Long banded spines that are blunt and harmless, and short dark ones that are not.
An urchin with two kinds of spine, which is the whole identification. The long primary spines are thick, hollow, blunt-tipped and boldly banded light and dark, standing out to fifteen centimetres or more; between them sits a dense understorey of much shorter, finer, dark spines that are sharp and barbed. The test reaches about 12 cm and colour varies from pale cream-and-brown banding to almost uniformly dark, but the reliable field mark is the anal sac at the top of the animal — a conspicuous pale balloon, usually white and spotted or streaked with black, which inflates and deflates and is visible from a distance. That sac separates it from Diadema setosum, which has an orange ring around the anus and five white dots on the test, and from the all-black Echinothrix diadema, in which the sac is dark.
By day it wedges under coral heads, rubble and jetty structure with only the spine tips showing, and comes out to graze algae after dark. The reason to stop at one is that the spine canopy is habitat. The urchin clingfish Diademichthys lineatus — long, dark, with a pale stripe along the flank — hangs head-down among the spines and is found on this urchin as often as on Diadema. The urchin shrimp Stegopontonia commensalis, striped lengthwise to match a spine, runs along the primaries; the zebra crab Zebrida adamsii sits on the test; cardinalfish of the genus Siphamia, juvenile shrimpfish and small filefish shelter between the spines. Getting a light in among them without touching, and waiting, is what shows the occupants, since everything in there moves to the far side of the urchin as a diver arrives.
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.