Diadema urchin can refer to several species of sea urchin. The most common urchins to take this name are Diadema setosum and Echinothrix diadema.
Both species are commonly found in the Indo-Pacific region, in water up to 70m in depth. D. setosum is more commonly found on sandy sea beds or damaged back reef, whereas E. diadema prefers intact back reef environments.
These two species can be differentiated as D. setosum can have radial white stripes, is slightly larger when full grown, and the opening of the anal opening is surrounded by a red or orange ring. E. diadema has a dark and light spotted anal sac.
It can also be used to refer to the genus, Diadema of sea urchins, although the species of this genus more appropriately take different common names.
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