Asterostegus maini McKnight, 2003

Nethupul, Hasitha, Stoehr, Sabine & Zhang, Haibin, 2022, Order Euryalida (Echinodermata, Ophiuroidea), new species and new records from the South China Sea and the Northwest Pacific seamounts, ZooKeys 1090, pp. 161-216 : 161

Asterostegus maini McKnight, 2003

Figures 18 , 19

Asterostegus maini McKnight, 2003: 386-389, figs 1, 2.

Astroceras elegans McKnight, 1989: 25 (non Astroceras elegans Bell, 1917).

Material examined.

China • 2 specimens; South China Sea , near Xisha Islands archipelago, seamount; 16°47.79'N, 113°15.04'E; depth 602 m; 31 Mar. 2020; Collecting event: stn. SC009; ‘Shenhaiyongshi’ msv leg; preserved in -80 °C; GenBank: OK044303 ; IDSSE-EEB-SW0075; IDSSE-EEB-SW0076 .

Description.

IDSSE-EEB-SW0076: disc diameter 32.2 mm, length of arms 240-250 mm, height of arm base 9.8 mm (Fig. 18A-M ).

Disc. Disc circular and slightly inflated radially, with sunken interradial margins (Fig. 16A ). Radial shields elongated, narrow, raised above the disc, extending nearly toward the disc center (Fig. 18B, C ). Distal half of radial shield periphery covered by 10-12 (0.44 to 0.75 mm in width) large, club-shaped granules (Fig. 18C ). Most of these granules cluster on distal end of radial shield (Fig. 18C ). Entire disc, including radial shields, covered by thick skin (Fig. 18A-C ). Adoral shields with slightly ovoid outline (Fig. 18E, F ). Teeth spearhead-shaped, accompanied by granular domed lateral oral papillae but not visible in wet specimen (Fig. 18D-F ). Proximally, adoral shields separated by triangular plate (Fig. 18F ) Oral shields absent, and single or double row of two to six rounded, square, or irregular oral interradial plates beyond adoral shields (Fig. 18E, F ). One median plate located between distal end of adoral shields, and slightly proximal to rest of oral interradial plates (Fig. 18E, F ). Lateral interradial surface of disc slightly vertical and covered by thick naked skin, two conspicuous genital slits inside a large opening (Fig. 18G ).

Arms. Arms simple, strong, and not branching (Fig. 18A ). Cross section of arm base slightly rectangular (7.2 mm in width and 8.5 mm high), but distal half of arm more cylindrical and narrower (Fig. 18H-J ). Proximal to middle region of dorsal arm slightly flattened, ventral surface slightly arched, and lateral surface vertical (Fig. 18J, K ). Whole arm concealed by thick skin (Fig. 18H-K ). Lateral arm plate on proximal half of arms bears three to five club-shaped granules (0.4-0.6 mm in width) (Fig. 18H ), decreasing in size and number along arm and vanishing on distal half of arm (Fig. 18I ). Ventral arm covered by naked skin (Fig. 18K ). First one to two tentacle pores lack arm spines; second or third pore with two or three arm spines, similar in size (Fig. 18D ). At proximal end of arms, arm spines short, thick, ovoid with more or less rounded tip with rough surface, at middle of arms club-shaped, transversely flattened (Fig. 18J, K ). Arm spines turn into hook with two to three secondary teeth at distal end of arms (Fig. 18L, M ).

Color. In live specimen, dorsal disc interradially dark brown but radial shields light brown. Ventral and lateral disc whitish brown, dorsal surface of proximal and middle regions of arms bright brown/red. Lateral and ventral surface of arms whitish brown, granules creamy white (Fig. 18 ).

Ossicle morphology.

Lateral arm plate with two arm spine articular structures, middle half of arm with large and wide, separated muscle and nerve openings, depression on inner side (Fig. 19A, B ). Arm spines large, short, flattened, and club-shaped with thorny surface (Fig. 19C, D ). Vertebrae with streptospondylous articulation. Vertebrae on proximal to middle half of the arm large, flat with deep ventral groove, no oral bridge (Fig. 19E, F ). Vertebrae on distal half of arm slightly longer, dorsally large median longitudinal furrow, and deep median longitudinal groove on ventral side, with oral bridge (Fig. 19G, H ).

Distribution.

417-602 m in depth. New Zealand (Cook Island), South China Sea.

Remarks.

Asterostegus maini was first described by McKnight (2003), with type locality Cook Islands, South Pacific Ocean, and Okanishi and Fujita (2014) redescribed it. However, this is the first record of Asterostegus maini since the holotype. Here we recorded two specimens from the South China Sea (disc diameter 26 mm and 32.2 mm) and both are larger than the holotype (disc diameter 22 mm). Currently, three species belong to the genus Asterostegus : A. maini McKnight, 2003, A. tuberculatus Mortensen, 1933, and A. sabineae Okanishi & Fujita, 2014. Asterostegus tuberculatus differs from A. maini in granules being scattered across the whole disc including the radial shield, and only two to three stump-like granules on the dorsolateral arm plate ( McKnight 2003; Okanishi and Fujita 2014). Asterostegus sabineae differs from A. maini in large stump-like granules on the radial shield, only one oral interradial plate, and one or two large stump-like granules on the dorsolateral arm plate ( Okanishi and Fujita 2014). The number of oral interradial plates and their arrangement are highly variable within and between individuals (Fig. 18E, F, N ). Previous studies on the holotype showed only one row with two to five interradial plates ( McKnight 2003; Okanishi and Fujita 2014). However, in our specimens, these are arranged in one or two rows with two to seven interradial plates in total (Fig. 18E, F, N ). Therefore, one of the key morphological characters in the genus Asterostegus , the arrangement of oral interradial plates has to be modified. One specimen from the present study (IDSSE-EEB-SW0075, 26 mm disc diameter) showed some morphological variations, such as: small granules on the periphery of the disc and on few small areas on the ventral disc (Fig. 18O, P ). These granules are extremely small compared to those on the radial shields and arms. In the specimens from the present study, first arm spines started from second or third arm segment, but in the holotype, it started from the fourth arm segment. However, except for these small morphological variations, both specimens were similar to the holotype description.

Figures

Figure 16. Asteroschema cf. lissum H. L. Clark, 1939 (IDSSE-EEB-SW 0079) A dorsal view B ventral view C dorsal disc D skin on the disc E lateral disc F ventral disc G dorsal arm (middle) H lateral arm (middle) I ventral arm (proximal) J outer arm spine (proximal) K arm spines (middle) L arm spines (distal). Abbreviations: as adoral shield, ass adoral shield spine, ars arm spine, go granular ossicle, gs genital slit, iars inner arm spine, j jaw, oars outer arm spine, ots oral tentacle scale, rs radial shield, t teeth, tp tentacle pore. Scale bars: 2 mm (A, B, G); 1 mm (C, E, F, H, I); 500 µm (J, K); 200 µm (D, L).

Figure 18. Asterostegus maini McKnight, 2003 (A-M IDSSE-EEB-SW 0076 N-P IDSSE-EEB-SW 0075) A dorsal view B center of dorsal disc C dorsal disc (distal end of radial shields) D ventral disc E, F oral frame G lateral disc H dorsal arm (proximal) I dorsal arm (distal) J lateral arm (proximal) K ventral arm (middle) L, M arm spines (distal) N oral frame O periphery of the disc P oral frame (small transparent ossicles). Abbreviations: as adoral shield, ars arm spine, go granular ossicle, gs genital slit, j jaw, mp median plate, oip oral interradial plate, rs radial shield, t teeth, tp tentacle pore, ts tentacle scale. Scale bars: 6 mm (A); 2 mm (B-H, N); 1 mm (I-K, O, P); 500 µm (L); 200 µm (M).

Figure 19. Asterostegus maini McKnight, 2003 (IDSSE-EEB-SW 0076) A, B lateral arm plate (external, internal) C, D arm spine (middle) E, F vertebrae (middle) E proximal view F distal view G-K vertebrae (distal) G proximal view H distal view I lateral view J dorsal view K ventral view. Abbreviations: asa arm spine articular structure, d dorsal, de depression, dist distal, mo muscle opening, ob oral bridge, prox proximal, v ventral. Scale bars: 800 µm (E, F); 500 µm (A, C, D, I, K); 300 µm (B, G, H, J).