Brittle Stars, Ophiuroidea, contains the largest number of species within the Echinodermata and occurs in a great variety of marine habitats, such as muddy substrates, living infaunally in sediments, under rocks, in the interstices of Sponges and Hard Corals, and on surfaces of various Animals such as Soft Corals. They are globally one of the dominant deep-sea megafaunal groups, but diversity in chemosynthetic ecosystems such as hydrothermal vents and hydrocarbon seeps remains poorly understood. Since 1985, 10 species of Ophiuroids, including Ophiacantha longispina, Ophiactis tyleri, Ophienigma spinilimbatum, Ophiocten centobi, Ophioctenella acies, Ophiolamina epraewas, Ophiomitra spinea, Ophioplinthaca chelys, Ophiotreta valenciennesi rufescens, and Spinophiura jolliveti, have been collected from chemosynthetic ecosystems. As our knowledge grows, we have gained better understanding of Ophiuroids within these settings, and it appears Ophiuroid diversity has likely been underestimated. In Japanese waters, surveys of Ophiuroids have recorded 346 species, which represent approximately three-quarters of the North Pacific Ocean Ophiuroid fauna. However, no Ophiuroid species have been recorded from chemosynthetic habitats around Japan to date.
In a paper published in the journal Raffles Bulletin of Zoology on 17 April 2020, Masanori Okanishi of the Misaki Marine Biological Station of the University of Tokyo, Moe Kato of the School of Natural System at Kanazawa University, Hiromi Kayama Watanabe and Chong Chen of the Japan Agency for Marine-Earth Science and Technology, and Toshihiko Fujita of the National Museum of Nature and Science, describe two new species of Ophiuroids from chemosynthetic ecosystems in Japanese waters.
Both new species are placed in the enigmatic Ophiuroid genus Ophiambix, which was originally erected by Edward Lyman in 1880 for the monotypic Ophiambix aculeatus, and is known from the deep waters (146–5,315 m) of the Pacific and Atlantic Oceans. Out of the four species of Ophiambix currently considered to be valid, only Ophiambix aculeatus has been recorded from Japanese waters. Ophiambix has a characteristically flat body and their arms are well differentiated from the disc in a manner superficially similar to that of Asteroids.
The first new species is named Ophiambix kagutsuchi, in reference to Kagutsuchi, the god of fire in ancient Japanese mythology, referring to the hot-vent habit of the new species. The species was found at a series of hydrothermal vents in the Okinawa Trough, as well as on sunken wood found in the Ryukyu Trench, southwest of Japan, at a depth range of 276–1,979 m.
Ophiambix kagutsuchi, holotype (NSMT E-13071). (A) Aboral body; (B) aboral disc and proximal portion of arm; (C) oral disc and proximal portion of arms, arrow heads indicate oral papillae; (D) proximal portion of aboral surface of arm; (E) proximal portion of oral surface of arm; (F) distal portion of aboral surface of arm; (G) distal portion of oral surface of arm. Abbreviations: ASh, adoral shield; ASp, arm spine; D, dorsal arm plate; GS, genital slit; L, lateral arm plate; OP, oral plate; OS, oral shield; RS, radial shield; SD, supplementary dorsal plate; Te, tentacle scale; V, ventral arm plate. Scale bars 1 mm. Okanishi et al. (2020).
The disk of Ophiambix kagutsuchi is pentagonal, and about 4 mm in diameter. The aboral surface covered by small granules of almost uniform size, separated from each other, approximately 50–70 μm in diameter. Removal of granules shows underlying plates are scalar, circular in outline, and imbricating, each approximately 250–350 μm in diameter. Radial shields are triangular, distally wider, 350 μm in width, and 350 μm in length, sharpen towards the centre, and completely concealed by the granules. On the oral surface, the adoral shields are parallelogram-shaped, wider than long, approximately 250 μm in width, and 120 μm in length, one overlapping the other. The oral plates are trapezoidal, approximately 250 μm in width, and 180 μm in length at the radial edge, 250 μm in length at the abradial edge, and in contact with each other. The oral shields are pentagonal, slightly rounded, with a convex distal edge, approximately 360 μm in width, and 60–260 μm in length. The interradial oral disc area is narrow, covered by scales under thick skin, and approximately 200–300 μm in length. The genital slits are narrow, almost extending from the edge of the oral shield to two-thirds the height of the oral interradial disc, and 0.1 mm in width. The oral papillae and teeth are rudimentary, very thin, narrow and flat ossicles, approximately 10 μm in length, forming a continuous horizontal row on oral edge of dental and oral plates. The teeth and oral papillae quite similar in shape but for descriptive purposes, the ossicles on top of the dental plate are called teeth and ossicles on oral edge of oral plate are called oral papillae. With the exception of the oral-most row of papillae, there are 6 to 7 thin and fan-shaped teeth forming vertical row on the dental plate. There is a second tentacle pore inside the mouth slit.
Ophiambix kagutsuchi, paratype (NSMT E-13070), scaning electron microscope images of ossicles. (A)–(D) Vertebrae from proximal portion of arm, distal view (A), proximal view (B), oral view (C), aboral view, green illustration indicates 'T'-shaped groove (D); (E)–(G) lateral arm plates from proximal portion of arms, oral view (E), adoral view (F), inner view (G) an arrow head indicates a perforation; (H) ventral arm plates from proximal portion of arm, inner view; (I), (J) arm spines from distal (I) and proximal (J) portion of arm, arrow heads indicate secondary teeth. Orientations: ab, aboral side; ba, basal side; dis, distal side; ex, external side; in, inner side; o, oral side; pro, proximal side. Abbreviations: AF, aboral muscle fossae; DL, dorsal lobe; ES, elongated structure; LAC, lateral ambulacral canal; MO, muscle opening; NO, nerve opening; OF, oral muscle fossae; VL, ventral lobe. Okanishi et al. (2020).
On the proximally arm, theaboral surface is covered by small granules the same as those on the aboral disc, these decrease in number and disappearing on arm tip. After removal of the granules, the exposed dorsal arm plates are oblong, longer than wide, the proximal edge is slightly wider than the distal edge. There is a pair of fan-shaped supplementary dorsal arm plates on both lateral sides of each dorsal arm plate and irregularly shaped smaller supplementary plates on the distal side of the dorsal arm plates separating them from each other. On the middle portion of the arm, these supplementary plates disappear and the dorsal arm plates are in contact, gradually decreasing in size toward the arm tip. The lateral arm plates are thin, and widely separated from each other throughout the arm. The tentacle pore forms a large hole. On the proximal portion of the arm, the ventral arm plates are almost square with a concave distal edge, and toward the arm tip, become oblong, longer than wide, with a distal concave edge, contiguous throughout the arms. There are three flat and pointed arm spines on the proximal portion of each arm, the middle one is longest, the same length as the corresponding arm segment, the aboral-most half length of the middle spine, and the oral-most spine is shortest, approximately one-fifth length of the middle spine. The arm spines decrease in number to one toward the arm tip, transforming into a hook-shape, approximately the same length as the corresponding arm segment. There is one small, rudimentary tentacle scale at each tentacle pore, although small, tentacle scales present on the distal portion of each arm.
Ophiambix kagutsuchi, paratype (NSMT E-13050), scanning electron microscope images of ossicles. (A), (B) oral plates of abradial side (A) and adradial side (B), partly cracking, arrows indicate neural grooves; (C), (D) adradial genital plates, aboral view (C) and oral view, partly cracking (D); (E) inner view of a radial shield; (F) external view of a dental plate. Orientations: ab, aboral side; cen, centre of disc; o, oral side; per, periphery of disc. Abbreviations: AbMA, abradial muscle attachment area; AdMA, adradial muscle attachment area; AG, articulation for genital plate; DAT, depression for aboral tentacle; DOT, depression for oral tentacle; DW, presumable depression for water ring canal. Scale bars 100 μm. Okanishi et al. (2020).
The second new species is named Ophiambix macrodonta, which is a Latin adjective which means to have large teeth, referring to the flat, wide teeth of the species. The species is known only from a hydrocarbon seep site on Kuroshima Knoll, southeast of the Yaeyama Islands, part of the Ryukyu Island chain, southwest of Japan, at a depth range of 638–644 m.
Ophiambix macrodonta, 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. Okanishi et al. (2020).
The disc of Ophiambix macrodonta is pentagonal, and 6 mm in diameter. The aboral surface is covered by polygonal scales, approximately 250–350 μm in diameter, arranged in a mosaic pattern. Each scale is 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. The radial shields are oval, 850 μm in length and 500 μm in width, and almost completely concealed by granules except on the margins. The adoral shields are curved, bar-like, wider than long, approximately 500 μm in length and 150 μm in width, and separated from each other. The visible part of the jaws is trapezoid, approximately 500 μm in length and 250 μm in width, and contiguous. The oral shields are elliptical in shape, longer than wide, slightly acute on both adradial edges, and approximately 300 μm in length and 650 μm in width. The interradial oral disc area is narrow, covered by polygonal and mosaic scales, approximately 200–300 μm in length as those on aboral disc. The genital slits are long and wide, almost extending to the disc edge from distal edge of the oral shield, and 0.3 mm in width. The oral papillae and teeth are rudimentary, very thin, narrow and flat ossicles, approximately 10 μm in width, forming a horizontal row on the oral edge of the dental plate and oral plate. The second tentacle pore is inside the mouth slit.
Ophiambix macrodonta, holotype (NSMT E-13059) (A)–(E), (G), (H) and a paratype (NSMT E-13060) (F), scanning electron microscope 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. Okanishi et al. (2020).
The hollowtype of Ophiambix macrodonta has two complete arms 25 mm and 26 mm, while the other three arms have lost their tips. The proximal portion of the arms is 1.5 mm wide and 1 mm high, and eliptical in cross section. The arms taper gradually towards the tip. The aboral surface is covered by dorsal arm plates and supplementary dorsal arm plates, with no granules. On the proximal portion of the arm, the dorsal arm plates are semicircular, and wider than long; the proximal edge is straight. There is one large, fan-shaped supplementary dorsal arm plate on both lateral sides of the dorsal arm plate. Three smaller, polygonal plates are at the distal edges of the dorsal arm plates and the two larger supplementary dorsal plates. Toward the arm tip, these supplementary plates decrease in size and gradually disappear with the dorsal arm plates becoming contiguous. The lateral arm plates are thin, and widely separated throughout the arm. On the proximal portion of the arm, the ventral arm plates are almost square, with slightly concave lateral edges and oblong. On the distal portion of the arm, the ventral arm plates are longer than wide, concave, and the lateral edges more pronounced. The ventral arm plates are contiguous throughout the arms. There are three flat arm spines on the proximal portion of arms, the aboral-most and middlevspines are flat and leaf-like, and the oral-most spine is cylindrical, narrow, and pointed. All three spines are equal in length to the corresponding arm segment. Toward the tip, the aboral-most and middle arm spines transform to a cylindrical and pointed shape, and the oral-most one transforms into a hook, approximately the same length as the corresponding arm segment. There is one small, triangular tentacle scale at each tentacle pore, although small, tentacle scales are present on distal portion of the arm.
Ophiambix macrodonta, 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. Okanishi et al. (2020).
The difference in body size of the two new species Ophiambix macrodonta (3.3 to 7.0 mm in disc diameter) and Ophiambix kagutsuchi (0.8 to 3.2 mm in disc diameter) suggests that these could be interpreted as different sizes of the same species. However, the smallest specimen of Ophiambix macrodonta (3.3 mm in disc diameter), and the largest specimen of Ophiambix kagutsuchi (3.2 mm in disc diameter), were similar in size but exhibited the full set of respective diagnostic characters. All examined specimens of Ophiambix kagutsuchi, were collected from hydrothermal vents or sunken wood environments, whereas those of Ophiambix macrodonta, new species, were collected only from hydrocarbon seeps. Therefore, Okanishi et al. consider that these two taxa are indeed separate new species that can be distinguished from each other by morphological characters and by different environmental preferences.
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