EURYALIDA LAMARCK, 1816

Stöhr, Sabine, 2005, Who’s who among baby brittle stars (Echinodermata: Ophiuroidea): postmetamorphic development of some North Atlantic forms, Zoological Journal of the Linnean Society 143 (4), pp. 543-576 : 545-546

ORDER EURYALIDA LAMARCK, 1816 FAMILY ASTERONYCHIDAE MÜLLER & TROSCHEL, 1840

ASTERONYX LOVENI MÜLLER & TROSCHEL, 1842

( FIG. 1A–Q )

Ophiuraster patersoni Litvinova, 1998

Ophiomyxa serpentaria – Stöhr, 2004 (postlarvae only) The habitus of the smallest postlarvae with a relatively large disc and short, not clearly offset arms is more similar to Asteroidea than Ophiuroidea . Among North Atlantic ophiuroid postlarvae this is a unique shape. The smallest individuals measure 1.8 mm dd, with two arm segments, which is the largest for any species known so far, but slightly smaller than those presented by Mortensen (1912). The postlarvae are characterized by large, slightly raised primary plates, a thick skin, evident particularly on the naked areas of the dorsal interradius, and large plates bordering the disc margin ( Fig. 1A ). The TP is bulbous, ending in three short thorns and oval RSs are present ( Fig. 1B ). The DP bears a rugose, triangular tooth and on each lateral edge a low, wide, rugose MP. The oral plates of adjacent jaws are connected by transparent skin ( Fig. 1C ). Growth is allometric, with arm length increasing faster than disc size.

At 2.5 mm dd, the TP is hollow and inflated with a wide distal opening ( Fig. 1I ). Narrow RSs are present in addition to the primaries ( Fig. 1D ), and the LAP bears two hook-shaped arm spines ( Fig. 1F ). The large marginal plates have been interpreted as LAP of the first arm segment, which move to the ventral side of the disc during development and form the ASs ( Mortensen, 1912; Stöhr, 2004; Fig. 1E ). The vertebrae seem to have an hour-glass-shaped articulation, but their undeveloped condition makes them difficult to interpret ( Fig. 1G, H ).

At 3.2 mm dd, long bar-like RSs are visible beneath the thick skin, overlaid by several round overlapping plates on their proximal half ( Fig. 1J ). In the centre of the disc, fragments of the primary plates with irregular edges are barely visible, perforated by large fenestrations. There are no DAPs and the LAPs bear two hook-shaped spines. The TP is bulbous, hollow and composed of several plates ( Fig. 1L ). The jaw bears a rugose apical tooth and 2–3 smaller rugose MP to either side ( Fig. 1K ). There is no ASS and the M is the only OS. The VAP is pentagonal with straight lateral and distal edges and proximal angle; the following plates are widely separated by the LAPs. Short bursal slits are present below the raised oral frame, next to the first arm segment. The ventral disc is naked, covered with thick skin. The vertebrae show an hourglass-shaped articulation ( Fig. 1P, Q ).

At 4 mm dd, the proximal ends of the RSs are overlaid by numerous plates and the primary plates have disappeared completely ( Fig. 1M ). The LAP bears four hooks, the ventral ones with two teeth and the dorsalmost spine with only a terminal tooth ( Fig. 1O ). The TP tapers to a narrow end with a small opening. There are still three small, widely spaced MP and the deeper teeth are larger and more blocklike than the first tooth ( Fig. 1N ). At this size, the animals can be identified using keys based on adult characters.

Remarks: The small postlarvae of Asteronyx loveni have been described as Ophiuraster patersoni by Litvinova (1998), but Stöhr (2004) showed that O. patersoni is a postlarva. Unfortunately, the growth series was then mistakenly associated with Ophiomyxa serpentaria . There are striking similarities between small A. loveni and O. serpentaria such as the thick skin, the serrated mouth papillae, the absence of dorsal arm plates and the hook-shaped arm spines (on the distal arm in adult O. serpentaria ). Recently obtained clearly identifiable larger juveniles of A. loveni suggested that the postlarvae previously presented as O. serpentaria are actually younger individuals of A. loveni , which changes the synonymy of O. patersoni once again. Mortensen’s (1912) description supports this conclusion.

The large ‘marginal plates’ of the small postlarvae were interpreted as ASs by Stöhr (2004), which concurs with Mortensen (1912). They lie next to the first ‘true’ arm segment, enclosing the LAP of that segment. RSs are present in the smallest individuals on top of the LAP of the first arm segment ( Fig. 1A ), which is unusual for such a young stage. However, these animals may not represent the earliest postlarva and examination of metamorphosing larvae is required.

Postlarvae of A. loveni are unmistakable among the species presented here. They have their closest affinities with Ophiuraster belyaevi Litvinova, 1998 from the Kerguelen Islands, which is probably the young of another species of Asteronyx .

ORDER OPHIURIDA MÜLLER & TROSCHEL, 1840 FAMILY OPHIOMYXIDAE LJUNGMAN, 1867

Figures

Figure 1. Asteronyx loveni postlarval development, note the thick, partly removed, skin. A, 1.8 mm dd postlarva dorsal; B, arm of same postlarva, note the radial shields; C, same individual ventral, note the large adoral shields; D, 2.5 mm dd postlarva dorsal; E, same individual ventral; F, arm of same individual lateral; G, vertebra proximal face; H, vertebra distal face; I, bulbous terminal plate; J, 3.2 mm postlarva dorsal; K, same ventral; L, arm distal, note the wide terminal plate; M, 4 mm dd postlarva dorsal; N, same ventral; O, arm spines of same; P, 3.2 mm postlarva vertebra proximal face; Q, distal face. Abbreviations: AS, adoral shield; LAP, lateral arm plate; M, madreporite; PP, primary plates; RPP, radial primary plates; RS, radial shield. Scale bars in millimetres.