Ophiactis cf. perplexa Koehler, 1897

Figs 36–37

Ophiactis perplexa Koehler, 1897: 327–328, pl. 7 figs 40–41.

Material examined

CHINA • 1 spec.; South China Sea, SE of Zhongsha Islands, seamount; 13°23.97′ N, 114°51.15′ E; depth 1618 m; 22 Sep. 2020; collection event: stn SC012; MSV Shenhaiyongshi leg.; preserved in 95% ethanol; GenBank: MZ 198767; IDSSE EEB-SW0013 • 1 spec.; same collection data as for preceding; GenBank: MZ 198768; IDSSE EEB-SW0014 • 1 spec.; same collection data as for preceding; IDSSE EEB-SW0063 .

Description (IDSSE EEB-SW 0013)

MEASUREMENTS. Disc diameter 7.2 mm.

DiSC. Sub-pentagonal or circular, pentamerous. Dorsal disc covered by large, coarse, irregular, polygonal overlapping scales, increasing in size arranged as a rosette in disc center (Fig. 36A–B). Radial shields longer than wide, completely separated mostly by a single series of two scales but sometimes three scales (Fig. 36A). Radial shield length less than half disc radius, distal edge with two smooth and pointed spines (one on each radial shield) (Fig. 36C). Ventral disc covered by small, irregular, overlapping scales (Fig. 36D–E). Oral shield lozenge-shaped with widely angular proximal end, wider than long (Fig. 36F). Adoral shields as large as oral shield, curved around lateral edges of oral shield, three times as long as wide (Fig. 36F). Wide tricuspid teeth. Single large, broad (wider than teeth), operculiform lateral oral papilla at proximal edge of adoral shield (probably adoral shield spine), covering second tentacle pore inside mouth angle (Fig. 36F). Genital slits conspicuous and extending from oral shield to disc periphery (Fig. 36G).

ARMS. Dorsal arm plates large, triangular, with straight distal edge, truncated proximal edge, twice as wide as long and contiguous (Fig. 36H). Ventral arm plates pentagonal, wider than long, completely separated (Fig. 36I). Lateral arm plate bears three arm spines with thick base, rounded, blunt tip, 1–1½ arm segment in length and middle one longest (Fig. 36J). Ventralmost spine thicker than others for first few arm segments. One large, broad, rounded tentacle scale, two thirds as long as ventral arm plate (Fig. 36I–J).

COLOR. Dorsal disc white with a few brown patches on scales. Disc periphery and distal part of radial shields light brown. Arms and ventral disc also light brown, but arm spines dark brown (Fig. 36A–B).

OSSiCLE MORPHOLOGY. Lateral arm plate with three well developed arm spine articulations, consisting of two unequal, subparallel, curved lobes (not volute-shaped). Arm spine articulation with two similar openings for muscle and nerve, but in some articulations nerve opening slightly larger than muscle opening (Fig. 37A–C). Vertebrae with well-developed zygospondylous articulation, narrow, shallow dorsal furrow, truncated far from distal articulation (Fig. 37D–H). Ambulacral groove deep, with hole in middle, without oral bridge (Fig. 37F).

Remarks

All specimens in the present study were found attached to a glass sponge species from a deep-sea seamount. Ophiactis perplexa was first described by Koehler (1897) and has not been redescribed since, but specimens were recorded in 2000, 2003 and 2007 (OBIS 2021). Our specimens are similar to the holotype description, but we noticed variations in some morphological characters that prevent us from fully associating these specimens with O. perplexa . In particular, the spines on the periphery of the ventral disc only appear in two of our specimens, and most specimens have disc spines at the base of the radial shields (Fig. 35C–E), but most of the species in the genus Ophiactis show this morphological variation among individuals. Ophiactis flexuosa Lyman, 1879 is related to O. perplexa, but is distinguished by the presence of spines on the disc, the shape of the radial shield and the pentagonal shape of the ventral arm plate. Another similar species is O. definita Koehler, 1922, recorded in deep waters from the South China Sea, Celebes Sea, Molucca Sea, Banda Sea, Solomon Sea, Coral Sea, Bismarck Sea and Australia. Ophiactis definita (currently accepted as O. brachygenys, see below) is clearly distinguished from O. perplexa by the absence of spines on the disc, a longer oral shield with a much smaller border and a pointed distalmost oral papilla. Ophiactis brachygenys H.L. Clark, 1911 has separated dorsal arm plates and a smaller disc border than O. perplexa, but rarely has a few spines on the disc (Fig. 38M).

Distribution

600–2000 m depth. East China Sea, Indian Ocean, Timor Sea, Tasman Sea (OBIS 2021).