Ophiocymbium rarispinum, 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 : 62-64

Ophiocymbium rarispinum sp. nov.

Figure 40G, H; 43; 67G

Material. Holotype, dried, ZMMU D-798, 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.

Etymology. From Latin rarus (rare, few) and spina (spine), in reference to few short scattered disk spinelets.

Description of the holotype. The disk, 9.2 mm dd, is flattened and rather rounded, not extending onto the proximal arm segments. The disk is covered by numerous small scales, slightly obscured by thin skin, giving the disk a wrinkled appearance. Few short spinelets are very sparsely placed in some areas of the dorsal disk. Radial shields are very small but conspicuous at the edges of the disk. Distally part of the articulating genital plate is also evident. The interradii are flattened, ventrally not covered with spinelets. There are no evident genital slits, but between arms and disk a long groove is present, which is superficially similar to the genital slit. A narrow adradial genital plate articulating with the radial shield is evident in the distal part of the ventral side of the disk, but does not support the walls of the genital slits. There is no abradial genital plate.

Each jaw bears a cluster of 4–6 short papillae apically. There is no larger middle apical papilla. Few other teeth are placed more dorsally on the dental plates, similar in shape and size to the apical papillae. The total number of teeth including apical papillae is 8–10, distributed in a dorsalwards tapering cone-shaped pattern. The oral papillae are represented by two well-defined block-shaped elevations. The third distalmost oral papilla is short and rounded. 1–2 small spiniform papillae also placed in some interradii lateral to the apical papillae, at the ventral edge of the dental plate. The adoral shield papillae are long, slightly flattened, very similar in size and shape to the arm spines, one papilla is placed in the middle of each adoral shield.

The oral shield is relatively narrow, triangular with straight or slightly convex rather long distal side, conspicuously longer than wide, completely separated from the first lateral arm plate by the adoral shields. Distally the oral shield adjoins a supplementary oral shield in form of a narrow elongated plate or just an elevation formed by adjacent interradial scales. The madreporic oral shield is considerably swollen. Adoral shield wing-shaped laterally, widely adjoining the arm, rapidly narrowing towards the mid-line of the jaws, retaining thus only a narrow bar between jaws and oral shield. True genital slits are barely conspicuous small openings under the adoral shields, but a long furrow between the most proximal arm segments and the ventral disk interradius forms a false slit.

Arm length is about 1.5 times the disk diameter. On all arms the most proximal dorsal plates are absent for 3–4 segments, covered only with thin skin, in which few small scales are embedded; vertebrae are clearly visible through translucent skin, or skin entirely absent exposing the vertebrae. 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 twice the length of the dorsal plate distally). All dorsal arm plates have pointed proximal and slightly convex distal edges. Many of the dorsal arm plates have a pit in the middle of the distal edge. Arms have weakly developed nodes but arm spine articulations are placed on lateral plates without forming an elevation. Between two adjacent lateral plates there is a narrow space of uncovered soft tissue. There are two flattened, apically pointed arm spines on the first arm segment, four on most proximal segments under the disk, four on the proximal free segments, and three distally. All spines are similar in size. There are no hook-shaped spines. The first ventral arm plate is broadly triangular, most further ventral arm plates have a characteristic pole axe shape throughout the entire arm length with straight proximal edge and convex distal edge with a small elevation. Ventral plates throughout the length of the arm are separated, except for first two segments. The tentacle pores are large throughout the entire arm length. There are no tentacle scales except for the first arm segments, where there is a single small scale placed on the inner part of the lateral arm plate, differing from the following arm spines only in size and absence of the defined articulation ridge.

Internal and microstructural characters. The radial shield is a small sub-rounded plate. The adradial genital plate is absent. Arm spine articulation belongs to the double-opening type ( Figs 43A–C). The muscle opening is placed on a distinct round or elongated elevation, some articulations slightly ribbed distally. The nerve opening is placed laterally below the level of the muscle opening. Ventralmost spine articulations have a rounded shape ( Fig. 43C). Vertebrae rather long proximally, not keeled and with well developed zygospondylous articulation ( Fig. 43I). The vertebral dorsal median groove is slightly distinct. Podial basins are large ( Fig. 43H).

Remarks. Ophiocymbium rarispinum sp. nov. has a similar arm spine articulations shape to O. cavernosum , but differs in having visible radial shields and small sparsely-placed disk spinelets.