Ophioscolex glacialis, MULLER & TROSCHEL, 1842

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 : 546-551

OPHIOSCOLEX GLACIALIS MÜLLER & TROSCHEL, 1842

( FIG. 2A–I )

The animals are covered with thick skin, transparent in small stages, obscuring the plates in the larger individuals. The smallest postlarva found has a dd of 0.5 mm, with a round, convex disc, and the short arms consisting of two segments are vertically erect above the disc ( Fig. 2A ). The dorsal part of the disc consists of five plates of different sizes and shapes. The plates overlap at the edges and meet in the centre, while the CPP appears to be lacking. The plate stereom is perforated by large oval fenestrations, which extend to the plate edge, giving it a jagged appearance. The TP of the arms bears 3–5 long, simple terminal spines. Each of the two arm segments bears two large arm spines on either side with several secondary teeth and a broader base. The jaw appears to end in the DP; no tooth is visible.

At 0.9 mm dd, a CPP can be recognized, surrounded by a circle of seven similar, round RPPs. The plate fenestrations are smaller than before, larger in the central part of the plates, and smaller on the margins ( Fig. 2B ). There are still only two multipointed, fenestrated spines to each side of an arm segment. The jaw is short, lacking oral papillae ( Fig. 2C ). The ASS is similar to the following arm spines.

In the larger postlarvae, the disc has a sac-like shape, which in combination with the often upwards directed arms results in a habitus similar to that of the ophiacanthid genus Ophiomyces . With four arm segments and 1.7 mm dd ( Fig. 2D–G ), the number of dorsal disc plates has increased, but individual plates cannot be traced back to earlier stages. RSs appear to be lacking. Each LAP bears three thorny spines and the TP bears a cluster of five spines ( Fig. 2F ). The ventral disc is covered with smaller round plates similar to the dorsal plates. A spiniform tooth has formed, accompanied by two shorter spiniform MP on the DP ( Fig. 2G ). On the AS, next to the second TPo, a spinelike ASS is present, although this has fallen off on most plates, leaving the spine articulation visible ( Fig. 2G ).

Until the animals possess about 11–12 arm segments, the arms tend to stand vertical above the high sac-like disc. In larger animals, the disc flattens and the arms retain a more horizontal position. At 3 mm dd there are two apical papillae forming a cluster with the tooth, widely separated from the single lateral papilla to each side of the jaw; all are spiniform ( Fig. 2H, I ). These characters are close to the adult state, allowing identification using common keys.

In adults, the thin glassy scales are deeply embedded in the skin. There is a cluster of spiniform papillae on the DP and 1–2 MP on the distal oral plate ( Fig. 2J, K ). The ASS is the only spine next to a TPo, all other pores being scale-less.

Remarks: The postlarva of Ophioscolex glacialis is unusual with its Ophiomyces -like ( Ophiacanthidae ) habitus, which distinguishes it clearly from other postlarvae known so far. However, the spiniform mouth papillae and the thick skin distinguish it from Ophiomyces . In Ophiomyces , the sac-like shape of the disc has been attributed to the lack of radial shields ( Paterson, 1985), which are thought to stabilize the disc. Possibly, the same is true for O. glacialis , because no radial shields can be distinguished among the disc plates of the postlarvae.

OPHIOSCOLEX PURPUREUS DÜBEN & KOREN, 1846

( FIG. 3A–J )

A single specimen with 0.6 mm dd and five arm segments is smallest among all postlarvae found of this species. The dorsal part of the disc consists of the CPP and five RPPs, with small RSs visible beneath the RPPs on the arm bases. The OSs are visible at the disc edge, one of them with a short stump-like protrusion identifying it as the M. The plate margins bear few short, conical, serrated spines with wide round bases. Plate fenestrations are rather small and the plate margins are almost imperforate ( Fig. 3A ). The first arm segment bears a small, triangular dorsal plate with convex distal edge; each segment has two short conical spines to either side with longitudinal rows of round fenestrations ( Fig. 3B ). The integument of the ventral side could not be completely removed and handling the single animal to scan both sides caused additional damage such as loss of spines, but the remaining ASSs appear to be short multifid stumps. The oval DP bears a block-like serrated tooth and a conical MP at each side ( Fig. 3C ).

The next largest individuals found have a dd of 0.9 mm. There are three short flat arm spines with several rows of perforations on the most proximal segment, and two on the following segments. Additional dorsal plates have formed in an irregular pattern, partly overlaying the RPPs. All plates are almost imperforate ( Fig. 3D ). The ASs are narrow, the triangular OSs are about as long as wide and the M still bears its protrusion. The DP is wider than long, with convex proximal edge and distal angle, bearing a small conical papilla to either side. In the mouth angle a longer, spiniform papilla arises from the oral plate. The ASS is flat, pointed leaf-shaped with a wide rounded base, situated close to the disc edge, pointing outwards and visible also from the dorsal perspective ( Fig. 3E ).

With increasing size, the number of dorsal plates increases, the CPP remaining the largest for some time ( Fig. 3F ), although it is no longer obvious in the adult. The ASS changes shape to a simple conical spine ( Fig. 3G ). The number of MP increases until they form a row along the jaw edges, but there is no cluster of apical papillae as in O. glacialis ( Fig. 3G, H ). The dorsal disc scales are increasingly obscured by thick skin and scattered short spines ( Fig. 3I ). The TPo bears a small spiniform scale. From about 2 mm dd, adult characters can be used for identification.

A diagnostic character of O. purpureus is the hookshaped distal arm spine, which could not be found in the postlarvae. However, as is often the case with Ophiomyxidae , most individuals were rather severely damaged and the distalmost tips of the arms were lost in the larger animals.

Remarks: Ophioscolex purpureus can readily be distinguished from O. glacialis by its flat habitus with horizontal arms, which may be explained by the presence of radial shields in O. purpureus . The size of plate fenestrations and the presence/absence of disc spines further distinguish the species from each other. Both can be distinguished from Ophioscolex tripapillatus Stöhr & Segonzac , in press, found off Iceland, which has a long spiniform tentacle scale in the postlarval stage and lacks disc spines.

FAMILY OPHIACANTHIDAE PERRIER, 1891 SUBFAMILY OPHIACANTHINAE PATERSON, 1985

Figures

Figure 2. Ophioscolex glacialis postlarval development. A, 0.5 mm dd postlarva dorsal; B, 0.9 mm dd postlarva dorsal; C, 0.9 mm postlarva ventral; D–G, 1.7 mm postlarvae; D, lateral; E, dorsal, note the irregular pattern of plates; F, arm dorsal; G, ventral, a spiniform tooth and two mouth papillae have formed; H, I, 3 mm dd juvenile ventral, note the large adoral shield spine; J, K, adult ventral. Abbreviations: ASS, adoral shield spine; DP, dental plate; OS, oral shield; others as in Fig. 1. Scale bars in millimetres.

Figure 3. Ophioscolex purpureus postlarval development. A–C, 0.6 mm dd postlarva; A, dorsal; B, arm dorsal; C, ventral; D, E, 0.9 mm dd postlarva; D, dorsal; E, ventral, note the leaf-shaped adoral shield spine; F, 1.5 mm dd postlarva dorsal; G, same ventral; H, 1.8 mm dd postlarva ventral; I, same dorsal. Abbreviations: CPP, central primary plate; DAP, dorsal arm plate; others as in Figs 1, 2. Scale bars in millimetres.