Diadema setosum
Spot the Long-spined Sea Urchin from the Yucatán Peninsula to Phuket. Tips on ID, look-alikes, and when to dive with this widespread Indo-Pacific species.
Safety warning
The spines carry venom and snap off easily in the skin. Do not touch it, and never rest a hand or knee on the reef near one; the spines drive in on the lightest contact.
If you're stung
Venomous spinesGeneral first-aid guidance, not a substitute for professional medical care. In an emergency, call your local emergency number.
Long, needle-thin black spines radiating outward from a rounded dark test (shell) are the key marker underwater. The spines are notably longer than those of most other urchins you will encounter on the same reef, often extending 20 to 30 cm. Look for a bright orange or yellow ring around the anal sac at the top of the test, a reliable field mark that separates it from other dark-spined urchins. The overall body is typically black to dark brown, making the orange ring stand out clearly against the reef.
Most often found tucked into crevices, under ledges, or wedged between coral heads during the day, with spines fanned outward as a passive deterrent. At night it becomes more active, moving out across open reef to graze on algae and organic film. It tends to ignore divers unless you get very close, at which point it may slowly orientate its spines toward the disturbance.
This species plays a significant role in controlling algae on coral reefs. After a mass die-off event in the Caribbean during the 1980s, algae bloomed rapidly across reefs that had previously been kept clear, demonstrating just how important these urchins are to reef health.
Comfortable across a water temperature range of 20 to 29°C, it is present on tropical and warm-temperate reefs throughout the year in the Northern Hemisphere with no concentrated season. In the Southern Hemisphere, sightings are most concentrated between September and January.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most commonly logged across tropical reefs in the Indo-Pacific and Caribbean, with strong populations in Mexico's Yucatán Peninsula, Thailand's Phuket and Phi Phi area, and Malaysia's Perhentian and Redang islands. The Philippines' Central Visayas, Honduras' Roatán, and Grand Cayman are also reliable spots, making this one of the most widely encountered urchins across both ocean basins. Florida and the US southeast round out the top regions for North American divers.
Mapping where Long-spined Sea Urchin is logged…
They are logged at dive sites across a large number of countries spanning the Caribbean, Indo-Pacific, and eastern Atlantic. Top spots include the Yucatán Peninsula in Mexico, Roatán in Honduras, Grand Cayman, Phuket and Phi Phi in Thailand, the Perhentian and Redang islands in Malaysia, and the Central Visayas in the Philippines.
In the Northern Hemisphere they are present year-round with no particular peak season, thanks to their broad temperature tolerance between 20 and 29°C. In the Southern Hemisphere, sightings are most concentrated from September through January. Night dives consistently produce more active individuals out foraging on the reef.
Look for a dark, rounded body covered in exceptionally long, thin black spines. The single most useful field mark is the bright orange or yellow ring around the anal sac on top of the test. No other common dark-spined urchin on the same reef reliably shows that ring.
They are not considered dangerous and will not actively attack. However, the spines are sharp, brittle, and can penetrate a wetsuit or bare skin if you brush against them. Standard reef etiquette applies: maintain good buoyancy control and avoid touching the reef.
It is a key grazer, scraping algae off reef surfaces and keeping algal growth in check. When populations crashed across the Caribbean in the 1980s due to disease, unchecked algae growth smothered corals across wide areas. Recovery of this species in affected regions is closely watched by marine biologists as an indicator of reef resilience.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
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