Ophiothrix fragilis (Abildgaard in O. F. Muller, 1789)

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 : 569

OPHIOTHRIX FRAGILIS

(ABILDGAARD, IN O.F. MÜLLER, 1789)

( FIG. 16A–N )

The postlarvae of this species were collected on adult conspecifics ( Fig. 16A ). The smallest individuals found measure 0.34 mm dd, with two arm segments. The high dorsal disc is formed by the large, flat, pentagonal CPP, and twice as wide as long RPPs ( Fig. 16A, B ). All plates have only a small number of scattered minute perforations in their otherwise solid stereom. Each LAP bears a large hook-shaped spine on a protruding articulation ridge. LAPs are evenly perforated by small holes. The TP is quite small and hollow, about half as long as the hooks of the distal segment. The DAP is triangular, slightly wider than long, with convex distal edge and proximal angle, almost imperforate. Large triangular plates are visible inter-radially at the disc edge. The DP bears a tricuspid tooth ( Fig. 16C ). The comma-shaped AS bears no spine. The first VAP is trapezoidal, longer than wide. OSs are not visible.

At 0.55 mm dd and four arm segments, few short trifid spines have formed on the inter-radial OSs ( Fig. 16D ). The proximal angle of the DAP is rounded off and the most proximal arm segment bears two straight spines instead of hooks, perforated by rows of small holes. The hooks of the distalmost segment are only half the size of those of the previous segments ( Fig. 16E ). Remaining hooks have a large terminal tooth and two secondary teeth along their ventral edge. A short spine has formed dorsal to the hooks ( Fig. 16F ). In the middle of the proximal edge of the tooth, a sharp thorn has formed ( Fig. 16G ). Two additional teeth have formed on the DP dorsal to the first. OSs are beginning to form at the disc edge.

At 0.8 mm dd and 10–11 arm segments, the distal half of the RSs can be seen above the arm, perforated by numerous small holes ( Fig. 16H ). A small IR2 has formed below the IR1 at the disc edge. The DAPs are longer than wide, triangular with convex distal edge and rounded proximal angle. The OSs are triangular, wider than long, with convex distal edge ( Fig. 16I ). The VAPs are rectangular, with straight to concave distal edge and concave lateral edges.

At 1.1 mm dd, the wide wedge-shaped k-plate has appeared ( Fig. 16J ). The first tooth papillae have formed on some DPs ( Fig. 16K ), apparently by fragmentation of the teeth.

At 1.3 mm dd, the dorsal disc is covered with numerous bifid and trifid spinelets (after removal of the integument only visible at the disc edge; Fig. 16L ). The proximal arm segments bear four erect spines with several thorns along their length, the dorsalmost half as long as the others ( Fig. 16N ). Only the distal segments still bear hooks. Each DP bears several papillae instead of teeth ( Fig. 16M ). The appearance of tooth papillae clearly identifies the animals as Ophiothricidae and in areas with no other species of the family, identification is unambiguous at this early stage.

In adults, only the ventralmost arm spine remains a short hook; all teeth have been replaced by papillae.

Remarks: Ophiothrix fragilis is identifiable in all growth stages by its characteristic arm hooks and the large imperforate CPP. The teeth appear to fragment during growth to form the tooth papillae. Postlarvae of this species, which has a planktotrophic larva, often aggregate on unrelated adult conspecifics ( MacBride, 1907).

FAMILY OPHIOCOMIDAE LJUNGMAN, 1867 SUBFAMILY OPHIOCOMINAE LJUNGMAN, 1867

Figures

Figure 16. Ophiothrix fragilis postlarval development. A–C, 0.34 mm dd postlarvae; A, on arm of adult; B, dorsal, note the large central plate (CPP); C, ventral, note the tricuspid tooth; D–G, 0.55 mm dd postlarvae; D, arm dorsal, note the small terminal plate (TP) and large hooks; F, arm lateral; G, ventral; H, I, 0.8 mm dd postlarvae; H, dorsal, radial shields (RS) and inter-radial plates (IR1, 2) have formed; I, ventral; J, K, 1.1 mm postlarvae; J, dorsal, note the k-plate; K, ventral, note the first tooth papillae (TPa); L–N, 1.3 mm postlarvae; L, dorsal with numerous disc spines; M, ventral with additional TPa; N, arm dorsal, note the serrated shape of the proximal dorsal spines. Abbreviations: as in Figs 1–3, 5. Scale bars in millimetres. Some dorsal plates have fallen off due to the bleaching process (H, J).