Thirty centimeters of needle-thin black spines around a body a third that size, and the spines track you. Diadema has light-sensitive cells across the whole test - it effectively sees with its skin - and it swivels its spines toward a shadow before anything touches it.
Those spines are brittle, barbed, and mildly venomous. They snap off inside a wound and cannot be pulled out whole, which is why an urchin injury is a matter of soaking in hot water, breaking the fragments up, and letting the body absorb what is left.
The reason to respect the animal is what happens without it. In 1983 a pathogen swept the Caribbean and killed around 97 percent of the Diadema there in a year. With the main grazer gone, algae overgrew the corals, and reefs that had been coral flipped to seaweed - a change that has still not reversed forty years later. It is the clearest demonstration on record that a spiny nuisance can be holding an entire ecosystem in place.
By day they pack into crevices, sometimes dozens together; at night they spread out over the reef and graze.
- extremely long thin black spines, far longer than the body
- blue iridescent line and blue spots on the test between the spines
- spines swivel to point at an approaching shadow
- test up to 10 cm, spines to 30 cm
