Asteroschema cf. bidwillae McKnight, 2000

Figures 8, 9

Asteroschema bidwillae McKnight, 2000: 24-27, fig. 8.

Material examined.

China • 1 specimen; South China Sea, Zhongsha Islands, seamount; 13°36.20'N, 113°33.74'E; depth 1515 m; 30 Mar. 2020; Collecting event: stn. SC025; ‘Shenhaiyongshi’ msv leg; preserved in -80 °C; IDSSE-EEB-SW0105 .

Description.

Disc diameter 13 mm, length of arms 195 mm, arm base width 4.5-5 mm (Fig. 8).

Disc. Disc circular, hexamerous, raised above arms, deeply swollen in center (Fig. 8A, B). Disc covered with dense, small, finely rugose granular epidermal ossicles (Fig. 8A-C). Granular ossicles dense and small in size in disc center, but slightly larger at distal edge (six or seven grains in 1 mm; Fig. 8C, D). Radial shields extending to center but proximal ends concealed by skin with granular ossicles, and distal ends raised above the disc (Fig. 8C). Granular ossicles around distal edge and periphery of disc larger and more irregular (Fig. 8C). Genital slits narrow, without ossicles and vertical on ventral interradii (Fig. 8E). Jaws elongated, mostly naked without granular ossicles (Fig. 8F). At apex of jaw flattened, pointed, and finely rugose teeth, and two to four granular tubercles that resemble lateral oral papillae (Fig. 8F). Ventral disc covered with widely separated small granular ossicles (four or six grains in 1 mm; Fig. 8B, F). Adoral shields connected to first ventral arm segment and concealed by widely separated small granular ossicles, but outline of shields clearly visible (Fig. 8F). Oral shields not discernible and adoral shield spine covered by ossicles (Fig. 8F).

Arms. Arms six, at base wide, not arched, dorsally flattened, and swollen in first few free arm segment (Fig. 8G). Arms distalwards from middle part narrowing and increasingly cylindrical (Fig. 8H). Swollen dorsal arm base covered with dense, large, irregular granular ossicles (four or seven grains in 1 mm), distalwards decreasing in size and becoming rounded (six or seven grains in 1 mm), and distally widely separated (Fig. 8G, H). Lateral arm plates covered with slightly separated granular ossicles. Ventral arm near base covered with granular ossicles similar to ventral disc (five or six grains in 1 mm), but becoming widely separated to completely naked along the arm (Fig. 8I, J). First two or three arm segments without arm spine (Fig. 8I). First arm spine appeared at third or fourth arm segment, and second arm spine at eighteenth or twenty-first segment (Fig. 8I-K). Inner arm spine cylindrical, with blunt thorny tip, one and a half arm segment in length (Fig. 8J, K). Outer spine half as long as inner spine in middle region, with thorny, pointed tip (Fig. 8K). Both arm spines equal in length at distal end of arm, and compound hook with 3-6 secondary teeth (Fig. 8L).

Color. In live specimen, reddish brown (Fig. 8).

Ossicle morphology.

Lateral arm plate with two arm spine articular structures, with slightly separated large muscle and relatively small nerve opening (Fig. 9A). A depression on inner side of lateral arm plate (Fig. 9B). Inner arm spine from proximal and middle half of arm with cylindrical, terminal projection, and upper part of spine covered with thorns (Fig. 9C). Distally, arm spine turns into compound hook with secondary teeth (Fig. 9D). Arm and disc concealed by rounded to slightly irregular granular ossicles (Fig. 9E). Vertebrae with streptospondylous articulation, dorsally a median large longitudinal furrow, ventrally with median deep groove with lateral ambulacral canals, podial basins small (Fig. 9F-J).

Distribution.

400-2000 m depth. New Zealand, Tasman Sea, Solomon Islands, South China Sea (OBIS 2021).

Remarks.

Asteroschema bidwillae was first described by McKnight (2000), with type locality New Zealand waters in the South Pacific Ocean. This is the first redescription since the original description. The specimens from our collection concur with McKnight’s description, but we noticed some differences such as: granular arrangement on radial shields, irregular ossicles on arm base, ossicles on ventral arm recorded nearly to middle region, and start of second arm spine. However, some of these variations may be related to size and maturity of the specimen (the holotype had a disc diameter of 5 mm). We hesitate to fully associate our specimen with Asteroschema bidwillae due to uncertainty with the morphological variation in A. bidwillae . The genus Asteroschema contains only two hexamerous species as far as known. Therefore, the closest one is Asteroschema wrighti McKnight, 2000, but it differs in characters of the radial shields, granulation on disc and arm, innermost arm spine, and start of second arm spine (Table 2). This is the first record of A. bidwillae from the North Pacific Ocean, if it is indeed this species.