Echinoderms
Diadem sea urchin
Diadema setosum
Epipelagic zone
Its long, thin black spines, able to break off and remain lodged in the skin, make it one of the sea urchins divers fear most, despite its essential ecological role.
🗺️ Geographic range
Coral reefs of the Indo-Pacific and the Red Sea.
📊 Population
No global population estimate available; locally abundant on Indo-Pacific and Mediterranean reefs (introduced species there), not evaluated by the IUCN.
Habitat
- Indian Ocean · Coral reef · Epipelagic zone · Permanent
- Red Sea · Coral reef · Epipelagic zone · Permanent
🥚 Reproduction
External fertilisation, with spawning synchronised to lunar cycles.
Behaviour
- Diet
- Algae, grazed off rocky and coral substrates.
- Hunting
- Moves slowly using its tube feet, mostly active at night.
- Communication
- None.
- Social life
- Can form dense aggregations.
Physiology
- Anatomy
- Long, thin, hollow spines that can exceed 30 cm.
- Respiration
- Dermal gills.
- Vision
- No eyes, but the entire body is light-sensitive.
- Adaptations
- Brittle spines that shatter under the skin, making them difficult to remove.
🔗 Web of marine life
Predators
⚡ Its extraordinary abilities
- Its entire body is light-sensitive, acting like one giant, distributed eye
- Plays an essential role in controlling algae populations on coral reefs
🏆 Records
Among the longest spines known in a sea urchin
Can exceed 30 cm in large individuals.
If you cross paths with this animal…
Risk: Moderate: its brittle spines cause intense pain and possible infection.
Behaviour: Not aggressive, dangerous only on direct contact.
Advice: Never walk barefoot on a reef, especially at night.
💡 Did you know?
- Grazes the algae that, without it, could smother an entire reef's corals.
- Some fish species feed safely between its spines.
