H). Lateral arm plates meeting above and below, but along arm becoming separated ventrally (Fig. 27H). Six sharp and thorny arm spines (Fig. 27J). Three uppermost spines longest, two to three arm segments in length, one to two dorsal arm spines smooth but others thorny (Fig. 27K). Two to three tapering ventral arm spines with rough tip (Fig. 24L). One tentacle scale, elongated, half as long as ventral arm plate, pointed (Fig. 27I).

COLOR. When alive, entire specimen light red, light brown when dry (Fig. 27A–B).

OSSiCLE MORPHOLOGY. Arm spine articulation well developed and placed at small angle to distal edge of lateral arm plate (Fig. 28A–B). Volute-shaped perforated lobe forms most articulations, but reduced in dorsalmost articulation (Fig. 28A–B). Arm spine articulation with large muscle opening (Fig. 28B). Ventral arm spine smooth, but with rough tip (Fig. 28C). Vertebrae with a well-developed zygospondylous articulation, with a broad dorsal surface, distally abruptly truncated, with a shallow dorsal medial furrow, podial basins slightly wider than long (Fig. 28D–H). Ambulacral groove widened in middle, without oral bridge (Fig. 28G).

Remarks

Ophiacantha bathybia was described by H.L. Clark (1911); additional specimens have been collected in 1980, 1981 and 2015 (OBIS 2021). Ophiacantha bathybia from the present study is quite similar to the holotype description, but it differs in the arm spine and arm plate characters. Our specimens were larger than the holotype (12 mm disc diameter) and some of the morphological differences may be caused by this size difference. The lateral arm plates in the holotype description are connected at both the dorsal and ventral edges, but in our specimens they were only connected by a ligament. Another morphological variation was the number of thorny spines, which differs from segment to segment and arm to arm.

Ophiacantha bathybia has no uniquely distinguishing characters, but appears to be different from other species of Ophiacantha by the size of the disc and characters of the disc covering, dorsal arm plates, arm spines and tentacle scale. Mature specimens have narrow, long radial shields and thorny disc stumps, but differ in arm spine and dorsal arm plate shape.

It has been recorded from abyssal depths, and its morphological characters are similar to those of other abyssal species of Ophiacantha, namely O. pacifica Lütken & Mortensen, 1899, O. sollicita Koehler, 1922, O. cosmica Lyman, 1878, O. frigida Koehler, 1907 and O. sociabilis Koehler, 1897, but these species have been recorded from Panama, the Southern Ocean and the Atlantic Ocean. Ophiacantha bathybia may be considered as a sister species to these abyssal species of Ophiacantha based on their morphological features.

Ophiacantha cosmica differs from O. bathybia by having up to eight arm spines, slender lateral oral papillae without a thick base and a blunt ventralmost tooth (Lütken & Mortensen 1889; Stöhr & O’Hara 2021). Ophiacantha sociabilis differs from O. bathybia by having four to five lateral oral papillae, a less enlarged distalmost papilla and wider jaws (Koehler 1897; Stöhr & O’Hara 2021). Ophiacantha sollicita differs from O. bathybia by having up to eight arm spines, conical disc spines, a notably thickened distal oral papilla and non-moniliform arms (Koehler 1922b). Ophiacantha frigida differs from O. bathybia by having a larger distalmost lateral oral papilla and usually two tentacle scales on the first tentacle pore (Koehler 1922b). Ophiacantha pacifica is highly similar to O. bathybia in morphological features of the oral frame, as well as the number and length of arm spines, but the ventralmost arm spines of O. bathybia have a rough tip (Lütken & Mortensen 1889) (Fig. 28C). However, these morphological differences vary and overlap among individuals between species of the genus Ophiacantha . Therefore, a molecular analysis from a wider range of localities is needed to understand the species boundaries.

Distribution

1602–3656 m depth. South China Sea, Alaska, British Colombia, Bering Sea, Shumagin Island, Makassar Strait and Northwest Pacific (OBIS 2021).