Ophiocoma echinata

Ophiocoma echinata, called the spiny ophiocoma, is a brittle star and the type species for its genus, Ophiocoma. It lives in the tropical west Atlantic, the Caribbean Sea, and the Gulf of Mexico.

Description

This is a large brittle star, with arms that can span up to 25 cm. The arms are slender, taper toward the tips, and are thickly covered in short spines. They are clearly set off from the central disc. The animal is dark in color with pale or cream markings, but red is never present on the arms.

Distribution and habitat

The species is common across the Caribbean down to about 30 meters deep. It hides under rocks, in cracks and crevices, beneath coral heads, and inside sponges. It also lives in seagrass meadows, on reefs, and on reef flats.

Behaviour

To feed, O. echinata holds some arms upright in the current, catching particles on its spines and moving them along channels to its mouth. The stomach, located entirely inside the disc, stores food. It uses its arms to burrow into sand and anchor itself in crevices. For movement, it lifts its disc off the bottom while its tube feet grip the surface.

Even without a brain—only a simple ring-shaped nervous system—it moves in a coordinated way: one arm leads while the others work together to push it forward. To change direction, it does not turn; instead, a different arm takes the lead. This shows that a radially symmetrical animal can perform fully coordinated, bilaterally symmetrical locomotion.

When a predator attacks, the brittle star may shed one or more arms. In a back-reef area of the Florida Keys, up to 47% of individuals had missing or damaged arms. It took about two years for those missing three arms to fully regrow them. Losing a limb instead of its life benefits the brittle star: though it must use energy for repair and regrowth, it can still breed later, and the lost arm becomes a renewable resource for the reef’s productivity.