Ophiomitrella clavigera

OPHIOMITRELLA CLAVIGERA (LJUNGMAN, 1865)

(FIG. 6A–K)

This species broods its young in the bursae and often arms of juveniles can be seen extending from the slits. The smallest embryo found still inside an adult is already quite well developed with 0.6 mm dd and about six arm segments (Fig. 6A). This animal is aberrant in having a tetramerous symmetry with only four arms. The dorsal disc is formed by numerous round, overlapping scales, among which no primary rosette can be distinguished. The scales are perforated by round evenly distributed fenestrations. Strong trifid spines are scattered across the scales. Each LAP bears three spines, the ventralmost of which is slightly hookshaped with two terminal teeth and a double row of secondary teeth along its ventral edge (Fig. 6B). The TP is strong, somewhat swollen with an irregular pattern of fenestrations. The jaws bear a triangular sharply pointed tooth and what seems to be an MP on at least one jaw (Fig. 6C). The ASS is straight, not tapering, with blunt end, pointing across the first VAP (Fig. 6C).

At 1 mm dd and 12 arm segments, additional disc scales have formed (Fig. 6D). A spine-like MP has formed to each side of the DP. A pointed spine-like scale covers each TPo.

At 1.3 mm dd, the DAP is shorter than half an arm segment, triangular, with convex distal edge. The ASs are rectangular, distally wider than proximally, their length 2.5¥ their greatest width. The OS is rounded triangular, with all edges convex. The ASS has shortened and moved closer to the mouth angle, pointing towards the slit (Fig. 6E, F).

The smallest free-living juveniles found have a dd of 1.5 mm and 13 arm segments. There seems to be a CPP, but all scales are of similar shape and size and it is unclear whether a primary rosette is present (Fig. 6G). Each LAP bears five serrated, tapering spines, but the hook shape of the ventral spine is no longer obvious. The ventral disc is formed of similar scales as the dorsal disc. The VAPs are almost square, with a concave notch in the distal edge; adjacent plates are widely separated by the LAPs. Tooth and MP have grown in length (Fig. 6H).

At 2.1 mm dd, rugose spinelets are scattered across the dorsal disc. The RSs are just visible above the arm, but hardly distinguished from the disc scales. There are six rugose arm spines (Fig. 6I), but they are still not as thick as in the adult. From about 3 mm dd, keys to adult specimens can be used.

In an adult of 5 mm dd the ASS is part of the row of MP and indistinguishable from them; one of the OSs is larger and identifiable by the hydropore in its centre (Fig. 6J). The arm spines are thick and strongly rugose (Fig. 6K).