Asteroschema domogranulatum sp. nov.

Figures 4, 5

Material examined.

Holotype: China • 1 specimen; South China Sea, East of Zhongsha Islands, seamount; 16°22.11'N, 113°6.01'E; depth 1742 m; 09 Aug. 2020; Collecting event: stn. SC028; ‘Shenhaiyongshi’ msv leg; preserved in -80 °C; IDSSE-EEB-SW0089.

Paratypes: China • 2 specimens; same data as for holotype; IDSSE-EEB-SW0090, IDSSE-EEB-SW0091 .

Diagnosis.

Radial shields straight, parallel, close together, and raised above the disc and arms (Fig. 4A). Disc concealed by large polygonal, slightly domed granular ossicles (Fig. 4C). Jaws elongated, apex covered with few granular ossicles, but distal half naked. Ventral disc covered with large polygonal plate-like ossicles but naked around distal half of jaws (Fig. 4E). Dorsal and lateral surface of arms covered with plate-like or granular ossicles but dense only on few arms segments beyond the arm base (Fig. 4F-H). Ventral surface of the arm naked except arm base (Fig. 4I-L).

Description of holotype.

Disc diameter 9 mm, length of arms 165 mm, arm base width 2.8-3.0 mm (Fig. 4).

Disc. Disc star-shaped, pentagonal, raised high above the arms, incised interradially and swollen on radial shields (Fig. 4A, B). Disc concealed by dense, large, polygonal, slightly domed ossicles (three or four grains in 1 mm; Fig. 4C). Radial shields bar-like, long, parallel, straight, adjacent pairs separated by narrow interradial disc, raised above the disc, and almost extending to center (Fig. 4C). Domed ossicles on distal half of radial shields larger (two or three grains in 1 mm) than in center (four grains in 1 mm; Fig. 4C). Genital slits narrow, vertical on interradii, dorsal half covered with ossicles similar to dorsal disc, ventral half similar to ventral disc (Fig. 4D). Jaws elongated, apex covered with few granular ossicles, but distal half naked (Fig. 4E). At apex of jaw a bluntly pointed tooth, at lateral edges a few granules that resemble lateral oral papillae (Fig. 4E). Ventral disc covered with large polygonal plate-like ossicles (three or four grains in 1 mm) except distal half of jaws (Fig. 4E). Adoral shields large but completely concealed by ossicles. Oral shields not discernible, and naked adoral shield spine (Fig. 4B, E).

Arms. Arms slender, arched at base, sub-cylindrical, increasingly cylindrical and narrower distalwards (Fig. 4F-H). Dorsal surface of arm base covered with large polygonal plate-like ossicles (three or four grains in 1 mm), then decreasing in size (five or six grains in 1 mm) and separated along the arm (Fig. 4F-H). Lateral plate covered with granular or plate-like ossicles, larger than on dorsal surface, and continuing to near base of arm spine (Fig. 4G, H). Distal half of arm laterally and dorsally covered with similar in size, separated granular ossicles (seven or eight grains in 1 mm; Fig. 4H). Ventral surface of arm base covered with polygonal plate-like ossicles (five or six grains in 1 mm), but after few arm segments from arm base completely naked (Fig. 4I-L). Tentacle pore at first arm segment without arm spine, but with small extended tube or sheath (Fig. 4I). Single arm spine from second arm segment with a second arm spine from nineteenth or twenty-second arm segment (Fig. 4I-L). Inner arm spine initially tapering to pointed thorny tip, middle half cylindrical, slightly club-shaped, one and a half arm segment in length, flattened, thorny (Fig. 4K-N). Outer arm spine half as long as inner spine in middle region, with thorny tip (Fig. 4N). Both arm spines similar in size at distal end, a compound hook with 3-6 secondary teeth (Fig. 4O, P).

Color. In live specimen, light brown color (Fig. 4).

Ossicle morphology of one paratype.

IDSSE-EEB-SW090: Lateral arm plate curved around vertebrae, with strong curved rib with one arm spine articular structure, with single, completely separated large muscle and nerve openings (Fig. 5A). A depression on inner side of lateral arm plate (Fig. 5B). In proximal and middle half of arm inner arm spine slightly swollen, flat, and thorny on distal arm. Outer arm spine nearly half the size of the inner one with thorny tip (Fig. 5C). Distally, both spines changing into compound hook with secondary teeth (Fig. 5D). Arm concealed by polygonal large granular or plate-like ossicles (Fig. 5E). Vertebrae with streptospondylous articulation, with deep groove between proximal and distal end, dorsally a median longitudinal furrow, ventrally with deep median longitudinal groove containing lateral ambulacral canals, no oral bridge (Fig. 5F-J).

Paratypes variations.

Disc diameter 6.5 and 8 mm, and both basically identical to holotype. However, the segment at which the second arm spine first appeared varied (14-20 free segments), but is considered intraspecific variation.

Distribution and habitat.

1742 m depth. Zhongsha Islands, the South China Sea. Attached to coral host.

Etymology.

The species name is derived from the Latin words domus, meaning dome, and granulatus, meaning granulated, referring to the domed granular ossicles on the disc.

Remarks.

The here examined new species was collected on a deep-sea seamount, attached to an unidentified coral species. It concurs with the group that has domed and plate-like granular ossicles, in the genus Asteroschema . This clade included only one species, prior to this study ( Asteroschema igloo Baker, 1980). Large polygonal plate-like ossicles were the most significant morphological character for delimiting most of the other Asteroschema species from A. domogranulatum sp. nov. (Table 2).

Asteroschema domogranulatum sp. nov. strongly resembles A. igloo . They are similar in size according to McKnight’s (2000) description (8 mm disc diameter). Therefore, here we include a comprehensive morphological analysis to distinguish A. domogranulatum sp. nov. from A. igloo such as (see also Table 2): in A. domogranulatum sp. nov. radial shields raised above the arms and disc, straight, parallel, with narrow gap, whereas in A. igloo distal ends of radial shields much wider apart, converging to center, in A. domogranulatum sp. nov. polygonal granular ossicles on dorsal disc, in center smaller than at distal edge, but in A. igloo concealed by polygonal or rounded domed ossicles, and in center large, domed, rounded ossicles, in A. domogranulatum sp. nov. teeth pointed but in A. igloo ventralmost one pointed and others blunt spearhead-shaped, in A. domogranulatum sp. nov. ventral disc covered with polygonal plate-like ossicles, and distal half of jaw naked but in A. igloo completely covered with compact polygonal or rounded domed ossicles, in A. domogranulatum sp. nov. only dorsal and lateral surface covered with plate-like or granular ossicles, dense only on few arm segments from arm base, and naked ventral arm except arm base but in A. igloo whole arm covered with dense, rounded or polygonal domed ossicles, in A. domogranulatum sp. nov. inner arm spine slightly swollen, blunt, flattened, and outer arm spine with thorny pointed tip but in A. igloo inner arm spine swollen, blunt, and outer arm spine with smooth pointed tip, in A. domogranulatum sp. nov. start of first arm spine at second arm segment, and second arm spine at nineteenth or twenty-second arm segment but in A. igloo first arm spine from third arm segment, and second arm spine starts at eighth or tenth arm segment (McKnight 2000). The most significant morphological characters of A. domogranulatum sp. nov. were the appearance of the radial shields, and the granulation of ventral disc and arms (Fig. 4).