A moving thicket of spines that a good number of small animals live inside.
A black urchin with a test to about 9 cm and needle-fine spines several times that length. It is separated from the other Diadema by two marks on the test itself — a bright orange ring around the anal cone at the centre of the upper surface, and five small white dots arranged between the ambulacra. Juveniles are banded black and white. The spines are brittle, hollow and finely barbed, and they break off in the wound, which is why an urchin injury is a nuisance out of proportion to its size.
Worth a page here as habitat rather than as an animal in its own right. The spine canopy shelters urchin shrimps of the genus Stegopontonia and Periclimenes, juvenile cardinalfish, small crabs and, on a good night, a juvenile filefish holding station head-down among the spines. Urchins aggregate on rubble and around jetty pilings, moving out to feed on algae after dark and gathering under cover by day. A diver going through a field of them is the standard way to find the associates, and the standard way to be spined; look before putting a hand down.
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.