Asteroschema cf. lissum H. L. Clark, 1939

Figures 16, 17

Asteroschema lissum H. L. Clark, 1939: 37-39, figs 1-3.

Material examined.

China • 2 specimens; 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; GenBank: OK044300; IDSSE-EEB-SW0079, IDSSE-EEB-SW080. Northwest Pacific • 1 specimen; near Mariana Trench, Southeast of Guam Island, seamount, 12°6.67'N, 141°37.27'E; depth 1160 m; 03 Sep. 2019; Collecting event: stn. SC033; ‘Shenhaiyongshi’ msv leg; preserved in -80 °C; GenBank: OK044299, OL712207; IDSSE-EEB-SW0081 .

Description.

IDSSE-EEB-SW0079: disc diameter 11.5 mm, length of arms 165 mm, arm base width 3-3.5 mm (Fig. 16).

Disc. Disc flat, slightly raised above arms, swollen in center (Fig. 16A, B). Disc covered with smooth, small granular ossicles (Fig. 16C, D). Granular ossicles dense and small in disc center (six or eight grains in 1 mm), but slightly larger and separated at distal end of radial shield (five or six grains in 1 mm) (Fig. 16C). Radial shields wide, similar in size, curved, slightly raised above disc but not meeting in center (Fig. 16A, C). Radial shields clearly recognizable under thin skin embedded with ossicles (Fig. 16A, C). Genital slits narrow, concealed by polygonal granular ossicles, and vertical on ventral interradii (Fig. 16E). Jaws elongated, mostly naked without granular ossicles (Fig. 16F). Flattened, spearhead-shaped teeth, and six to seven granular ossicles that resemble lateral oral papillae at apex of jaw (Fig. 16F). Ventral disc covered with widely separated small granular ossicles (six or seven grains in 1 mm), but mostly covered by translucent thin skin (Fig. 16F). Adoral shields large, distal edge convex, concealed by thin skin with scattered small, smooth granular ossicles (Fig. 16F). Oral shields not discernible and oral tentacle pore naked without ossicles (Fig. 16F).

Arms. Arms slightly arched, circular, from middle to distal half narrower and more cylindrical (Fig. 16G). Dorsal arm base covered with granular ossicles similar to dorsal disc (six or eight grains in 1 mm), increasingly separated and decreasing in size along the middle segments of the arm (seven or eight grains in 1 mm), distally widely separated (five or six grains in 1 mm) (Fig. 16G, H). On lateral arm plates, granular ossicles widely separated but continuing to base of arm (Fig. 16H, K). Only one or three ventral arm segments near arm base covered with few granular ossicles similar to ventral disc (six or seven grains in 1 mm), then completely naked along the arm (Fig. 16I, J). First arm spine appears at second arm segment, second arm spine at ninth or thirteenth segment (Fig. 16I-K). Inner arm spine initially short, thick with thorny pointed tip, at middle arm cylindrical, taller, one and a half arm segment in length, with flattened thorny tip (Fig. 16J, K). Outer arm spine half as long as inner spine in middle region, with smooth to thorny tip (Fig. 16K). Both arm spines equal in length at distal end of arm, and compound hook with five or six secondary teeth (Fig. 16L).

Color. In live specimen, reddish brown but radial shields slightly lighter in color due to thin skin (Fig. 16).

Ossicle morphology.

Lateral arm plate with two arm spine articular structures, with large muscle and nerve openings (Fig. 17A, B). Inner arm spine from proximal and middle half of arm cylindrical, with thorny tip (Fig. 17C). Distally, arm spine turns into compound hook with secondary teeth (Fig. 17D). Arm and disc concealed by less dense, wider, and short granular ossicles (Fig. 17E). Vertebrae with streptospondylous articulation, dorsally a median longitudinal furrow, ventrally with deep median longitudinal groove with lateral ambulacral canals, podial basins relatively small (Fig. 17F-J).

Distribution.

797-1515 m depth. Maldives, South China Sea, Northwest Pacific.

Remarks.

Asteroschema lissum was first described by H. L. Clark (1939), with type locality Maldives waters in the Indian Ocean. This is the first redescription after the original description. The specimens from our collection were close to H. L. Clark’s description but we noticed some differences, such as: characters of radial shields, and granular ossicles at ventral disc and base of arm. We hesitate to fully associate our specimens with Asteroschema lissum or propose a new species, due to uncertainty of these morphological variations. Some of these variations may be affected by size, maturity, and environment (holotype disc diameter 7.5 mm). Asteroschema lissum strongly resembles A. hemigymnum, A. intectum, and A. sublaeve by having similar granular density, and almost naked ventral disc and arms but differs in characters of the radial shields, start of second arm spine, granulation pattern on the disc and arm (Table 2). This is the first record of A. lissum from the South China Sea and the North Pacific Ocean, if it is indeed this species.