Ophiocomina nigra

The black brittle star, or black serpent star, Ophiocomina nigra, is a marine invertebrate from the order Ophiurida. It lives in the northeastern Atlantic Ocean, the North Sea, and the Mediterranean Sea. This species is a large brittle star, with five slender arms reaching up to 125 mm in length and a distinct central disc up to 25 mm wide. Its color is typically black or various shades of brown, though pale individuals are occasionally seen.

The upper disc is covered in fine granules that hide the underlying plates, while on the underside, granules are only on the outer part, leaving the plates visible near the central mouth. The mouth is surrounded by Aristotle's lantern, a feeding structure with five toothed jaws, each bearing oval jaw plates. A comb-like row of spines runs down each side of the arms, giving them a bristly look. On the upper arm, each segment has a broad plate with five to seven spines; on the underside are tube feet, but these lack suckers.

Distribution and habitat

This brittle star ranges from Norway south to the Azores, across the North Sea, the Mediterranean Sea, and into the Indian Ocean. It lives on rocks, boulders, and gravel in the neritic zone down to about 100 meters, though it is occasionally found deeper. It tolerates low salinity.

Biology

O. nigra is a predator, scavenger, and filter feeder. It uses its arms to capture pieces of organic detritus and small invertebrate prey, pushing them into its mouth.

It can scavenge carcasses or graze on algal films. As a suspension feeder, it raises one or more arms into the current. A net of mucous threads among the arm spines traps plankton and other floating organic matter, which is rolled into balls and moved to the mouth by ciliary currents and tube feet.

Breeding occurs in June in the Firth of Clyde, Scotland. A large female is often found with a smaller male clinging above or below her.

Fertilization happens in the water column through chance meeting of gametes. The larvae are planktonic and settle after several months. The species grows slowly and lives long, not reaching maturity until three or four years old.

Ecology

O. nigra often lives with another brittle star, Ophiothrix fragilis. Many individuals of both species sometimes form dense communities, with hundreds per square meter.