Brittle stars are starfishlike echinoderms (phylum Echinodermata) in the family Ophiodermatidae, whose star-shaped bodies are radially symmetrical and are supported by a hard endoskeleton made of calcium salts. Brittle stars are closely related to basket stars, and more distantly related to starfish, sand dollars, and sea urchins. Brittle stars are named for the ease by which their arms fall off when touched; these animals, known collectively as ophioroids, are also called serpent stars (ophis means snake in Greek) because their long arms resemble serpents. The brittle stars comprise the largest number of species (about 1,000) of echinoderms and are found on the seabed in all of the world's oceans. Brittle stars also inhabit the dark high-pressure environments on the floor of the abyssal zone, the deepest part of the ocean where few other living things can survive. Some species of brittle star can swim, but most species simply crawl along the ocean floor. Brittle stars in shallow seas tend to avoid light and prefer to hide in dark crevices, becoming more active at night, or they inhabit the ocean depths where it is always dark. Brittle stars usually have five long, thin, jointed arms covered with spines; sometimes they have six or seven arms. Brittle stars can be as large as 2 ft (0.6 m) in diameter or as small as a few millimeters. Ophioroids are usually a drab green, grey, or brown, but some have variegated color patterns. Some species of brittle stars glow in the dark-a bright green luminescence appears if they are disturbed. Others can change their color.
The arms of brittle stars are attached to a central disklike body that houses on the underside the mouth and jaws, stomach, and saclike body cavities called bursae, which are peculiar to ophioroids. The mouth is surrounded by five moveable jaw segments, making the mouth opening look like a star. Some brittle stars are carnivorous, while others feed on small particles of plankton. The food enters the mouth and goes directly into the stomach, and there is no intestine and no anus; absorption and excretion are carried out instead by the five pairs of bursae located at the base of each of the arms. Water circulates in the bursae and there is an exchange of respiratory gases and excretion of wastes. The coelomic walls of the bursae contain the gonads that discharge sex cells in the water for fertilization. The larva, which develops from a fertilized egg, is called an ophiopluteus and is free swimming in the plankton until it transforms into the juvenile stage when it settles on the bottom of the ocean. Brittle stars are either male or female, although a few individuals are hermaphrodites. Brittle stars spawn at the end of summer, and most species release their eggs into the plankton and invest no parental care thereafter. They can also reproduce asexually; if an arm breaks off and it still has a small piece of the central disk attached, the arm can regenerate into a whole new brittle star.
Brittle stars are among the most active of the echinoderms and, unlike starfish and sea urchins, can move easily and quickly. The arms of a brittle star reach out in pairs to pull the animal along. Each arm is supported by a central internal skeletal support (ossicle). Like starfish, brittle stars have tube feet, but those of brittle stars lack suckers. The tube feet help more with feeding than with locomotion. Like other echinoderms, brittle stars can regenerate lost parts. If an arm is broken off, a new one grows in its place within months. If an arm is injured, the star can cast off the injured arm (autotomy) and then eventually grow a new one.