Amphiura ungulata Madsen, 1970

Stöhr, Sabine & Alme, Øydis, 2015, Sometimes two arms are enough — an unusual life-stage in brittle stars (Echinodermata: Ophiuroidea), Zootaxa 3994 (3), pp. 425-432 : 427-429

Amphiura ungulata Madsen, 1970

Figures 1 , 2

Material examined (adults/juveniles). 7AN-01(2/0), 7AN-06 (2/0), 5C-10 (2/0), 5C-14 (2/0), 5C-16 (4/0), 5C-17 (1/0), 7GH-13 (1/2), 7GU-04 (3/4), 7GB-03 (7/9), 7GB-07 (1/17), 7GB-08 (0/5), 7LI-10 (3/18), 7LI-12 (0/6), 6 N- 0 1 (1/16), 5SP-03 (1/0), 5SP-05 (1/0), 7SP-07 (0/2), 7SL-02 (2/2), 7SL-04 (0/2), 2012404-SL29 (0/41).

Diagnosis (after Madsen, amended). Species of Amphiura characterized by three arm spines, the middle and ventral ones appearing hoof-shaped in cross section due to a deep longitudinal furrow. Tentacle scales lacking, distal oral papilla spiniform. Juveniles exhibit unequal growth of arms and disc, at first only two grow to great length, later the remaining three arms develop. The disc is at first narrow and elongate, later growing to its round symmetrical shape.

Distribution. Coastal waters off West Africa, from northern Angola to Morocco, at 15–50 m, type locality off Liberia at 22 m ( Madsen 1970). Records from India need to be verified.

Description. Adults. (Based on several specimens). Dorsal disc often lost, diameter of oral frame about 1 mm, disc diameter about 2–3 mm ( Fig. 1 A). Disc more or less round, covered by numerous small round to irregular oval imbricated scales, with minute perforations ( Fig. 1 A, B). No primary plates discernible. Radial shields bar-like narrow and long, partly overlapped by disc scales, pairs contiguous along entire length ( Fig. 1 B), sometimes missing at a radius. Five arms, all broken in all specimens, sometimes two longer and wider than other three. Greatest arm width some distance from disc, under disc narrowest. Dorsal arm plates about twice as wide as long, oval, convexly bent around the arm ( Fig. 1 C). Three arm spines. Ventralmost spine longest, as long as an arm joint, slightly bent, with longitudinal furrow, hoof-shaped in cross section, edges with small thorns pointing distalwards ( Fig. 1 C). Middle spine wider, slightly shorter, with deep furrow, hoof-shaped in cross section, thorns along edges. Dorsal spine shortest, thicker than other spines, but compressed, about half as long as an arm joint, pointed, hollow, closed at tip, with two wide lateral slits with thorny borders. Oral papillae consisting of a pair of rounded oval infradentals, a high buccal scale and a short spiniform distal papilla ( Fig. 1 D, E). Ventralmost tooth square. Oral shields rounded pentagonal with convex proximal and distal edges, madreporite larger and bulging ( Fig. 1 E). Adoral shields wing-like reaching around the oral shield. Ventral arm plates rounded pentagonal with convex distal edge, proximal angle and concave lateral edges, separated by lateral arm plates ( Fig. 1 F). No tentacle scale.

Juveniles. Specimens with elongated oval disc 1 mm long, 0.3 mm wide ( Fig. 2 A–G): Disc strongly elongated, oval, covered by numerous oval to round scales with small perforations, primary plates are not distinguishable. Radial shields beginning to form at the disc edge above the arms. Two arms, opposite to each other at short disc edges, broken at 4–5 mm length. Dorsal arm plates bell-shaped, stereom with small perforations, neighbouring plates separated from each other by lateral arm plates. Oral frame pentamerous with five jaws. Ventralmost tooth large, triangular. Oral papillae consisting of buccal scales along each jaw edge and a pair of short infradentals laterally at the dental plate. No adoral shield spine on narrow adoral shields. At three radii, two adjacent to each other, one opposite across the disc, a bulbous, hollow terminal arm segment is present, separated from the corresponding first ventral (oral) arm plate by few small scales. Oral shields rounded triangular, madreporite not distinguishable. Three arm spines, ventral and middle spine almost as long as one joint, with deep longitudinal furrow for 3/4 of spine length, from a thickened base to tip, causing the appearance of a hoof-shape in transect. Numerous thorns along the lateral edges of the furrow, pointing distalwards. Dorsal spine thicker, shorter, with three thorny longitudinal ridges terminating in a pointed tip. Ventral arm plates on proximal arm just separated by lateral arm plates, longer than wide, with slightly convex proximal and distal edges, laterally concave for tentacle pore; stereom tuberculous. Tentacle pores large, lacking scales. Lateral arm plates meeting dorsally and ventrally, spine articulations with parallel lobes. A second specimen of the same size shows one wider oral shield with a hole at its proximal end, possibly the madreporite with hydropore.

Specimen with disc 1.53 mm long, 0.61 mm wide ( Fig. 2 H): Similar to smaller specimens, but three underdeveloped arms have grown to about 20 segments (two broken). Ventral disc covered by scales similar to dorsal disc. Mouth parts close to adult condition, including the presence of a short, spiniform adoral shield spine (= distalmost oral papilla), but madreporite not obvious.

Specimen with disc 2.85 mm long, 0.9 mm wide ( Fig. 2 I, J): Five arms, two larger at opposite short edges of disc, two smaller adjacent at one long edge, third short arm in between these at opposite long disc edge. Bar-like narrow, long radial shields visible above longest arms. Ventralmost spine longest, longer than a joint. At intact arm a bulbous terminal segment. Distal oral papilla pointed spiniform, at proximal end of adoral shield (= adoral shield spine). Infradentals in final position.

Figures

FIGURE 1. Morphology of Amphiura ungulata, adults. (A) dorsal disc, (B) radial shields, (C) arm dorsolaterally with spines, (D) ventral aspect, disc lost, (E) oral frame, (F) ventral arm. M, madreporite. SEM images, scale bars in millimetre.

FIGURE 2. Morphology of Amphiura ungulata, juveniles. (A) elongated dorsal disc, lacking primary plates, (B) dorsal arm, (C) ventral overview, (D) oral frame and pattern of arms, note the terminal plates of three undeveloped arms marked by arrows, (E) ventral arm, (F) hoof-shaped spines, (G) specimen with possible madreporite, (H – J) the three underdeveloped arms are growing in larger juveniles. M, madreporite, TP, terminal plate. Scale bars in millimetre. SEM images except O, P.