Ophiacantha anomala, G. O. SARS, 1871

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 : 551-552

OPHIACANTHA ANOMALA G.O. SARS, 1871

( FIG. 4A–H )

This hexamerous species is viviparous, with the embryos developing deeply embedded in the gonads (sometimes together with unidentified Myzostomatidae), not just lying in the bursae as in many other brooding species. All embryos are covered with thick skin and their disc is often compressed with arms vertical. The smallest embryo found measures 0.6 mm dd and its six arms consist of three segments and a flattened TP with a blunt thorny distal end and large round fenestrations in irregular longitudinal rows ( Fig. 4A ). Several plates of different sizes and irregular shape form the dorsal disc, in a pattern which may consist of a CPP and six RPPs, but not a typical primary rosette ( Fig. 4B ). The plates bear no spines at this stage. The plate stereom is a meshwork of large irregular fenestrations. Each arm segment bears a strong serrated spine to either side. The jaws terminate in a pointed spiniform tooth.

At 0.9 mm dd and four arm segments, the dorsal disc plates have a more compact stereom with smaller round perforations and imperforate margins. Short trifid disc spines with wide round base are distributed irregularly across the disc surface. There are two arm spines on each LAP, erect, and half as long as a segment ( Fig. 4C ).

From 1.3 mm dd, the dorsal disc is formed by numerous overlapping scales, among which the primaries are indistinguishable ( Fig. 4D ). The scales bear multifid stumps. There is a long pointed tooth and three conical MP, the distalmost arising from the AS ( Fig. 4E ). An increasing number of rugose granules obscure the disc scales at 1.6 mm dd ( Fig. 4F ).

The smallest free-living stages found have a dd of about 2 mm and can be identified using adult characters. The OSs are pentagonal, the M distinguishable as the largest one ( Fig. 4G ). The ASs are distally flaring, separating the OS from the first VAP. The VAPs are wider than long, with strongly convex distal edge, the first plate smaller than those following. There is a single pointed scale at each TPo. Later during development, additional MP are formed, which are pointed spiniform, erect, forming a cluster on the jaw ( Fig. 4H ). The ASs open in a wide angle at the proximal edge of the OS, no longer separating it from the arm.

Remarks: Upon dissection, fewer than 50% of the largest individuals carried embryos and rarely more than one per bursa, up to seven in total per individual, usually fewer. However, because of the rarity of the species and the damage necessary to extract the deeply embedded embryos, no quantitative analysis was conducted. Contrary to most other six-armed species, O. anomala does not reproduce asexually by fission.

SUBFAMILY OPHIOTOMINAE PATERSON, 1985

Figures

Figure 4. Ophiacantha anomala postlarval development, stages taken from brooding animal are termed embryo, free-living stages postlarvae. A, B, 0.6 mm dd embryo dorsal; C, 0.9 mm dd embryo dorsal; D, 1.3 mm dd embryo dorsal; E, 1.3 mm embryo ventrolateral; F, 1.6 mm dd; G, 2 mm dd postlarva ventral; H, 3 mm dd juvenile jaw. Scale bars in millimetres.