Diadema antillarum

Identification: the round, slightly flattened test is up to 10 cm wide and 5 cm high. The thin, hollow, brittle spines are usually 10 to 12 cm long and up to 30 cm in large individuals, giving an overall diameter that can exceed 50 cm. They are black, sometimes grey or white; juveniles have white-and-black banded spines. Iridescent blue bands outline the plates on top of the test. It resembles Diadema setosum, which lives in the Indo-Pacific; the very similar D. ascensionis and D. africanum share the rest of the warm Atlantic.

In the wild it lives in the tropical western Atlantic, the Caribbean, the Gulf of Mexico and south to Brazil, as well as the Canary Islands and Madeira, from 0 to 400 m but mostly at 1 to 10 m, on reefs, rock, seagrass and sand. It is gregarious, reaching about twenty individuals per square metre. Very sensitive to light, it picks a shaded shelter to wedge into by day and roams about a metre from it at night to graze. It eats mainly algae, sometimes seagrass; starving urchins turn carnivorous. It is the most important herbivore on Caribbean reefs: its grazing stops algae from smothering corals. Juvenile cardinalfish, shrimps and the crab Percnon gibbesi shelter among its spines.

Reproduction: it releases over a million eggs per spawning, around the full moon. Fertilisation takes place in the water and needs a high enough urchin density; the larvae spend a few weeks in the plankton. It can be raised in the laboratory and released: nurseries exist in Jamaica and Puerto Rico.

Status: in 1983 an epidemic of unknown cause killed more than 97% of D. antillarum from Panama to the Bahamas; reefs became covered in macroalgae and corals declined. Recovery remained partial and uneven. Since January 2022 a new mass mortality (US Virgin Islands, Saba, St Eustatius and elsewhere) was traced in 2023 to a ciliate close to Philaster apodigitiformis, named D. antillarum scuticociliatosis. The species has not been evaluated by the IUCN and is not CITES-listed.