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Diadema - overview
Diadema is a genus of tropical sea urchins.
These echinoids are quite common in shallow waters throughout all the tropics, as well in the Atlantic (D. antillarum, D. ascensionis) as in the Indian (D. setosum, D. savignyi, D. paucispinum) and Pacific oceans (D. setosum, D. savignyi, D. paucispinum, D. palmeri, D. mexicanum). They usually live at snorkelling depths in coral ecosystems, but can be found as deep as 70m. They are mainly grazers, but can eat a wide variety of foods.
These species all share a distinctive shape, with a rather small test (5-10cm) but very long and slender spines (up to 30cm in D. setosum). However, some related species such as Centrostephanus longispinus have a similar shape, and in a lesser extent some species of Echinostrephus or cidaroids (with stouter spines).
Most species have a particular pattern of electric blue on the aboral part of the test, called "iridophores" : it is especillally obvious on D. savignyi. Like all diadematids, they wear a characteristic "anal papilla" at the top of the test. Still a characteristic of the family, some of their spines are venomous and painful, but not really dangerous.