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Echinoderms - Starfish & Brittle Stars

Starfish or sea stars are star-shaped Echinoderms belonging to the class Asteroidea. About 1,500 species of starfish occur on the seabed in all the world's oceans, from the tropics to frigid polar waters. They are found from the intertidal zone down to abyssal depths, 6,000 m (20,000 ft) below the surface.

Starfish are marine invertebrates. They typically have a central disc and five arms, though some species have more than this. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown.

Brittle stars or ophiuroids are Echinoderms in the class Ophiuroidea closely related to the Starfish. They crawl across the sea floor using their flexible arms for locomotion. The ophiuroids generally have five long, slender, whip-like arms which may reach up to 60 cm (24 in) in length on the largest specimens.

The Ophiuroidea contain two large clades (family tree), Ophiurida (brittle stars) and Euryalida (basket stars). Many of the ophiuroids are rarely encountered in the relatively shallow depths normally visited by humans, but they are a diverse group. Over 2,000 species of brittle stars live today. More than 1200 of these species are found in deep waters, greater than 200 m deep.

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