Spiny Urchin

Diadema antillarum Philippi

Cidaris antillarum Philippi, 1845:355. [For a synonymy, see Mortensen, 1940:269. For the biology of this species see Lewis, 1964, and Randall, Schroeder, and Starck, 1964. For illustrations, see Kier and Grant, 1965, pi. 2: figs. 4–7.]

This urchin occurs both on the reef and in the lagoon. It lives throughout the length of the transect in cavities in the coral or rock. It is most common in the Patch Reef Zone with a density of 6–7 per square meter. Rare at greater depths, no specimens were observed deeper than 40 meters. However, it has been reported elsewhere at far greater depths. Large herds of 20–40 specimens occur in profusion in the lagoon’s Thalassia beds.