Ophioplexa condita, 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 : 79-85

Ophioplexa condita gen. et sp. nov.

Figures 4J, K, N, P–S; 17L–O; 54A–E; 55A–H; 56; 67F

Material. Holotype, dried, ZMMU D-788, R / V “ Akademik Kurchatov”, cruise 11, sta. 896, 05.12.1971, 56° 52' S 24° 59' W – 56° 51' S 24° 59' W, depth 5651– 5530 m, Sigsbee trawl. One paratype, dried, ZMMU D- 789, same sta. as holotype. One paratype, ethanol, ZMMU D-790, same sta. as holotype. One paratype, mounted on SEM stub, ZMMU D-791, same sta. as holotype. One paratype disarticulated and mounted on SEM stub, ZMMU D-792, same sta. as holotype. Two paratypes, dried, ZMMU D-793, R / V “Akademik Kurchatov”, cruise 11, sta. 870, 29.11.1971, 55° 7' S 25° 2' W – 55° 8' S 25° 1' W, depth 4704– 4680 m, Sigsbee trawl.

Etymology. from Latin conditus, hide, concealed, in reference to some morphological (small concealed radial shields and genital plates) and ecological (abyssal environment) features of the new species.

Description of the holotype. The disk is damaged, approximately 18 mm dd, presumably flattened when alive, round and not indented interradially. The disk plates are numerous, most of them elongated and narrow, irregular-rhomboidal and polygonal, entirely devoid of spines and granules. There is a thin skin layer, which is conspicuous in ethanol specimens, but almost undetectable when dried. Radial shields are very small and entirely concealed under disc scales (see a detailed radial shield description under internal and other characters below). The interradii are slightly swollen, ventrally covered with numerous scales similar to the dorsal ones. Each jaw bears three to five irregularly placed, narrow, spiniform ventralmost teeth, which are not distinguished from more dorsally placed teeth, nor from adjacent lateral oral papillae. Distal oral papillae and adoral shield papillae are similar in size and shape, and placed along both edges of a jaw in a characteristic slightly curved row around the second tentacle pore, approximately 5–6 in number (including 2–3 adoral papillae in the strict sense). Thus there is a continuous series of papillae from the teeth to the adoral shield papillae. Deeper in the mouth opening on the jaws there is a distinct group of papillae, placed laterally to the other oral papillae, 2–4 in number. Numerous spiniform teeth are arranged in a dorsalwards tapering coneshape on the dental plate. There are 17–20 teeth (including “apical papillae”). The oral shield is wide, triangular, with straight or slightly convex, distal edge, about as long as wide, completely separated from the first lateral arm plate by the adoral shields. Distally the oral shield is contiguous with a supplementary oral shield in form of a narrow elongated plate or just an elevation formed by adjacent interradial scales. The madreporite is similar in shape to the others, but slightly larger and has a swollen round area in the middle, conspicuously smaller than the entire shield. Adoral shields wing-like laterally, widely adjoining the arm, rapidly narrowing towards the jaws, thus retaining only a narrow bar between jaws and oral shield. True genital slits are small openings under the adoral shields, but long furrows between the proximalmost arm segments and the ventral disk interradius form false slits. Arm length is about 1.4 times the disk diameter.

The dorsal arm plates are well developed but thin, fan-shaped, moderate in size. Few proximal plates are wide, rapidly narrowing towards the middle of the arm, contiguous along most of the arm length. The proximal edge of the dorsal arm plate is almost straight throughout the length of the arm, but triangular on the basal segments. A few proximal dorsal plates have a pointed proximal angle, whereas the majority of them possess a short straight proximal edge. Two arms show irregular bands of numerous elongated scales intermingled with the lateral and dorsal plates. These proximal scales are similar to dorsal disk scales. The second dorsal arm plate near the disk is narrow, elongated transversally, bar-shaped or oval. On two arms the proximalmost dorsal plates are absent for 4–5 segments, whereas in one arm small scales almost entirely conceal the first two proximal segments. Arms are weakly noded and form a low lateral ridge on which the arm spine articulations are placed. There are two or four arm spines on the most proximal segments under the disk, six to seven rather adpressed spines on the free proximal and middle segments, and five distally. Dorsal and ventral spines are similar in size. Arm spines are flattened, pointed apically. 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 contiguous. The tentacle pores are larger and more conspicuous proximally than towards the distal end of the arm. Basally on the part of the lateral plate that encircles the tentacle pores, three flattened, apically rounded tentacle scales are placed. The tentacle scales are shorter than the arm spines, but conspicuous and large enough to cover the whole tentacle pore. Many of the tentacle scales are broken or have fallen off. Towards the distal end of the arm the number of tentacle scales is reduced to two, and the inner scale is shorter than the outer one. The ventral arm plates do not bear any tentacle scales.

Paratype variations. A paratype, from sta. 870, 8 mm dd, differs from the holotype in having an almost undamaged irregularly-pentagonal disk, evenly covered with numerous, mostly elongated scales, which continue dorsally onto 1–2 proximal arm segments. Almost all dorsal arm plates, except for the 1–2 most proximal, have a pointed, narrowly triangular proximal edge, and are separated by lateral plates, to a small degree proximally, but up to half of the dorsal plate length distally. The pattern of the oral papillae is similar to the holotype, but the papillae are significantly fewer, and the curved row of oral papillae around the second tentacle pore is evident only in some interradii. Numerous spiniform teeth do not differ from the 3–4 apical papillae. Two, rarely three papillae on the adoral shield. Oral shields are more lengthened than in the holotype, and the madreporite is just an inconspicuous spot within the oral shield. Most of the oral shields have a remarkable feature of one or two rather conspicuous scales overlapping the shield distally, as a continuation of the supplementary oral shield – several scales adjacent distally to the oral shield. In other characters the paratype is similar to the holotype.

Another paratype, also from sta. 870, 5.5 mm dd, differs from the holotype and other paratype in size, an irregularly pentagonal disk, evenly covered with numerous polygonal and oval scales, which are continued dorsally on 2–3 proximal arm segments. The pattern of the oral papillae is also distinct from the holotype and from the paratype from the same station (870), but these differences clearly reflect the juvenile nature of the present specimen. The papillae are significantly smaller in number, and the curved row of oral papillae around the second tentacle pore is indistinct and can be traced only in two interradii. One or two distalmost oral papillae are block shaped. At least one papilla on the adoral shields is present in one interradius, whereas in the other interradii it has most probably fallen off. Oral shields are more elongated in length than in the holotype, and the madreporite is just an inconspicuous spot within one oral shield. Most of the oral shields are overlapped by a conspicuous scale distally. On the most proximal lateral arm plates under the disk there are 2 spines, on the proximal lateral plates 4–5 spines, towards the middle 5–6. Spines are flattened, all similar in size. Tentacle scales are similar to the holotype, but many are broken or have fallen off. In the middle part of the arm the number of tentacle scales is reduced to two.

Internal and microstructural characters. Radial shield and genital plate articulate very tightly, unlike most extant ophiuroids, and have the appearance of an entire plate broken by a fissure ( Figs 4J, K, P). The articulation surface of the radial shield is distinctly excavated ventrally, with a distal groove ( Figs 4R, S). The articulation surface of the adradial genital plate lacks distinct condyles ( Fig. 4Q). The abradial genital scale is absent. Jaws are slightly elongated ( Figs 56O, P). Adradial sides of the jaws distally bear few sharp straight folds ( Fig. 56P). The dental plate is large with rounded sockets and irregular folds ( Figs 56M, N). Arm spine articulations compressed transversally ( Figs 55G–H; 56A–H). Some proximal articulations may have a distinct nerve opening ( Figs 56D–E). Few ventralmost articulations on the proximalmost segments have a distinctive slit-shaped appearance ( Figs 56C–E). Vertebrae short, distinctly keeled and with well developed zygospondylous articulation ( Figs 17L–O). Vertebral dorsal median groove almost indistinct ( Fig. 17L). Podial basins are large ( Fig. 17M).