Astrogymnotes oharai, Stöhr, Sabine, 2011

Stöhr, Sabine, 2011, New records and new species of Ophiuroidea (Echinodermata) from Lifou, Loyalty Islands, New Caledonia, Zootaxa 3089, pp. 1-50 : 16-18

Astrogymnotes oharai sp. nov.

Figures 4E, F, 6

Type material. Holotype, in 80% ethanol, sta. 1461, reef Shelter, 20°54.0'S, 167°02.1'E, 100–120 m, collected by dredging, [ MNHN IE- 2009-9204]; skeletal elements on SEM stubs [ MNHN IE- 2009-9205].

Comparative material. Astrogymnotes irimurai Baker et al., 2001 , holotype, Japan, Izu Archipelago, Nii-jima Island, 15 m, on Antipathes sp. B, scuba, collector I. Soyama, 25 June 1997, [ NSMT E- 3700]; 2 paratypes, Japan, Izu-Oshima Island, Akino-hama, 27 m, collector I. Soyama, 4 May 1997 [ NSMT E- 3702 and 3705]

Etymology. This species is named for Timothy D. O’Hara, eminent Australian ophiuroid expert, who once taught me the basics of ophiuroid taxonomy.

Holotype description. Disk diameter 5 mm, six arms, three larger and three smaller ones, indicating a fissiparous nature. Two arms broken off close to the disk, the others near their tip. Disk interradially deeply incised. Disk and arms covered with thick skin, in which scattered round, low granules are embedded, concentrated on the radial areas. Radial shields not visible, but indicated by granule-covered ridges. Scraping away the skin on one of these ridges proved the existence of bar-like long radial shields, widely separated from each other. No other scales or plates present.

Smaller granules form a dense cover on the dorsal side of the gradually tapering arms. Dorsal arm plates are obscured by skin. First arm segment under the disk spine-less, the following three bear a single spine each, which increases in length from a short stump to rod-like. From segment five there are two spines along the arm, of about equal length, longer than an arm segment, distally the ventral spine is shorter. Spines smooth, tapering towards a blunt tip.

Ventral disk covered by naked skin. Each of the fully developed jaws bears a pointed triangular apical tooth and three to four smaller, conical, lateral papillae at each edge. On the regenerating jaws the conical oral papillae form a cluster. All plates obscured by skin, including oral shields and madreporite, which are however visible when the skin is scraped off. Ventral arms likewise covered with thick skin, obscuring any plates, with small round to oval embedded granules. Ventral arm spines flat, wing-shaped on the proximal arm, rounded rectangular with indented distal edge on the distal arm. A single, semi-erect tentacle scale resembling a short arm spine or granule. Genital slits open, about 1.5 arm segments long; a strong abradial genital plate supports the outer edge.

Vertebral articulation zygospondylous with large proximal dorsal side flanges and smaller distal ones. Arm spine articulations on lateral arm plates vary in shape from round depressions with several larger holes to horseshoe shaped elevations with large hole towards the closed end. The corresponding articulation at the proximal end of the spine of curved shape with flat, denser surface and central hole. Isolated dorsal arm plate proximally wingshaped, twice as wide as long, curving inwards at about mid-length to a narrower distal part, with convex proximal and distal edges. Ventral plate about as wide as long, lateral edges straight, proximal and distal edges with deep notches.

FIGURE 6. SEM images of skeletal details of the distal arm of Astrogymnotes oharai sp. nov., holotype MNHN IE- 2009-9205. A, vertebra, distal face; B, vertebra, proximal face; C, vertebra, lateral aspect; D, lateral arm plate, external, note the weakly developed spine articulations; E, lateral arm plate internal aspect; F, arm spine, articulation; G, dorsal arm plate; H, ventral arm plate. Scale bars 0.1 mm.

Colouration of dorsal disk golden brown with darker irregular patches, white granules. Arms uniformly darker brown, in some radii a broad dark median line continues onto the arms. Ventral disk with large dark spots, arms and jaws light golden brown with small white spots that correspond to the granules; an interrupted darker brown median line or series of spots along the arms.

This animal was collected without its host, but there is no reason to assume that it was not associated with black coral like all its conspecifics.

Remarks. The arm spine articulation of Astrogmynotes has not been shown yet and the articulations found on the new species differ from those of Ophiomyxa and Ophiolycus ( Martynov 2010b) in being less pronounced and having a more porous stereom. It may still be a variation of the articulation typical for Ophiomyxidae , but further study is needed. The genus Astrogymnotes formerly included four species, the pentamerous A. thomasinae Baker et al., 2001 and A. hamishia Baker et al., 2001 , and two hexamerous ones, A. irimurai Baker et al., 2001 and the type species A. catasticta H. L. Clark, 1914 ( Baker et al. 2001) . The colour pattern of the new species resembles the holotype of A. irimurai , from which it differs by its smaller and more sparsely distributed granules, and the longer, thinner and smooth arm spines that don’t turn into hooks. A paratype of A. irimurai showed a darker, almost black colour with larger white patches than the holotype and differed even more from A. oharai . In that specimen, the madreporites or oral shields were obscured, whereas they are obvious, covered only with thin skin and white in colour in the holotype. Another paratype had three small, obviously regenerating, arms and three large ones, an indication of the fissiparous nature of the species. Astrogymnotes oharai differs from A. catasticta in the colour pattern and length of arm spines. The intraspecific variation of A. oharai is not known since only a single specimen was found, but it is probably fissiparous and somewhat variable in colouration. The largest species is A. catasticta , which reaches a disk diameter of 9 mm, but A. thomasinae with a holotype of 7 mm may possibly reach a similar size, while the remaining species have only been found at sizes of about 4–5 mm dd. Unusual among ophiuroids, the ventral side is often more brightly coloured in these species, and they orientate themselves with the ventral side outwards, dorsal side towards their coral host.

Distribution. The depth distribution of the species of Astrogymnotes is probably dependent upon that of their hosts, the black coral Antipathes spp., and varies greatly, with A. irimurai occurring at the shallowest depth at 15 m and A. hamishia at 1225 m at the greatest depth. The new species was found at medium depth of 100–120 m, similar to A. catasticta , which has been found at 55–205 m depth (Liao & Clark 1995). So far, the widest geographic distribution has been documented for A. catasticta , found off Western Australia (type locality), the Philippines and Southern China ( Clark 1911; Liao & Clark 1995). Astrogymnotes irimurai has only been found in Japan. The remaining two species were described from Kermadec Islands; A. hamishia has also been found in New Caledonia (O'Hara & Stöhr, unpublished result).