The black urchin with needle-like spines up to twice as long as its body, tucked into cracks on rocky reefs and coral. It's harmless if you keep your hands and feet off it, but its brittle, hollow spines break off in skin and hurt.
A round, black body (test) bristling with long, thin, sharp spines, black with a brown tint, that can be up to twice as long as the body is wide. The spines are hollow, fragile and lined with tiny teeth. On the body, look for two parallel lines of iridescent blue dots running from the top toward the widest point of the shell, with no white dots between them; on top sits a small black anal cone. Young urchins have banded spines in black, brown, purple and white.
Diadema mexicanum lives along the Pacific coast of the Americas from the Gulf of California to Peru, including offshore islands such as the Revillagigedos, Clipperton, Cocos, Malpelo and the Galápagos, from the shoreline down to 113 m. It shelters in crevices and holes that protect it from predators and often gathers in groups. It is a grazer: by eating algae it keeps them from smothering corals and frees space for young corals to settle, but when urchins become too many, their scraping erodes the reef itself. In the Gulf of California its numbers are usually low, from about 0.003 to 0.02 per m² at Santa Rosalía, Loreto and Isla Espíritu Santo, compared with more than 3 per m² on some reefs of Huatulco, Oaxaca.
Sexes are separate. In the Gulf of California the reproductive season runs from April to November, with spawning around the full or new moon, and activity increases when the water is above 23 °C. Its predators include the guineafowl puffer (Arothron meleagris) and its relative Arothron hispidus, porcupinefish, the Mexican hogfish and several triggerfishes.
After the 1982–83 El Niño killed 50 to 90% of the corals in Costa Rica, Panama and the Galápagos, urchin numbers exploded: at Isla Uva, Panama, densities went from under 3 to 50–80 per m², and up to 150 per m² were recorded.
Source ↗At the base of the Isla Uva reef, Diadema removed an average of 4.38 kg of reef carbonate per m² a year, about 78% of all the erosion by bottom animals there. In Mexico the effect has been smaller.
Source ↗After about 80% of the corals in Huatulco National Park died in 2023, green algae covered the dead skeletons and urchin numbers, including Diadema mexicanum, boomed.
Source ↗A study of 12 reefs from Mexico to Panama found that coral growth outpaced urchin erosion everywhere except Bahía Culebra, Costa Rica; the balance depends on protection, overfishing and pollution.
Source ↗In the Gulf of California, Diadema mexicanum reproduces from April to November and spawns around the full or new moon.
Source ↗You'll see this urchin while snorkeling or diving on rocky reefs and coral around Cabo San Lucas, the bays of the tourist corridor and Cabo Pulmo, usually wedged into cracks and under ledges. The real risk is touching one: watch where you put your hands and feet, especially when walking in or out of the water over rocks, don't grab rocks to steady yourself, and keep your fins and knees off the bottom. Don't handle urchins at all.
If you're spiked: get out of the water and soak the area in hot water, no hotter than 45 °C (113 °F), for 30 to 90 minutes, testing the temperature on your own skin first, since the injured spot may not feel heat properly; the venom is broken down by heat. Pull out any spines you can see with tweezers, but don't dig for deep ones or squeeze the wound, because the spines crumble. Wash gently, use antiseptic, and don't close the wound with tape or glue. A dark mark from the spine's pigment without a visible spine usually fades within about 48 hours. See a doctor soon in any case, and urgently if a spine went into a joint or near a nerve, if you have trouble breathing, weakness or feel faint, or if pain lasts more than 5 to 7 days, which suggests infection or a spine left inside. Make sure your tetanus shot is up to date.
| Sport fishing (NOM-017-PESC-1994, 2013 update) | Only fish may be taken; sea urchins cannot be collected |
|---|---|
| Mexico (NOM-059) | Not listed |
| IUCN Red List | Not evaluated |
| Cabo Pulmo National Park | Collecting wildlife only with a SEMARNAT permit for research; keep 2.5 m from rock and coral |
Don't touch: the spines break off in skin. Soak a sting in hot water and see a doctor.
The crowned sea urchin (Centrostephanus coronatus) also has long, thin spines, usually dark or banded, and young Diadema are easily mistaken for it; adult Diadema are solid black with blue dots on the body. The slate-pencil urchin (Eucidaris thouarsii) has thick, blunt spines, often crusted with growth, and hides between or under rocks.
Sea urchin spines carry venom that causes pain and swelling, and the brittle spines break off in the skin. Soaking in hot water helps because the venom is broken down by heat.
Soak the area in hot water up to 45 °C for 30 to 90 minutes, remove visible spines with tweezers, wash gently and see a doctor. Get urgent care if a spine is in a joint or you feel faint or short of breath.
No. Divers Alert Network warns against unproven remedies; use hot water, gentle cleaning and medical care.
Top illustration: Top Anglers. Photos: iNaturalist contributors, credited on each photo.
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