[2] O. nigra is a large brittle star with five narrow arms up to 125mm long and a quite distinct central disc which is up to 25mm wide.

[8] Numerous individuals of these two species sometimes form dense communities with hundreds of brittle stars per square metre.

These beds may extend several hundred square metres over sandy and pebbly sediment on the sea floor and contain millions of brittle stars.

The vast social agglomerations can be advantageous in enabling the brittle stars to filter feed in fast moving currents which would otherwise sweep them away.

Other echinoderms likely to be in the same vicinity (and enjoying the feast) are the predatory starfish, Asterias rubens, Luidia ciliaris and Crossaster papposus and the sea urchins, Echinus esculentus and Psammechinus miliaris.

On rocky outcrops among the seething mass of brittle stars the soft coral, Alcyonium digitatum, the hydroid, Nemertesia antennina and the sea anemone, Metridium senile, are often found.

Another sea anemone, Urticina felina, may be semi-buried in the sediment and surrounded by an area clear of brittle stars.