Diadema urchin can refer to several species of sea urchin

Sea urchin

Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...

. The most common urchins to take this name are Diadema setosum

Diadema setosum

Diadema setosum is a species of long-spined sea urchin belonging to the family Diadematidae. It is a typical sea urchin, with extremely long, hollow spines that are mildly venomous. D. setosum differs from other Diadema with five, characteristic white dots that can be found on its body...

and Echinothrix diadema

Echinothrix diadema

The Diadema Urchin is a long spined urchin, with a diameter of about 8 cm. It is generally black in colour, and always dark. The spines are closed at the tip; the anal sac is spotted light and dark....

.

Both species are commonly found in the Indo-Pacific

Indo-Pacific

The Indo-Pacific is a biogeographic region of the Earth's seas, comprising the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two in the general area of Indonesia...

 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

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

, Diadema

Diadema (genus)

Diadema is a genus of sea urchins of the Family Diadematidae and is one of the most abundant, widespread, and ecologically important shallow water genera of tropical sea urchins. It is found in all tropical oceans, although is ubiquitous in the Indo-Pacific region, where it inhabits depths down to...

of sea urchins, although the species of this genus more appropriately take different common names.