Granja-Fernández et al. (CC BY 4.0)
Mini Brittle Stars
Ophiactis savignyi and other small ophiuroids
Tiny detritus-eating brittle stars that stay around 2 cm across and clean crevices no other cleanup animal can reach.
Identification
Very small brittle stars, about 2 cm (under 1 in) from arm tip to arm tip, in white, grey, green, brown, or cream. A pinhead-sized central disc carries five — often six — long, thin, jointed arms with fine spines. They live tucked inside rock and sponge, waving one or two arms out of holes to catch food, and multiply by splitting in half and regrowing the missing side.
Often confused with
Pest starfish such as Asterina stars, and larger predatory sea stars. The tell is body plan: a mini brittle has long, skinny, snake-like arms clearly set off from a tiny central disc, and it rows itself away quickly when exposed. Asterina stars are chunky, fleshy stars with short, stubby, webbed arms and a thick body, and they crawl slowly on their tube feet. Six arms on a mini brittle is normal fission reproduction, not a warning sign.
Natural predators
No natural predators are listed.
Field notes
Excellent micro-cleanup for rockwork: they consume detritus and stray food particles in tight spaces and reproduce steadily in mature tanks and refugiums. Fish food value aside, a thriving population signals a well-fed, stable system.