A sea star inflated into a pentagon, with a pair of shrimps living on its underside.
A sea star whose arms are so short and whose body is so inflated that the adult is a stiff cushion or pentagon rather than a star, to about 30 cm across. The upper surface is covered in low granular tubercles and coloured in browns, greens, reds and yellows in a mottled pattern; the underside is flat and pale, with the ambulacral grooves and tube feet running out along five lines. Juveniles are flat and conventionally star-shaped and look nothing like the adults.
Feeds on encrusting algae, detritus and some hard coral by everting its stomach over the surface. It is one of the standard hosts here: the commensal shrimp Zenopontonia soror lives on it, usually two or three animals on one star and coloured to match it, and small parasitic snails of the family Eulimidae are often attached near the grooves. The shrimps sit on the underside, which is why a cushion star is turned over by guides more often than it should be; a light run round the margin will usually show a shrimp without lifting the animal at all.
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.