The buried sea star of shallow sand, often found stacked in pairs.
A five-armed sea star to about 15 cm across, grey-brown with darker markings radiating along the arms and a paler underside, flattened, with a neat double row of squared marginal plates down each side of every arm. The tube feet end in suckers, which separates it from the sand stars of the genus Astropecten on the same ground; those have pointed tube feet and a fringe of long marginal spines.
It lives buried in shallow sand and seagrass with only a faint five-pointed outline showing at the surface, and it feeds on organic material in the sediment. Densities are often high, and a stretch of shallow sand that looks empty can hold dozens within a few square metres.
Pairing is the behaviour it is known for. A smaller male climbs onto a female with his arms set alternately between hers and the two stay joined for days or longer before spawning, so stacked animals are a common sight at some sites and seasons. Recorded from the shallows to about 15 m.
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.