A flattened urchin under a felt of very short spines, ploughing just below the sand surface.
An irregular echinoid flattened into a disc or a low oval, to about 12 cm, covered on both surfaces in a dense felt of very short spines that moves continuously and gives the animal the look of velvet rather than of an urchin. The upper surface carries a five-petalled pattern of paired pore rows around the centre. Live colour is grey, fawn, brown or purple, and the outline is usually blurred by a film of sediment.
The animals live in the top few centimetres of sand, moving forward slowly and sorting fine organic particles out of the sediment with the spines and tube feet, which pass the material along tracts on the underside to a mouth at the centre. That central mouth is the working separation from the heart urchins, which share the ground and the habit but are heart-shaped in outline with the mouth set well forward.
What most divers see is the bare test after death: a thin, brittle, chalk-white disc marked with the petal pattern, sometimes in numbers where current has collected them. Living animals are found by the shallow depressions and the raised trails they leave in the surface. Recorded from about 2 to 30 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.