(Abildgaard, in O.F. Müller, 1789)
Common Brittle Star
Body with a central disc of up to 20 mm in diameter and 5 bristly arms of about 5 times that length; colour highly variable, from dark violet to white, yellow, orange and reddish, spotted, often with arms banded.
The dorsal side of the disc is covered with short spinelets or longer spines, except for the conspicuous, naked radial shields. The ventral side of the disc is covered with short spinelets, except for the proximal part (O. fragilis disc).
The dorsal arm plates are naked and have a more or less distinct longitudinal keel. The ventral arm plates are almost rectangular, with a small proximal depression and the outer edge being slightly concave. In general, there are 7, erect and often serrated, pairs of arm spines per segment. The tube foot pores are partially covered by one small tentacle scale (O. fragilis arm).
On top of the jaw there is a group of tooth papillae and a vertical row of broad teeth. There are no mouth papillae. Two pairs of tube-feet within the mouth (O. fragilis mouth).
This species prefers hard substrata including sand and shell bottoms and is often found in empty shells or under stones, from shallow water down to 350 m.
In the North Sea it is common on all coasts. It is distributed from Lofoten and Iceland to the Mediterranean and the Azores, and apparently along the whole African coast down to the Cape of Good Hope.