Arms that snap off on purpose, wrapped around a sponge or a sea fan by the dozen.
An ophiuroid with a small central disc a centimetre or two across and five long thin arms fringed with glassy spines, purple, brown or banded. Brittle stars are not sea stars: the arms are sharply set off from the disc, they move by whipping rather than by tube feet, and the animal is fast — a disturbed brittle star crosses open ground quicker than anything else on the bottom of its size.
The name is literal. The arms detach at pre-formed breakage planes under attack, and regrow; a population will contain many animals with arms at several stages of regeneration. Most species here are suspension feeders, climbing onto sponges and sea fans at night and holding the arms into the current, and by day they pack into crevices and the interiors of sponges in numbers — a barrel sponge cut open by a storm often proves to have been full of them. Recorded from the shallows to about 40 m. They are also the principal food of the tiger shrimp Phyllognathia ceratophthalma.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.