(Abildgaard, in O.F. Müller, 1789)
Black Brittle Star
Body with a central disc of up to 25 mm in diameter and five arms of about 5 times that length. Colour varying from black to brown, grey or pink, often it is more or less spotted.
The dorsal side of the disc is smooth and completely covered by small granules which cover the scales and radial shields; on the ventral side only the outside is covered by granules, and the scales are visible on the inner part of the disk (O. nigra disc).
The dorsal arm plates are broad, contiguous, with the outer edge slightly rounded. The proximal ones are sometimes covered or surrounded by granules. The ventral arm plates have a distinct obtuse angle, the outer edge is rounded or slightly concave in the middle. There are 5-7 smooth arm spines per segment, about as long as two arm joints. In the proximal part, the tube foot pores are partially covered by 2 tentacle scales, in the distal part only 1 (O. nigra arms).
The mouth shields are variable in shape, but are usually broader than long, oval, with a rounded peak on the outer side. There are 10-15 tooth papillae over a single vertical row of teeth on top of the jaw, and 4 long, thin mouth papillae on each side of the jaw. Two pairs of tube-feet within the mouth (O. nigra mouth).
This species is mainly found living on stones and rocks at depths between 0-400 m, but it appears to be rather rare below ca. 100 m. Its occurrence is usually associated with Ophiothrix fragilis. Tolerant of brackish conditions.
In the North Sea this species is found on the British east coast as far south as Durham. Elsewhere it is distributed from Norway and the British west coasts down to the Azores and the Mediterranean.