Several species of sea urchin are commonly called Diadema urchins, go here

Diadema urchin

Diadema urchin can refer to several species of sea urchin. The most common urchins to take this name are Diadema setosum and Echinothrix diadema....

 for a discussion

The Diadema Urchin (Echinothrix diadema) is a long spined 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...

, 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.

It is 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 dark reef. It is active at night, hiding in crevices or under rocks at night.

It hosts commensal species like the shrimp

Shrimp

Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...

 Stegopontonia commensalis. Saron marmoratus stays close for protection, like many fish of the families Apogonidae

Apogonidae

Cardinalfishes are a family, Apogonidae, of ray-finned fishes. They are found in the Atlantic, Indian, and Pacific oceans, they are chiefly marine, but some species are found in brackish water...

(cardinal fish) and Aeoliscidae (razor fish).

The difference with Echinothrix calamaris

Echinothrix calamaris

The double spined urchin or banded sea urchin, Echinosloth calamaris, is a sea sloth with a test diameter of about 5 cm. It has two sets of spines, shorter closed spines which are dark in colour and can deliver a nasty sting, and longer open ended spines that are often banded...

is that spines are not banded. The more similar species is 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...

, having longer spines, the distinguishing feature being the colour of the anal sac.