Ophiocymbium tanyae, 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 : 57-58

Ophiocymbium tanyae sp. nov.

Figures 4H; 12C–E, H; 17P–S; 27J, K; 40E, F

Material. Holotype, dried, ZMMU D-799, R / V “Vityaz”, sta. 7407, 29° 18' N 143° 15' E – 29° 17' N 143° 15' E, Izu-Bonin Trench, depth 6740–6850 m, Sigsbee trawl. Four paratypes, ethanol, ZMMU D-800, same sta. as holotype. One paratype, disarticulated and mounted on a SEM stub, ZMMU D-801, same sta. as holotype.

Etymology. This species is named in honour of neurophysiologist and my wife Tanya Korshunova (Moscow, Institute of Higher Nervous Activity and Neurophysiology) for her generous assistance and patience.

Description of the holotype. The disk measures 9.5 mm dd, rather flattened, pentagonal with rounded angles. Numerous disk plates, most of elongated shape, narrowed, irregular-rhomboidal or polygonal, entirely devoid of spines and granules. Disk scales continue dorsally and extend onto the first arm segment. Skin layer is thin and not conspicuous. Radial shields and both genital plates are absent. The interradii are slightly swollen, ventrally with numerous scales. Each jaw bears one larger spiniform apical papilla and 1 –2 smaller adjacent ones. Apical papillae are distinct from the teeth, the latter are narrower and longer and only one in number in most interradii except a single jaw with two teeth. The proximalmost oral papillae are small, spiniform, typically placed in pairs. Three papillae are placed distally on each side of the jaws and characteristically block-shaped, appearing rather as modified, originally spiniform, papillae attached to the jaws over almost their entire length. The distalmost oral papillae are shorter than the middle one, which is rather long and rectangular, but pointed distally, the most proximal block-shaped papilla is more pointed, and in some interradii almost spiniform. The second tentacle pore is covered by two similarly sized, flattened and rounded scales, which are placed on the adoral shield and thus can be equally considered as both adoral shield spines or tentacle scales. The oral shield is wide, triangular or somewhat rhomboidal, with straight or slightly convex distal edge, as long as wide, completely separated from the first lateral arm plate by the adoral shields. Distally the oral shield adjoins a supplementary narrow plate or just an elevation formed by adjacent interradial scales. The adoral shield is wing-shaped laterally, widely adjoining the arm, narrowing towards the midline of the jaws, retaining a moderately narrow bar between the jaws and oral shield. True genital slits are barely conspicuous small openings under the adoral shields, but long furrows between the most proximal arm segments and the ventral disk interradius form a false slit.

Arm length is almost equal to the disk diameter. The dorsal arm plates are well developed but rather small, triangular, narrowing and turning pentagonal towards the middle of the arm, well separated on all segments (for about the length of the dorsal arm plate proximally and twice the dorsal plate distally). All dorsal arm plates have pointed proximal and straight distal edges. Arms have weakly developed nodes but arm spine articulations are placed on lateral plates without forming an elevation. On the most proximal segments under the disk there are two conical spines, on the proximal and distal segments there are three (rarely four) spines. The second dorsal spine is slightly longer than others. The first ventral arm plate is triangular. Further ventral arm plates have a characteristic pole-axe shape throughout entire arm length. Medially ventral arm plates have a distinct elevation, making the plates somewhat roundly carinate. The proximal edge of the ventral arm plate is straight, whereas the distal edge is convex with a small pit in the middle. Ventral plates throughout the length of the arm are separated, slightly so on the proximal segments, and similar in length distally. The tentacle pores are large. On the first three arm segments under the disk there are distinct large tentacle scales, placed on the lateral plates. On the first segment there are two flattened rounded tentacle scales, whereas on the second and third segments the scales become more spiniform. The tentacle scales are similar in size to the arm spines and capable of covering a considerable part of the tentacle pore. On the remaining segments tentacle scales are entirely absent. There are no tentacle scales on the ventral arm plates.

Paratype variations. A paratype in ethanol, from the same station as the holotype, 12.0 mm dd, is similar to the holotype, but differs in minor ways. The first 1–2 proximal segments near the disk lack dorsal arm plates and small scales, exposing the vertebrae. There are no teeth on any dental plates except for the conspicuous middle apical papilla and 1–2 adjacent smaller apical papillae. Oral shields are smaller than in the holotype, triangular or irregularly oval. The madreporite has lost its usual shape and is markedly swollen. On the most proximal lateral arm plates under the disk there are two spines, on proximal and middle lateral plates three (occasionally four) flattened apically pointed spines.

Internal and microstructural characters. Radial shield and both genital plates are absent ( Fig. 4H). Jaws are elongated ( Figs 12C–D). Adradial sides of the jaws distally irregularly folded ( Fig. 12D). Dental plate without folds, with few small rounded sockets ( Fig. 12E). Arm spine articulations are poorly developed and clearly belong to the single-opening type ( Fig. 27J, K). The unevenly rounded or slightly subparallel muscle opening lacks a ribbed distal area and a distinct lobe. The nerve opening is not evident. Vertebrae rather long, not keeled and with well developed zygospondylous articulation ( Fig. 17P, R). Vertebral dorsal median groove absent ( Fig. 17P). Podial basins are large ( Fig. 17Q).

Remarks. Ophiocymbium tanyae sp. nov. resembles O. cavernosum in general disk appearance but differs from it in several important characters. All arm segments of O. cavernosum are entirely devoid of conspicuous tentacle scales (except for the first segment bearing a very small spiniform scale), whereas Ophiocymbium tanyae sp. nov. has a conspicuous, wide, flattened tentacle scale on the first three proximal segments. The absence of the tentacle scales in O. cavernosum is confirmed in the present study for the holotype and several specimens from the Atlantic and Pacific oceans. Another important difference of Ophiocymbium tanyae sp. nov. is the complete absence of adradial and abradial genital plates and the radial shields, confirmed for all studied specimens ( Fig. 4H). In contrast, O. cavernosum has a small, but evident adradial genital plate and radial shield ( Fig. 4G); confirmed also for the holotype.