Brittle Stars — Class Ophiuroidea

Approximately 18 species of ophiuroids, including Ophiacantha bidentata and Amphiura sundevalli, occur on sandy and muddy substrates in arctic waters. Ophiuroids, also known as brittle stars, are typically brown or grey in colour and are differentiated from sea stars by their longer, thinner arms that distinctly radiate from the central disc. Although brittle stars usually have only five arms, these arms may be branched, giving the appearance of a multitude of limbs. Hard, protective plates cover the arms and central disc of brittle stars, and plates also surround their mouth to form a type of chewing apparatus. Brittle stars are filter feeders, scavengers, deposit feeders, or carnivores. Filter feeders trap particles in the water by lifting up their arms and waving them around. Algae and other particles stick to mucous strands, located between the spines on their arms, and are swept along to their mouth. Brittle stars that feed on larger prey, such as dead animals, grasp their prey with an arm and move the food towards their mouth. Predatory brittle stars feed on polychaetes, molluscs, and crustaceans.

The most mobile of the echinoderms, brittle stars are able to "crawl around" by holding their central disc above the bottom as they move their arms forward. They use the spines on their arms to provide traction while digging into the substrate.

Like sea stars, brittle stars, can cast off an arm and regrow it later. They do this as a defence mechanism if they are disturbed or seized by a predator. If a break occurs at a point on the arm beyond the central disc, the brittle star can then regenerate the lost portion.

Fertilization among brittle stars usually takes place after the sex cells have been released into the water, but some species may care for the developing eggs until they hatch into young brittle stars. Larvae develop into adults while in the water column, after which time they settle to the bottom.