Ophiocymbium antarcticus, Martynov, 2010

Martynov, Alexander, 2010, Reassessment of the classification of the Ophiuroidea (Echinodermata), based on morphological characters. I. General character evaluation and delineation of the families Ophiomyxidae and Ophiacanthidae 2697, Zootaxa 2697, pp. 1-154 : 60-62

Ophiocymbium antarcticus sp. nov.

Figure 42

Material. Holotype, USNM 1106231 , South Atlantic Ocean , Scotia Sea , South Sandwich Islands, NE of Candlemas Island, Cr. 575, sta. 38, 22 May 1975, 57° 00,24' S 26° 10,06' W depth 2740–2757 m, Trawl- Blake. No paratypes.

Etymology. This species is named after Antarctic.

Description of the holotype. The disk, ca. 13 mm dd, is pentagonal. The disk plates are numerous, somewhat obscured by thin wrinkled skin when dried, extending across the first 1–3 arm segments, substituting for the dorsal arm plates. Radial shields are very small irregular plates, similar in size to disk scales, placed at the edge of the disk and scarcely visible. The whole dorsal side of the disk is devoid of spinelets. The interradii are slightly swollen. Each jaw bears apically a cluster of 4–6 spiniform teeth or apical papillae. The 2–6 lateral oral papillae together with the adoral shield papillae are placed along both edges of a jaw in a characteristic, slightly curved row around the second tentacle pore. The adoral shield papillae are long, slightly flattened or rounded, similar in size and shape to the arm spines, 2–3 in number in most interradii. There is a distinct gap between adoral papillae and apical papillae. Between lateral oral papillae and second tentacle pore the jaws form an indistinct rectangular border, which is superficially similar to the blockshaped oral papillae. Beyond this border deeper on the jaws there is small triangular papilla, not evident in all radii. Numerous spiniform teeth are distributed somewhat irregularly. There are approximately 10–12 teeth (including apical papillae) on each dental plate. The oral shield is wide, with straight or slightly convex distal edge, as long as wide, completely separated from the first lateral arm plate by the adoral shields. The madreporic oral shield has a conspicuous swollen oval area occupying the distal portion. The adoral shield is poorly developed, laterally having a narrow wing, widely adjoining the arm, rapidly tapering towards to the midline of the jaws, thus retaining only a narrow inconspicuous bar between the jaws and oral shield. Genital slits are long furrows opening widely proximally, not supported by genital plates.

Most arms are broken off. The length of the longest arm piece is 13 mm. On all arms the most proximal dorsal plates are absent for two segments. The dorsal arm plates are well developed but rather small, triangular, well separated on all segments (for about half of the length of the dorsal arm plate proximally and length of the dorsal plate distally). On most proximal segments the dorsal arm plates are somewhat irregular, and have traces of fusion from several pieces and additional small plate distally. All dorsal arm plates have pointed proximal and convex distal edges. Arms have weakly developed nodes, but arm spine articulations are placed on lateral plates without forming an elevation. Laterally between two adjacent lateral plates there is a narrow space of uncovered soft tissue. There are two spines on the first arm segment, three to four on most proximal segments under the disk, five on the proximal free segments, and distally four to three spines. The first dorsal spine is slightly longer than the other ones proximally and medially. Spines are slightly flattened to round, hollow, pointed apically. There are no hook-shaped spines. The first ventral arm plate is broadly triangular, the second has straight distal and proximal edges; further ventral arm plates have a characteristic pole-axe shape throughout most of the arm length, with an almost straight proximal and slightly convex distal edge. Ventral plates throughout the length of the arm are contiguous. The tentacle pores are large throughout the entire arm length. Basally on the parts of the lateral plate encircling the tentacle pores, there are two long, subequal, flattened, pointed tentacle scales, only slightly shorter than the adjacent arm spines. Two tentacle scales persist for at least the three most proximal segments. On most arm segments there is one tentacle scale. Distally the tentacle scale gradually decreases in size. The tentacle scales are shorter than arm spines, but conspicuous and capable of covering the whole tentacle pore. Many of the tentacle scales are broken or have fallen off. There are no tentacle scales on the ventral arm plates.

Internal and microstructural characters. The radial shield is a small sub-rounded plate. The adradial genital plate is absent. Arm spine articulations are well defined and belong to the double-opening type ( Fig. 42E). The muscle opening is on a distinct round or elongated elevation, some articulations are slightly ribbed distally. The nerve opening is placed laterally below level of the muscle opening. The ventralmost artciulations have a peculiar oblique slit-shaped shape ( Fig. 42F). Vertebrae are rather short, not keeled, with well developed zygospondylous articulation ( Fig. 42G–H). The vertebral dorsal median groove is not distinct. Podial basins are large ( Fig. 42I).

Remarks. Ophiocymbium antarcticus sp. nov. is clearly distinguished from both Ophiocymbium cavernosum and Ophiocymbium tanyae sp. nov. by the presence of a distinctly pentagonal disk, less evident distal block-shaped oral papillae, and the shape of the arm spine articulations and vertebrae. Ophiocymbium antarcticus sp. nov. is somewhat similar to Ophiocymbium ninae sp. nov. but differs in lacking disk spinelets, the shape of the tentacle scales and the arm spine articulation details. The disc of Ophiocymbium antarcticus sp. nov. is almost twice as larger as the largest known example (7 mm dd) of 26 known O. ninae sp. nov. specimens.