If you dive the Caribbean, the long-spined sea urchin is the black pincushion wedged into a crevice at the base of the reef, its spines swinging toward your shadow. It is also the animal reef scientists most want back.

What is a long-spined sea urchin?

Diadema antillarum is a regular (round) sea urchin in the family Diadematidae. Its body, or test, is a dark dome a few inches across, but the animal looks far larger because of its spines. According to species accounts, most sea urchin spines are 1–3 cm long, while this species usually carries spines of 10–12 cm, with very large individuals reaching about 30 cm. Adults are typically black; juveniles often have banded black-and-white spines, and an occasional adult shows grey or banded spines.

Where does Diadema antillarum live?

It lives in the tropical and subtropical western Atlantic, including Florida, the Bahamas, the Gulf of Mexico and the whole Caribbean. Most individuals sit on coral reefs at 1–10 m, tucked into holes and crevices so that only the spines show. Urchins that cannot find a hole live in the open on reef flats, seagrass edges and rubble. For an overview of where other urchins live, see habitat of sea urchins.

Why are long-spined urchins so important to coral reefs?

They are lawnmowers. Diadema grazes turf and fleshy algae off dead coral and rock at night, leaving bare, clean surfaces where coral larvae can settle. Before 1983, dense populations kept Caribbean reefs cropped close, often leaving a visible “halo” of grazed rock around their shelters.

When the urchins disappeared, algae took over. The 1983 mass mortality led to widespread algal overgrowth on Caribbean reefs, which reduced coral cover and biodiversity and lowered reef resilience. Overfishing of parrotfish and surgeonfish, the other main grazers, made it worse. Read more on grazing in do sea urchins eat algae.

What happened in the 1983 die-off?

Starting in 1983, an unidentified pathogen swept through the Caribbean, from the Bahamas south to South America. More than 97% of long-spined urchins died, and in many places the figure was close to 98–99%. The cause was never confirmed at the time. Recovery was patchy and slow; four decades later most reefs still had far fewer urchins than before.

What caused the 2022 die-off?

In early 2022, long-spined urchins in the US Virgin Islands and then across the eastern Caribbean and Florida began losing spines and dying within days. A team led by Mya Breitbart at the University of South Florida identified the cause in April 2023: a single-celled scuticociliate, a free-living ciliate in the genus Philaster. It was the first time researchers had pinned a cause on an urchin die-off this quickly. The population that was hit was still recovering from 1983.

Is the long-spined sea urchin dangerous?

It is not deadly, but it is the urchin most likely to hurt a Caribbean diver or snorkeler. The spines are thin, hollow and brittle. They pierce skin easily and snap off inside, and the tissue around them carries a mild toxin, so a puncture throbs and can leave black marks from spine pigment. Most fragments dissolve or work their way out in days; deep or joint punctures, or signs of infection, should be seen by a doctor. Wear gloves only where reef rules allow, watch your hands and knees, and never kneel on the reef. See can you touch sea urchins for first aid basics.

How does Diadema behave?

Diadema is mainly nocturnal. By day it shelters in holes, often in groups, and swings its spines toward moving shadows; the spines are sensitive to changes in light. At night it moves out to graze. Small fish such as juvenile cardinalfish and shrimps shelter among the spines. Predators include triggerfish, spiny lobsters, helmet snails and some wrasses. See what eats sea urchins.

How does the long-spined urchin reproduce?

Sexes are separate. Males and females release eggs and sperm into the water, where fertilization happens, and the larvae drift in the plankton for weeks before settling on the reef. Because the adults now live far apart on many reefs, spawning success drops, which is one reason recovery after 1983 was so slow. More detail is in how do sea urchins reproduce.

Can long-spined urchins be restored?

Restoration groups in Florida, the Caribbean and elsewhere now raise Diadema from larvae in hatcheries and relocate wild juveniles to reefs, in the hope of restoring grazing. Florida programs have worked on spawning and rearing the species in captivity. Results are encouraging at small scales, but the 2022 die-off showed how quickly gains can vanish.

• Hatchery rearing: spawning adults and raising larvae to juveniles for outplanting.
• Translocation: moving juveniles from high-settlement areas to reefs that need grazers.
• Monitoring: reporting sick or spineless urchins quickly so outbreaks are tracked.

How is it different from other long-spined urchins?

In the Indo-Pacific, Diadema setosum and Diadema savignyi look similar and are also called long-spined urchins; D. setosum has small white spots and an orange ring around the anal cone. In the Caribbean, the only urchin with spines this long is D. antillarum. Compare it with the stubby pencil urchin and the short-spined pincushion urchin, or browse types of sea urchins.