Identification: the slightly oval test is about 5 cm across, but with its spines the animal reaches up to 30 cm. The primary spines, 10 to 15 cm long, are hollow, thick and often blunt, usually banded white and black or brown (sometimes all white, purple in juveniles, or black); they are so fragile you can crush them between two fingers. The short, thin, yellow to dark brown secondary spines carry venom glands. On top they leave five bare, star-shaped zones, often blue, and a large pale, speckled anal cone. Diadema setosum and D. savignyi are larger and more uniformly black (D. setosum has five white spots and an orange ring around the anus); Echinothrix diadema has a small black anal cone, a blue sheen and sturdier spines. A dark Indian Ocean form, greenish with a reddish test, may be a separate species.
In the wild it ranges from the Red Sea and East Africa to French Polynesia, including Hawaii, from the surface to 70 m (90 m according to SeaLifeBase), in lagoons, seagrass beds, outer reef slopes and channels. By day it hides in crevices and under rocks; at night it feeds on algae, invertebrates, debris and carrion, becoming more carnivorous with age. Its photoreceptors let it point its spines at a threat. Juvenile cardinalfish (Apogonidae) shelter among its spines; in Hawaii it often hosts the crab Echinoecus pentagonus.
Reproduction: the sexes are separate and gametes are released into the water; the larvae spend a few weeks in the plankton. Adults can live up to 6 years. It can hybridise with Echinothrix diadema.
Status: it has not been evaluated by the IUCN and is not CITES-listed. It is often shown in public aquariums and frequently seen in the aquarium trade.
