Echinothrix diadema

The long, hollow spines of Echinothrix diadema are brittle and can easily break off in the skin, causing pain and potential infection, though they are not technically venomous. Some members of the Diadematidae family also possess pedicellariae—small pincer-like structures among the spines—that can deliver a mild stinging sensation to bare skin.

The iridescent blue or green spots visible on the body of Echinothrix diadema are not true eyes but are believed to function as simple photoreceptors, allowing the urchin to detect light and shadow. These eyespots help the urchin sense approaching predators and navigate toward shelter during daylight hours.

Echinothrix diadema is primarily omnivorous, grazing on algae, detritus, and small organisms scraped from reef surfaces using its specialized mouth structure known as Aristotle's lantern. Their algae-grazing behavior makes them ecologically important on Indo-Pacific coral reefs, where they help control algal growth.

Echinothrix diadema can be kept in reef aquariums, where it will graze on algae and help keep live rock clean. However, its long spines can irritate or damage corals if the urchin brushes against them, and aquarists should ensure the system has sufficient algae or provide supplemental feeding to sustain the animal.