To most snorkelers, the long-spined sea urchin is just a black pincushion to avoid stepping on. To reef scientists, it is one of the most important animals in the Caribbean. Each night it slowly grazes the reef, scraping away the algae that would otherwise smother young corals, and few other grazers do the same job as thoroughly.

Its story is also one of the most dramatic in Caribbean ecology. In 1983 a mysterious disease wiped out almost the entire population across the region, and in 2022 a second die-off hit the animals that had slowly come back. Every long-spined urchin you see on a dive in Punta Cana is a survivor, and a good sign for the reef around it.

Quick Facts

Name: long-spined sea urchin, Diadema antillarum; also called the black sea urchin
Size: the round body (the test) is about 10 cm (4 in) across; the spines are commonly 10–30 cm (4–12 in) long
Diet: algae, from fine turf to fleshy seaweeds, and some seagrass
Activity: mostly nocturnal; shelters in crevices and under ledges by day
Depth: most abundant in shallow water, roughly 1 to 10 m (3 to 33 ft), though it occurs deeper
Range: tropical western Atlantic, including the whole Caribbean, Florida and Bermuda

How to Recognize a Long-Spined Sea Urchin

This is the easiest urchin on the reef to identify. Look for a black or very dark body bristling with extremely long, thin, needle-like spines, often several times longer than the body is wide. The spines move constantly, and if you watch closely they swing toward any shadow that falls across the urchin, including yours. Juveniles often have spines banded in black and white or gray.

Other Caribbean urchins look quite different:

Rock-boring urchin (Echinometra lucunter): small, with short, stout spines, usually tucked into a hole it has worn into the rock.
West Indian sea egg (Tripneustes ventricosus): a large dark body covered in short white spines, often found on seagrass.
Slate pencil urchin (Eucidaris tribuloides): few, thick, blunt spines like pencils.

If the spines are long, black and thin as needles, you are looking at Diadema.

Behavior: Why Urchins Matter as Grazers

By day, long-spined urchins wedge themselves into crevices, under ledges and between coral heads, often in small groups. At night they spread out and graze, scraping algae off rock and dead coral with a five-toothed jaw on their underside. That nightly mowing keeps patches of reef clean, and clean, hard surfaces are exactly what coral larvae need to settle and start a new colony.

Herbivores are the reef's maintenance crew. Parrotfish and surgeonfish do much of this work by day, and we explain how that balance works in our deep dive on why parrotfish matter. The long-spined urchin is the night shift, and it can reach into tight spaces where fish cannot. When the urchins disappeared from Caribbean reefs, the effect was obvious: algae spread across many reefs, and coral recovery slowed.

Two Die-Offs: 1983 and 2022

The first die-off began off Panama in January 1983. Over roughly the next 13 months, a disease spread with the surface currents through the Caribbean, Florida and Bermuda. Across the region, populations fell by more than 93%, and by 97–98% in many studies. The cause was never identified. Recovery was very slow: a review published about 30 years later estimated that densities were still only around 12% of pre-1983 levels.

The second die-off began in early 2022, first reported from St. Thomas in the US Virgin Islands. Within months, sick urchins with drooping spines were reported across much of the Caribbean, including the Greater Antilles, and losses of 85–95% or more were recorded at affected sites. This time scientists found the cause. In 2023, a large international team published in Science Advances that a single-celled parasite, a scuticociliate most similar to Philaster apodigitiformis, causes the disease, now called Diadema antillarum scuticociliatosis. The USGS has a good summary of the research.

Where to See Long-Spined Sea Urchins in Punta Cana

We still find long-spined urchins on our local reefs, but numbers vary from site to site and they are far from the dense carpets older divers describe from before 1983. The best places to look are shallow reefs with plenty of crevices. At El Niño, at 6 m (20 ft), check the gaps under coral heads; at Cuevitas, at 10 m (33 ft), look in the small caves and swim-throughs, where you often see a cluster of black spines poking out of the shadows. You will also find the urchin on our species guide.

Tips for Divers and Snorkelers

Keep your buoyancy under control near the reef, especially in swim-throughs and narrow channels. The spines are brittle and break off in skin easily.
Never put your hands or knees on the reef. Urchins hide in exactly the gaps you might reach into.
Snorkelers entering from shore should wear fins or water shoes and avoid standing on rocks and reef.
Never move, turn or "rescue" an urchin. Every one left on the reef is doing important work.

First Aid for Urchin Spines

A spine puncture hurts, but it is rarely serious. If it happens:

Get out of the water and look at the wound. Gently remove any spines that are sticking out with tweezers. They snap easily, so do not squeeze or dig.
Soak the area in hot water, as hot as you can comfortably tolerate but no hotter than about 45 °C (113 °F), for 30 to 90 minutes. Test the temperature with your other hand first. Heat helps ease the pain.
Wash the wound with soap and clean water and keep it clean while it heals.
Dark purple or black dots around the wound are often dye from the spines, not broken pieces, and usually fade within a few days.
See a doctor if a spine is in or near a joint, there are many punctures, the area becomes red, swollen or hot, or the pain does not settle.

Frequently Asked Questions

Are long-spined sea urchins poisonous?

The spines can carry a mild toxin that adds to the sting, but the main problem is the brittle spines breaking off in the skin. The urchin is not aggressive and only hurts people who touch it.

Are long-spined sea urchins recovering?

Slowly and unevenly. After 1983 they took decades to partly come back, and the 2022 die-off set many places back again. Restoration projects across the Caribbean are now raising and restocking urchins.

Why are they so important for coral reefs?

They graze algae that compete with coral, keeping surfaces clear for young corals to settle. Reefs that lost their urchins often became covered in algae.

What should I do if I step on a sea urchin?

Remove the visible spines gently, soak the area in hot water and keep it clean. Get medical help if a spine is near a joint or the wound looks infected.

New to diving? Our Discover Scuba Diving experience takes you to shallow reefs to 6 m (20 ft), where these night gardeners hide by day.