Uniformly dark long spines, with a dark anal sac rather than a pale spotted one.
An urchin with a test to about 10 cm and long spines that are uniformly dark, black or dark purple with a blue-green iridescent sheen in a torch beam, and not banded. It has the same two kinds of spine as Echinothrix calamaris: long, thick, hollow primaries over a dense understorey of much shorter, finer, barbed secondaries. The separation from E. calamaris is the banding of those primaries, and the anal sac at the top of the animal, which here is dark rather than the conspicuous pale spotted balloon that E. calamaris carries. Diadema setosum is also black and long-spined, but its spines are finer and it has an orange ring around the anal cone and five small white dots on the test.
By day it wedges under coral heads, ledges and rubble with only the spine tips showing, and it grazes algae over open ground after dark, sometimes in loose aggregations. Like the other long-spined urchins it is worth treating as habitat: cardinalfish, juvenile shrimpfish, urchin shrimps and small crabs shelter between the spines, and they are found by working a light in among the spines without contact and waiting, since the occupants move to the far side of the urchin as a diver arrives.
Recorded from the shallows to about 30 m on rubble, reef edges and around jetty structure.
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.