Echinothrix diadema is similar to E. calamaris, but has a smaller, black anal bulb and usually fine banding on the thin spines. Large individuals may turn all black. This species is common, often under rocks or in algae patches. Large black ones are common wedged into holes and crevices on shallow lagoon pinnacle reefs.
The large black urchins often host tiny red shrimp that align themselves along the spines. They are especially visible under a diver's flashlight at night. The shrimp have been tentatively identified as Cristimenes zanzibaricus.
Created 15 June 2018