Ophioplinthaca cf. lithosora (H. L. Clark, 1911)

Figs 12, 13

Ophiocamax lithosora H. L. Clark, 1911: 191-193, fig. 89.

Ophiomitra lithosora: Matsumoto 1917: 131.

Material examined.

China • 1 specimen; South China Sea, near Xisha Islands archipelago, seamount; 16°47.79'N, 113°15.04'E; depth 602 m; 31 March 2020; Collecting event: stn. SC009; Shenhaiyongshi msv leg; preserved in-80 °C; GenBank: OK043834 IDSSE-EEB-SW0111 .

Comparative material.

Japan • Holotype specimen; East China Sea, Osumi Islands, Kuchnioerabu Island; 30°22'N, 129°08.5'E; depth 660 m; 13 Aug 1906; Collecting event: stn. 4918; R/ V Abatross, North Pacific Expedition leg; preserved dry; USNM 25622. • 1 paratype specimen; North Pacific Ocean, Wakayama, Honshu Island, Shiono Misaki; 33°25.17'N, 135°37.33'E; depth 446-463 m; 29 Aug 1906; Collecting event: stn. 4967; R/ V Abatross, North Pacific Expedition leg.; preserved dry; USNM 26220. • 1 specimen; S off Daiozaki, Kumanonada; 34°05.12'N, 136°51.24'E to 34°05.05'N, 136°50.5'E; depth 475-494 m; 25 May 1994; Collecting event: stn. KN25; R/ V Tansei-Maru KT-94-07 leg.; gear 3 m ORE beam trawl; preserved in ethanol; NSMT E-7943.

Description.

Disc diameter 20 mm, arm base width 4.5 mm (Fig. 12).

Disc. Sub-pentagonal and incised interradially, creating five wedge-shaped lobes over each arm base in contrast to sunken center and interradii of disc (Fig. 12A, B). Disc scales small, irregular, overlapping, some scales bear more than one low stout spine, these spread across entire disc except radial shields (Fig. 12C). Spines in disc center, 0.9-1.4 mm high, thick, with cylindrical to rounded base, tapering to sharp point, with truncated or pointed tip, and additional irregular flanged thorns arising from lateral margins along spine. Proximal end of disc spines, 0.8-0.9 mm high, thick, base cylindrical, with thorny pointed tip (Fig. 12D-G). Peripheral disc spines decreasing in size, conical, with smooth or slightly thorny pointed tip (Fig. 12F, G). Radial shields large, 2 × as long as wide, acute proximal end, much wider convex distal end, completely separated (Fig. 12H). Ventral disc covered by small, overlapping disc scales without spines (Fig. 12B, I). Genital slits conspicuous and extending from oral shield to periphery of disc (Fig. 12B, I). Oral shield wide fan-shaped, 2 × as wide as long, with rounded proximal end, concave lateral margins, convex to wavy distal edge, and lateral angle connected to first lateral arm plate (Fig. 12I). Madreporite as long as wide, proximal end with wide angle. Distal edge strongly convex (Fig. 12I). Adoral shield 2 × as long as wide, with straight lateral margin, pair of shields proximally connected (Fig. 12I). Adoral shields connected to first lateral and ventral arm plates (Fig. 12I). Jaw longer than wide, bearing four to five elongated, pointed leaf-like lateral oral papillae, cluster of up to three pointed tooth papillae, and small, 4-6 granules covered by adoral shield spines in some jaw slits (Fig. 12I). Two adoral shield spines, one larger, oval, scale-like flat, other one smaller, situated at lateral margin of adoral shield at second tentacle pore (Fig. 12I).

Arms. Dorsal arm plates pentagonal, wider than long, with truncated proximal end, weakly convex proximolateral margins, straight lateral margins, and convex to slightly wavy distal margins, on proximal to middle arm segments contiguous, but distally separated (Fig. 12K). First ventral arm plate nearly square, connected to second ventral arm plate (Fig. 12J). Following ventral arm plates twice as wide as long, with obtuse proximal end, straight proximolateral margins, slightly concave lateral edges, straight distal edge, and widely separated (Fig. 12L). Lateral arm plates meeting only below (Fig. 12K-M). Up to seven arm spines, three dorsal and four ventral; dorsal arm spines two to three arm segments in length, thick, with smooth or rugose surface and lateral thorns; ventral arm spines shorter, two arm segments in length, smooth, or slightly rugose, thick, with pointed tip (Fig. 12L). First tentacle pore covered by two or three thickened, oval, blunt tentacle scales (Fig. 12J). Subsequent seven to eight tentacle pores covered by single similar oval scale (Fig. 12J). Further tentacle pores covered by one small scale, with rounded base and spinules at tip (Fig. 12L).

Color. In live specimen, orange-brown dorsal disc, pale color in arms and ventral disc, arm spines orange, disc spines and papillae red (Fig. 12).

Ossicle morphology. Arm spine articulations well developed, five in number, and placed at slight angle to distal edge of lateral arm plate. Volute-shaped perforated lobe forms dorsal and distal part of articulation, with large muscle opening and small nerve opening, and decreasing in size ventralwards (Fig. 13A). Distal half of inner side of lateral arm plate with group of small, irregular perforations parallel to row of spine articulations; a continuous ridge and a prominent knob close to ventral edge form vertebral articulation, shaped like a broad, nose-shaped beak (Fig. 13B). Dorsal arm spine thorny, with several longitudinal rows of perforations and widely spaced small thorns (Fig. 13C). Ventral arm spine short, rough, thorny surface with truncated apex (Fig. 13D). Vertebrae with streptospondylous articulation, short, broad podial basin at proximal end and narrow small distal end (Fig. 13E-I). Dorsal end of vertebrae distally acute and proximally flattened with longitudinal groove along midline (Fig. 13E, F). Ventral end of vertebrae with broad ambulacral groove and lateral ambulacral canals (Fig. 13G-I).

Distribution.

462-663 m depth, South China Sea, East China Sea, Japan Sea.

Remarks.

Ophioplinthaca lithosora was described by H. L. Clark (1911) in the genus Ophiocamax Lyman, 1878, and is currently accepted in Ophioplinthaca ( Stöhr et al. 2021). However, we could not find the taxonomic act that transferred it to Ophioplinthaca and assume that this decision may never have been formalized in a publication. We agree that it belongs in this genus on account of its deeply incised disc. Matsumoto (1917) considered O. lithosora in Ophiomitra Lyman, 1869, despite noticing the incised disc and enlarged marginal disc scales. These genera and Ophiocamax share indeed many other characters, but by molecular data they have not been found to be closely related and at least Ophiomitra may be polyphyletic (Christodoulou et al. 2019). Ophioplinthaca lithosora was previously recorded from the East China Sea and Japan Sea. It is easily recognized within the genus Ophioplinthaca by radial shield, oral frame, and tentacle scale characters (Table 3).

Ophioplinthaca cf. lithosora from the present study concurs with the holotype description in most respects, but it differs slightly by having contiguous dorsal arm plates, long spines in the center of the disc and few granules in the mouth angle at only some jaws. These granules are present at the second tentacle pore at the adoral shield at all jaw angles in the holotype, paratype and an additional specimen, all of which we examined from digital photographs. They are more obvious than in any other species of Ophioplinthaca and H. L. Clark (1911) included them in the series of oral papillae, which explains his high number of up to 15 lateral papillae at a single jaw edge. They may perhaps form a funnel around the tube foot. None of these specimens has long disc spines. However, the information about morphological variations within O. lithosora is still incomplete (H. L. Clark 1911), because it appears to be a rare species of which only few specimens are known. Therefore, these small morphological changes may represent intraspecific variation between O. lithosora specimens. We still cautiously refrain from fully associating our specimen with this species, due to the small differences between species in Ophioplinthaca .