Ophiambix macrodonta, Okanishi & Kato & Watanabe & Chen & Fujita, 2020

Okanishi, Masanori, Kato, Moe, Watanabe, Hiromi Kayama, Chen, Chong & Fujita, Toshihiko, 2020, Large populations of two new species of Ophiambix (Echinodermata, Ophiuroidea) discovered on Japanese hot vents and cold seeps, Raffles Bulletin of Zoology 68, pp. 196-213 : 202-206

Ophiambix macrodonta , new species

(New Japanese name: Ōba-kaede-kumohitode) ( Figs. 5 –10)

Holotype. R / V NATSUSHIMA, ROV HYPER-DOLPHIN: NSMT E-13059 (1 specimen, dry), cruise NT04-03, Dive #0300, Kuroshima Knoll seep site, Ryukyu Islands, Sakishima Islands, Okinawa Prefecture, southwest of Japan, 24°07.822′N, 124°11.538′E, 638 m, 25 April 2004.

Paratypes. NSMT E-13060 (1 specimen, dry) , NSMT E-13061 (1 specimen, dry) , NSMT E-13062 (1 specimen, dry) , NSMT E-13063 (1 specimen, dry) , NSMT E-13064 (1 specimen, dry) , NSMT E-13065 (1 specimen, ethanol) , NSMT E-13058 (131 specimens, ethanol) , NSMT E-13057 (108 specimens, ethanol), same locality as the holotype . NSMT E-13066 (6 specimens, ethanol), cruise NT04-03, Dive #0299, Kuroshima Knoll seep site, Ryukyu Islands, Sakishima Islands, Okinawa Prefecture, southwest of Japan, 24°07.833′N, 124°11.558′E, 641 m, 24 April 2004 . NSMT E-13067 (12 specimens, ethanol), cruise NT04-03, Dive #290, Kuroshima Knoll seep site, Ryukyu Islands, Sakishima Islands, Okinawa Prefecture, southwest of Japan, 24°07.828′N, 124°11.56′E, 641 m, 13 April 2004 .

R / V NATSUSHIMA, DOLPHIN 3 K : NSMT E-13068A (7 specimens, ethanol) , NSMT E-13068B (1 specimen, ethanol), collected with slurp gun, cruise NT02-07, Dive #0558, Kuroshima Knoll seep site, Ryukyu Islands , Sakishima Islands, Okinawa Prefecture, southwest of Japan, 24°07.812′N, 124°11.366′E, 644 m, 22 May 2002 .

Diagnosis. Aboral disc covered by small granules in a circular arrangement around periphery of disc scales, but arms not covered by such granules. Arm spines on proximal portion of the arms flattened and leaf-like. Teeth flattened, plate-shaped and forming a horizontal row on oral-most edge of dental plate. Two tentacle scales on each tentacle pore, spiniform, conspicuously smaller than tentacle pore (width/ length: 1/3–1/4) on proximal portion of arm ( Table 1).

Etymology. The specific epithet “macrodonta” is a Latin adjective which means to have large teeth, referring to the flat, wide teeth of the new species.

Description of holotype (NSMT E-13059). Disc. Pentagonal, 6 mm in diameter ( Figs. 5A, B , 6A ). Aboral surface covered by polygonal scales, approximately 250–350 μm in diameter, arranged in mosaic ( Figs. 5D , 6B ). Each scale covered by small granules of uniform size, approximately 40–60 μm in diameter, in contact with each other and forming two or three circular rows on the periphery of each scale ( Figs. 5D , 6B, C ). Radial shields oval, 850 μm in length and 500 μm in width, almost completely concealed by granules except on the margins ( Figs. 5D , 6B ). Adoral shields curved, bar-like, wider than long, approximately 500 μm in length and 150 μm in width ( Fig. 5F, G ), separated from each other. Visible part of jaws trapezoid, approximately 500 μm in length and 250 μm in width, contiguous ( Fig. 5G ). Oral shields elliptical in shape, longer than wide, slightly acute on both adradial edges, approximately 300 μm in length and 650 μm in width ( Fig. 5G ). Interradial oral disc area narrow, covered by polygonal and mosaic scales, approximately 200–300 μm in length as those on aboral disc ( Figs. 5G , 6E ). Genital slits long and wide, almost extending to the disc edge from distal edge of oral shield, 0.3 mm in width ( Figs. 5F, G , 6E ). Oral papillae and teeth rudimentary, very thin, narrow and flat ossicles, approximately 10 μm in width, forming a horizontal row on oral edge of dental plate and oral plate ( Figs. 5H , 6E ). Second tentacle pore inside the mouth slit ( Figs. 5H , 6E ).

Arms. Five, two complete arms 25 mm and 26 mm, while the other three arms have lost their tips. In proximal portion, 1.5 mm wide and 1 mm high, shape of section elliptical. Arms tapering gradually toward the arm tip.

Aboral surface covered by dorsal arm plates and supplementary dorsal arm plates, no granules ( Figs. 6G , 7A ). On proximal portion of the arm, dorsal arm plates semicircular, wider than long, proximal edge straight ( Figs. 6G , 7A, B ). One large, fan-shaped supplementary dorsal arm plate on both lateral sides of the dorsal arm plate ( Figs. 6G , 7B ). Three smaller, polygonal plates at distal edges of the dorsal arm plates and the two larger supplementary dorsal plates ( Figs. 6G , 7A ). Toward the arm tip, these supplementary plates decrease in size and gradually disappear with dorsal arm plates becoming contiguous ( Fig. 7C ). Lateral arm plates thin, widely separated throughout the arm ( Figs. 5F , 7D, E ). On proximal portion of the arm, ventral arm plates almost square, slightly concave lateral edges ( Figs. 5E , 6H , 7D ) and oblong. On distal portion of the arm, ventral arm plates longer than wide, concave lateral edges more pronounced ( Fig. 7E ). Ventral arm plates contiguous throughout the arms. Three flat arm spines on proximal portion of arms, aboral-most and middle spines flat and leaf-like, and the oral-most spine cylindrical, narrow, and pointed ( Figs. 6G, H , 7A, D ). All three spines equal in length to corresponding arm segment ( Figs. 6G, H , 7A, D ). Toward the tip, the aboral-most and middle arm spines transforming into cylindrical and pointed shape, and oral-most one transforming into hook, approximately same length as corresponding arm segment ( Fig. 7C, E, F ) (see detailed descriptions of each ossicle in next section). One small, triangular tentacle scale at each tentacle pore ( Fig. 7D ), although small, tentacle scales present on distal portion of the arm ( Fig. 7E ).

Description of ossicle morphology of holotype (NSMT E-13059) and a paratype (NSMT E-13060, NSMT E-13061, NSMT E-13064, NSMT E-13068B). Vertebrae streptospondylous-like articulation with zagapophyses framing the water vascular canal on proximal side ( Fig. 8A ); with wing-like muscle flanges on oral-distal side ( Fig. 8 A–D); longitudinal groove on oral side and a pair of lateral ambulacral canals opening inside the groove ( Fig. 8D ); “T”- shaped groove on aboral side ( Fig. 8C ); narrow and thick, long lateral shield covering lateral-oral side ( Fig. 8 A–D), forming a large depression around oral fossae on proximal side ( Fig. 8A ). Adoral shield bar-like, boomerang-shaped, flaring to distal side ( Fig. 8E, F ). Adradial genital plates long and bar-like, slightly thinner on inner side, approximately five times longer than wide ( Fig. 8G ), having slit like articulation on external edge ( Fig. 8H ).

The first lateral arm plate oblong, wing-like, inner side wider than external side, tentacle notch distinctively concave on inner distal position (Fig. 9A). Excluding first lateral arm plate, lateral arm plates on proximal portion of arm oblong, but inner side thinner than external side (Fig. 9B). A perforation on centre of inner side (Fig. 9B). Three equally sized spine articulations on distal edge (Fig. 9C, D), composed of equally sized dorsal and ventral lobes that are horizontally parallel (Fig. 9C, D). Nerve and muscle openings between the lobes, separated by thin bridge (Fig. 9C). On middle portion of arm, lateral arm plates oblong ( Fig. 8E ) with thin leaf-like projection to proximal side (Fig. 9F, H). On oral side, one well-defined elevated structure on external side (Fig. 9E). The elevated structure with finer meshed stereom than surrounding area (Fig. 9E). No distinct perforation observed on inner side (Fig. 9E). Three spine articulations on distal edge (Fig. 9G, H), composed of horizontal dorsal and ventral lobes, generally parallel but opening toward external side (Fig. 9G). Aboral-most articulation largest, decreasing in size orally (Fig. 9G). Dorsal lobes basically larger than ventral lobes (Fig. 9G, H). Nerve and muscle openings between the lobes, separated by bridge (Fig. 9G, H). Granules on aboral side of the disc cone-shaped, having some dozen thorns on aboral side (Fig. 10A, B). On proximal portion of arm, aboral-most and middle arm spines flattened while oral-most spine cylindrical (Fig. 10C, D). On distal portion of arm, arm spines transforming into hook with maximum of three secondary teeth (Fig. 10E). Ventral arm plates oblong, longer than wide, slightly concave on both lateral and distal edges (Fig. 10F).

Oral plates irregularly oblong, with notches for water ring canals and oral tentacles on aboral periphery and oral central part, respectively (Fig. 10G, H). A depression for an aboral tentacle located just below the notch for water ring canal (Fig. 10H). Abradial and adradial muscular attached area on middle portion of abradial side (Fig. 10G) and central portion (Fig. 10H) of adradial side, respectively. A presumable depression for water ring canal on aboral central part of abradial side (Fig. 10G). Radial shield circular, slightly pointing to periphery of the disc, with two small and large depressions, indicating articulations for adradial genital plates at periphery of disc on inner side (Fig. 10I). Dental plate oval, stereom mesh more dense on aboral side than on oral side, without distinctive articulations for teeth (Fig. 10J).

Colour in life. Uniform creamy white colour, with brown tentacles ( Fig. 6D ).

Distribution. Japan: Only known from the hydrocarbon seep site on Kuroshima Knoll, southeast of Yaeyama Islands, Ryukyu Islands, southwest of Japan. Depth range 638–644 m ( Fig. 1 ).

Remarks. Ophiambix macrodonta , new species, can be distinguished from the other four congeners and Ophiambix kagutsuchi , new species, described above, by the following four features (see also Table 1):

(1) Armament of aboral body. Aboral disc of Ophiambix kagutsuchi , new species, O. devaneyi , and O. epicopus are covered by small granules which are in contact with each other, whereas those of O. aculeatus and O. meteoris are covered by spines. Ophiambix macrodonta , new species, also possesses small granules circularly arranged to surround periphery of disc scales on aboral surface of disc. Aboral arms of Ophiambix kagutsuchi , new species, O. aculeatus , O. devaneyi , and O. epicopus are covered by small granules, whereas those of O. meteoris are covered by spines, and those of O. macrodonta , new species, are not covered by any granules or spines.

(2) Shape and arrangement of teeth. Teeth of Ophiambix macrodonta , new species, and O. kagutsuchi , new species, are flat, forming a horizontal row on the oral-most edge of the dental plate, but those of O. aculeatus , O. devaneyi , O. epicopus , and O. meteoris are spiniform, scattered on dental plate without forming a regular row.

(3) Shape of arm spine. The second from oral-most arm spines are flat and leaf-like in Ophiambix devaneyi , O. epicopus , and O. macrodonta , new species, slightly flat, but not leaflike, and relatively long in O. kagutsuchi , new species, and cylindrical and spiniform in O. aculeatus and O. meteoris .

(4) Number, size, and shape of tentacle scales. Ophiambix macrodonta , new species, and O. kagutsuchi , new species, have one small (1/4–1/5 length of the corresponding arm segment), spiniform (width/length: 1/3–1/4) tentacle scale, O. epicopus has one small (1/3 length of the corresponding arm segment), spiniform (width/length: 1/4) tentacle scale, O. devaneyi has two large (2/3–1/2 length of the arm segment), spiniform (width/length: 1/3) tentacle scales, O. meteoris has four or five, large (1/2–1/3 length of the arm segment), spiniform (width/length: 1/3–1/5) tentacle scales, and O. aculeatus has three small (approximately 1/8 length of the arm segment), spiniform (width/length: 1/1) tentacle scales on each tentacle pore on proximal portion of arm.

Figures

Fig. 5. Ophiambix macrodonta, new species, holotype (NSMT E-13059). A, aboral body; B, oral body; C, aboral disc and proximal portion of arm; D, aboral periphery of disc and proximal portion of arm; E, oral disc and proximal portion of arms; F, oral periphery of disc and proximal portion of arm; G, a jaw; H, top of a jaw, arrow heads indicate teeth and oral papillae. Abbreviations: ASh, adoral shield; DP, dental plate; GS, genital slit; L, lateral arm plate; OP, oral plate; OS, oral shield; RS, radial shield; Te, tentacle scale; V, ventral arm plate. Scale bars = 1 mm.

Fig. 6. Ophiambix macrodonta, new species, holotype (NSMT E-13059) (A–E, G, H) and a paratype (NSMT E-13060) (F), SEM images. A, aboral disc and proximal portion of arm; B, aboral periphery of disc and proximal portion of arm; C, granules (arrow heads) on aboral disc; D, oral disc and proximal portion of arms; E, jaws, an arrow head indicates teeth or oral papillae; F, a jaw from lateral view, arrow heads indicate teeth and oral papillae; G, proximal portion of aboral surface of arm, arrow heads indicate supplementary plates; H, proximal portion of oral surface of arm. Abbreviations: ASh, adoral shield; ASp, arm spine; D, dorsal arm plate; GS, genital slit; RS, radial shield; V, ventral arm plate.

Fig. 7. Ophiambix macrodonta, new species, holotype (NSMT E-13059). A, proximal portion of aboral surface of arm, a part enlarged in (B), arrow heads indicate supplementary plates; C, distal portion of aboral surface of arm; D, proximal portion of oral surface of arm; E, distal portion of oral surface of arm; F, distal portion of lateral surface of arm. Abbreviations: ASp, arm spine; D, dorsal arm plate; L, lateral arm plate; Te, tentacle scale; V, ventral arm plate. Scale bars = 1 mm.

Fig. 8. Ophiambix macrodonta, new species, holotype (NSMT E-13059) (A–D), a paratype (NSMT E-13061) (E, F), and a paratype (NSMT E-13064) (G, H), SEM images of ossicles. A–D, vertebrae from proximal portion of arm, proximal view (A), distal view (B), aboral view, green indicates “T” shaped groove (C), oral view (D); E, F, a pair of adoral shields, oral views; G, H, adradial genital plate, aboral (G) and oral (H) views. Orientations: ab, aboral side; dis, distal side; o, oral side; pro, proximal side. Abbreviations: AF, aboral muscle fossae; LAC, lateral ambulacral canal; LS, lateral shield; OF, oral muscle fossae; SA, slit-like articulation; Z, zygapophyses.

Fig. 1. A, known distribution of Ophiambix aculeatus, O. devaneyi, O. epicopus, O. kagutsuchi, new species, O. macrodonta, new species, and O. meteoris; B, enlarged area, sampling localities of examined specimens of Ophiambix aculeatus, O. epicopus, O. kagutsuchi, new species, and O. macrodonta, new species, in this study.