Astroceras aurantiacum, 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 : 12-15

Astroceras aurantiacum sp. nov.

Figures 4A–D, 5

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- 2009-9202]; skeletal elements on SEM stubs [ MNHN IE- 2009-9203].

Etymology. The specific name is derived from the Latin word for orange, alluding to the colour pattern of this species.

FIGURE 4. A–D, Astroceras aurantiacum sp. nov., holotype MNHN IE-2009-9202. A, dorsal aspect showing colour pattern; B, ventral overview; C, ventral disk, lower two sectors lightly bleached to expose adoral plates; D, dorsal arm and disk section, bleached to reveal rib-like processes (arrow) of the lateral plates and radial shields. E, F, Astrogymnotes oharai sp. nov. holotype MNHN IE-2009-9204. E, dorsal aspect; F, ventral aspect. Scale bars 1 mm.

Holotype description. Disk diameter 4.5 mm, five non-branching arms, strongly coiled. Four arms proximally inflated, the fifth narrower, all tapering gradually beyond the inflated part, where they are as wide as high, ventral side flat. Arms almost touching, leaving only a narrow interradial space. Dorsal disk with smooth, round, low granules on the radial shields, embedded in thick skin, interradially naked, no scales visible. After bleaching of a radius, large radial shields exposed, almost as long as the disk radius, pairs contiguous, forming most of the dorsal disk. Dorsal arms covered with similar granules, proximally as four longitudinal rows, distally as three. Lateral arms naked. Proximally arms dorsally slightly noded, after bleaching, rib-like processes on the lateral arm plates are exposed. These support the skin, forming a chamber for the gonads. Two short, stump-like arm spines at lateroventral edge of arm, ventral spine longer than dorsal one. Spines on proximal arm thorny, distally turning into hooks with three, distalmost two, long thorns of about equal length, lamina indistinct. Granules on distal arm rugose (after bleaching).

FIGURE 5. SEM images of skeletal details from distal part of arm of Astroceras aurantiacum sp. nov., holotype, MNHN IE- 2009-9203. A, hook spine; B, granules on dorsal arm; C, lateral arm plate distal aspect of spine articulation; D, lateral arm plate external, spine articulation to the left; E, lateral arm plate, internal, spine articulation to the left; F, distal vertebra, distal face; G, vertebra, lateral aspect,; H, vertebra, ventral aspect, note the roof-like bar covering the proximal part; I, vertebra, proximal face. note the closed ventral nerve canal. Scale bars 0.1 mm.

Ventral disk and arms covered by thick naked skin, obscuring all plates. Jaws with large triangular apical papilla and five rectangular, massive teeth. At the lateral jaw edge a cluster of up to seven granules or papillae, a larger one higher up at the jaw. Second tentacle pore inside mouth angle, without scale or spine. Third tentacle pore lacking spine, fourth and fifth with one spine each, two spines on following segments. In each ventral interradius, almost vertically, a deep depression with two short bursal slits. After bleaching of an interradius, the adoral shields are just visible, triangular, distally 2.5 times as wide as proximally, with straight lateral edges and slightly convex distal edge, filling the whole interradius. Oral shields not visible, possibly absent. Gonads extend into arms.

Vertebrae from the distal arm closed over their proximal ventral half by a bridge-like flat structure, firmly fused to the sides of the vertebra and separating the lateral arm plates. Articulation streptospondylous. No sign of ventral arm plates anywhere along arm. Lateral arm plate from distal arm twice as long as high, curved in a wavelike shape at its distal edge to form the spine articulations, that consist of two round flat shapes projecting from the lower edge, with the upper edge roof-like above them. No regular dorsal arm plates, but ossicles embedded in the deeper layers of the skin on the middle to distal arm; none proximally in area of ribs.

Colouration dorsally white with narrow orange lines, dividing dorsal disk into sections, two of which are completely orange, the others white. Thin transverse orange lines at each arm segment junction, some wider bands of orange irregularly distributed along dorsal arms. Ventral disk and arms uniformly white.

Remarks. The closed vertebrae of streptospondylous type clearly place this species in the family Euryalidae . Its generic placement is more difficult. There are three euryalid genera with non-branching arms, Astroceras , Asteromorpha and Asterostegus . Asterostegus is characterized by dorso-lateral ridges on the arms, bearing prominent tubercles and by ventral disk plates distal to the adoral shields ( Fell 1960). Neither of these characters is present in the new species. In Asteromorpha the arms are covered with a dense coat of granules dorsally and laterally ( Fell 1960), while the new species has granules only on the dorsal arm. Astroceras has been diagnosed as having a naked skin with at most some tubercles or spines on the proximal arm ( Mortensen 1933a), but in Astroceras elegans (Bell, 1917) the arms may bear a dense cover of granules ( Baker 1980) and A. kermadecensis Baker, 1980 has a dense granulation all over the disk and arms, lacking any larger tubercles. The granulation is thus not a good character to differentiate these genera and Baker (1980) suggested that the structure of the lamina of the hook spines in Astroceras and Asteromorpha may be used to differentiate between both genera. Asteromorpha has a distinct smooth lamina, while Astroceras has an indistinct one, which concurs with the new species. The spine articulations of the new species are different from all previously presented ones ( Martynov 2010a).

Distribution. Known only from the type locality.