Silax cf. abditus (A. M Clark, 1970)

Stöhr, Sabine & O’Hara, Timothy D., 2021, Deep-sea Ophiuroidea (Echinodermata) from the Danish Galathea II Expedition 1950 - 52, with taxonomic revisions, Zootaxa 4963 (3), pp. 505-529 : 525-527

Silax cf. abditus

Figure 5

Material examined

EAST PACIFIC OCEAN •> 100 specimens; off Nicaragua; 09°23’N, 089°32’W; 3570 m; 06 May 1952; Galathea II stn. 716; dark, muddish, clay; NHMD-86700, 868859, SMNH-186206, SMNH-186209 • 4 specimens; Gulf of Panama; 05°44’N, 079°30’W; 2950–3190 m; 12 May 1952; Galathea II stn. 724; dark clay, stones; NHMD-868701.

Descriptive remarks

Disc diameter 16 mm arms missing, except for short proximal parts, complete length unknown. Dorsal disc covered by numerous small round scales, six larger plates in disc centre probably primary rosette widely separated by scales, devoid of granules or thorns. Radial shields semicircular, proximal end slightly narrower than distal, pairs separated by several disc scales, distally connecting. Dorsal arm plates oval, twice as wide as long, consecutive plates separated by lateral plates. Up to eight arm spines proximally, middle spines longest, tapering pointed, smooth, slightly longer than a segment. Articular structures for spines on lateral arm plates with bent parallel lobes, of amphiurid type.

Ventral disc covered by small round scales similar to dorsal disc. Bursal slits slightly longer than half interradius length, abradial edge lined by larger scales. Jaws with stout block-like infradental papillae below the wide, straight-edged teeth. Lateral oral papillae comprised of two wide scales (possibly modified adoral shield spines), and between these and the infradentals the tip of the buccal scale, extending from its position higher up on the oral plate. Oral shield spearhead-shaped, as long as greatest proximal width, short distal lobe. Madreporite about twice as large as other oral shields, with several hydropores. Adoral shields isosceles triangular, bordering lateral sides of oral shield, not meeting proximally. Ventral arm plates quadrangular with central keel, lateral edges concave. Tentacle pores only partially covered by single wide low tentacle scale, weakly curved, placed at ventral arm plate edge, and across junction between lateral and ventral arm plate on proximal segments.

Variations

In some specimens, the oral shields lack the distal lobe and have an isosceles triangular shape. Tentacle scales may be lacking distalwards from segment four, but it is unknown if this is a natural variation or damage from handling. Some specimens had only seven or six arm spines proximally. These specimens were unusually fragile for larger ophiuroids, but it is unknown if this is typical for the species or caused by rough handling or inadequate initial preservation (most likely in formalin). Many of the larger disc and arm plates cracked during drying or were already cracked. Arms were mostly missing, except segments close to the disc edge and those under the disc.

Remarks

These specimens exhibit much morphological similarity to Triplodia abdita , the only specimen of which originated from the Gulf of Panama, where also some of our specimens were collected. This genus was established by A.M. Clark (1970) for this specimen, which appears to lack a buccal scale. However, the oral papillae (and tentacle scales) within Silax species can vary greatly in shape, and they are often malformed. The tip of the buccal scale appears between infradental and second adoral shield in our specimens, but due to the small size of these ossicles, they are hard to recognize without SEM. Consequently, here we treat the only species T. abdita as an extreme morphotype of Silax . Unfortunately, the holotype of T. abdita appears to be missing from the MCZ (A. Baldinger, personal communication) and morphological details cannot be verified. The type figure shows two supplementary plates on the distal side of the oral shield. However, these may be misrepresented enlarged disc scales that are positioned close to the oral and adoral shields as we find in some of our specimens ( Fig. 5E, F ). This uncertainty precludes us from fully identifying our material as S. abditus at this point.

Silax cf. abditus resembles S. verrilli with regard to the oral skeleton, dorsal disc, and keeled ventral arm plates, but S. verrilli has four to five arm spines and no tentacle scales. Silax daleus has a similar oral skeleton but three arm spines, flat ventral arm plates and a smaller tentacle scale. Silax cernuus (Lyman, 1879) and Silax patulus (Lyman, 1879) also have only three arm spines, whereas S. magnificus ( Koehler, 1908) possesses four spines. The differences are small and arm spine numbers may vary somewhat within species. The presence or absence of tentacle scales among individuals of the same species seems more inconsistent than in other ophiuroid genera, but they may be removed by rough handling during collection.

Figures

FIGURE 5. Silax cf. abdita, Galathea II stn. 716, from lot NHMD-868859. A, dorsal disc section, B, dorsal arm, C, central dorsal disc, D, dorsolateral arm, E, ventral aspect, F, jaws with madreporite, G, proximal ventral arm, H, dorsal disc, I, ventral aspect, J, dorsal arm, K, lateral proximal arm. A–G, SEM images, SMNH-189206, 189209, I–K, digital photos of dried speci- men, NHMD-868700. AdS, adoral shield, CPP, central primary plate, DAP, dorsal arm plate, IPa, infradental papillae, LAP, lateral arm plate, M, madreporite, OPa, oral papillae, OS, oral shield, VAP, ventral arm plate. Scale bars = 1 mm.