Ophionereis degeneri ( A.H. Clark, 1949 )

Stöhr, Sabine, 2011, New records and new species of Ophiuroidea (Echinodermata) from Lifou, Loyalty Islands, New Caledonia, Zootaxa 3089, pp. 1-50 : 32-34

Ophionereis degeneri ( A.H. Clark, 1949)

Figure 14A , 15

Material. Sta. 1455: 1 spm; sta. 1456: 2 spms; sta. 1450: 1 spm; sta. 1465: 3 spms (1 used for SEM); sta. 1467: 2 spms.

Description. At 4.5 mm dd dorsal disk covered by round, imbricating scales of uniform size. Radial shields small, overlapped and separated by scales. Dorsal arm plates trapezoidal, twice as wide as long, distal edge shorter than proximal, contiguous. Stereom with bead-like structure. Supplementory dorsal arm plates half-circle shaped, conspicuous, as long as lateral edges of ventral plates. Three blunt arm spines, shorter than an arm joint.

Ventral disk covered by scales similar to dorsal disk. Oral shield triangular to spearhead-shaped, longer than wide, madreporite larger, possibly deformed in figured specimen. Aboral edge of bursal slit lined with granule-like genital papillae along its entire length, visible at distal edge of radial shield from above. Ventral arm plates axeshaped, twice as wide as long, dorsal edge twice as wide as proximal edge, concave, lateral edges indented for tentacle pores. Single large oval tentacle scale.

Dental plates rectangular, flat, externally four tooth sockets visible, two dorsal ones with a pair of deeper fenestrations, only dorsalmost socket completely perforating plate, visible internally, ventral sockets low depressions. Oral plates with smooth irregular rectangular abradial muscle attachment area, adradial distal “hinge” with numerous horizontal bars and grooves. Proximal lateral arm plates 1.5 times as wide as long, spine articulations u-shaped, internally a small hole near tentacle pore. Vertebrae zygospondylous, with proximal depression and distal projection (keel), dorsal surface with longitudinal groove. Radial shield triangular, distal edge shorter than proximal edge, lateral edge (towards adjacent shield) longest, with distal strong ball-like condyle.

Colouration in ethanol a mottled light and dark brown on dorsal disk, radial shields lighter, arms variegated light brown with dark bands.

Remarks. One of the largest spms with a dd of 4.5 mm was selected for examination in comparison with O. porrecta . Ophionereis degeneri is superficially similar to O. porrecta , but it can be distinguished by a number of characters. The dorsal disk scalation is coarser and lacks the differentiation into smaller central scales and larger marginal radial scales observed in O. porrecta (see below). The supplementary dorsal arm plates are conspicuously larger, the arm spines shorter and at least proximally the ventral spines are flat and wide with a truncated tip, in contrast to the conically tapered spines of O. porrecta . The distal edge of the ventral arm plates is concave, while it is slightly convex in O. porrecta . The oral shield is longer than wide, while it is as wide as long in O. porrecta . Also the internal skeleton shows differences in the perforations of the dental plate, the shape of the oral plate and the radial shield, but since only one specimen of each species has been examined, more data are necessary to determine intraspecific variation of these characters. In addition, O. degeneri does not grow larger than 7 mm dd (Clark & Rowe 1971), whereas O. porrecta reaches a size of more than 10 mm dd.

Devaney (1974a) synonymized O. degeneri with O. porrecta without explanation. Guille and Ribes (1981) considered their small specimens as juveniles of O. porrecta , although they exhibited characters of O. degeneri and apparently no comparisons with young O. porrecta were made. The Lifou material shows that at a size of 4.5 mm dd both species can be distinguished without doubt and there is no reason to believe that smaller specimens of O. porrecta would show characters of O. degeneri that then somehow vanish during further development. In the Lifou material, the smallest specimen of O. porrecta measured 3.2 mm dd and was clearly identifiable by its disk scalation, longer arm spines and smaller accessory arm plates. The size range of O. degeneri was 2.6– 4.5 mm dd and all of these showed the distinguishing characters of the species. The size of disk scales is usually relatively larger in small juvenile brittle stars, which can cause difficulties in identification, particularly at stages that have not yet developed the supplementary arm plates. In large specimens the size of these supplementary plates varies and may approach either species. However, the shorter arm spines in O. degeneri and the larger marginal disk plates in O. porrecta are reliable characters.

According to T. O'Hara, O. degeneri is similar to O. lineata H.L. Clark, 1946 , O. terba Baker & Devaney, 1981 and O. novaezelandiae Mortensen, 1936 (recognized as different from O. fasciata Hutton, 1872 by O'Hara), but they all have small accessory dorsal arm plates (O'Hara, personal communication).

Distribution. This is a species of shallower shelf areas, at Lifou it was found at 15–90 m depth. Its true geographic distribution is unknown due to the taxonomic confusion, which has probably resulted in it being reported as O. porrecta in many cases. Both species occur in sympatry at least in part of their range. With just nine specimens of O. degeneri found at Lifou, the species seems to be more limited in its distribution and possibly narrower in its ecological requirements or more difficult to collect due to a cryptic life-style (living in holes in dead coral; T. O'Hara personal communication). This is a new record for New Caledonia.

Figures

FIGURE 15. SEM images of Ophionereis degeneri, 4.5 mm disk diameter. A, dorsal aspect; B, dorsal interradius; C, dorsal arm; D, lateral arm; E, ventral aspect; F, ventral arm; G, dental plate, external; H, dental plate, internal; I, oral plate, abradial; J, oral plate, adradial; K, proximal lateral arm plate, external; L, proximal lateral arm plate, internal; M – O, proximal vertebra; M, distal face; N, proximal face; O, dorsal; P, radial shield, internal. Dental plates orientated with dorsal end up; oral plates with distal end to the left; radial shield with proximal end to the left. Scale bars in millimetre.

FIGURE 14. A, Ophionereis degeneri, dorsal aspect. B, Ophionereis porrecta, dorsal aspect. C, D, Ophiomastix caryophyllata, juvenile; C, dorsal aspect; D, ventral aspect. E, Ophiocoma erinaceus, probably night colouration. Scale bars in millimetre.