Asterina Sea Star - Asterina sp.
Picture taken March 2006, Nikon D70 Digital SLR, Sigma 105mm macro lens

This is a fairly common star found in many reef aquaria. The come as hitch hikers on live rock.

They are common considered Asterina sp. but that identification has not been confirmed to my knowledge.

They are very small (size of a US Penny) cushion stars nearly always white or gray and irregular looking due to asexual reproduction (fission) by dropping legs. It is very rare to see a symmetrical looking Asterina. Typically they have 1 to 3 fully developed legs. The picture above has 1 fully developed leg with a secondly almost fully developed.

This star can multiply very rapidly in aquaria almost to the point of becoming a pest. There are several different identical looking species of Asterina (... well they are likely to be different species). At least one of them is reported to be SPS coral eater and another is reported to be a zoanthid eater. I've never personally seen this behavior. Mine have always seemed to surface films on the glass and rocks.

If Asterina star population does become a problem, sea stars from the family Nardoa have been reported to consume Asterina stars.