Black Brittle-Star

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. The general colour is black or varying shades of brown, but pale coloured specimens occasionally occur. The upper surface of the disc is covered with fine granules which obscure the plates which cover the surface. On the underside the granules are restricted to the outer portion and the plates are visible towards the central mouth. This is surrounded by the feeding apparatus known as Aristotle's lantern with five toothed jaws each with oval jaw plates. There is a comb-like arrangement of spines down either side of the arms giving them a bristly appearance. On the upper side, each arm segment is covered by a broad plate with 5 to 7 spines. On the underside there are tube feet but these have no suckers.

O. nigra occurs in the eastern Atlantic Ocean from Norway south to the Azores, the North Sea and the Mediterranean Sea and in the Indian Ocean. It is found on rocks, boulders and gravel in the neritic zone down to about one hundred metres but is occasionally found at greater depths. It is tolerant of low salinity levels.

O. nigra is often found living in association with another brittle star, Ophiothrix fragilis. 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. These can be either O. nigra or O. fragilis, or a mixed community of the two. 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. Often the bed consists of adults and newly settled juveniles, with intermediate sized individuals living elsewhere.