Identification: the test is about 7 cm across (8 to 10 cm at most) and the spines can reach nearly 30 cm. It is usually black, with iridescent blue lines on top; some individuals have white or banded spines, especially juveniles. Diadema savignyi, which shares the same reefs, has stronger iridescent blue or green lines and no orange ring around the anal cone. Diadema antillarum lives in the Atlantic and the Caribbean, not in the Indo-Pacific.
In the wild it lives on reefs, rocky bottoms, sand flats, seagrass beds and mud, from 0 to 70 m. By day it hides in crevices, especially under strong light; at night it grazes encrusting algae on dead coral and rubble, which keeps algae in check on the reef. When a fish approaches, it points its spines towards it. The Banggai cardinalfish hovers above it and shelters among its spines.
Reproduction: gametes are released into the water. Near the equator it spawns all year round; elsewhere spawning is seasonal, with water above 25 °C and the phases of the moon as possible cues.
Status: the species has not been evaluated by the IUCN and is not CITES-listed. It reached the eastern Mediterranean (recorded in Turkey in 2006) and has become invasive there. Since 2022, mass mortalities have hit Diadema urchins in the Mediterranean, the Red Sea, the western Indian Ocean and Oman in 2023, where a ciliate of the genus Philaster was detected in sick D. setosum.
