Diadem urchin

Scientific name : Echinothrix diadema

Long spined urchin, test diameter about 8 cm. Dark colour, generally black. The spines are pointed and closed at the tip. Dark and light spotted anal sac.
Found in the back reef, hiding in crevices or under stones during the day, active at night.
The difference with Echinothrix calamaris is that spines are not banded. The more similar species is Diadema setosum, having longer spines. The colour of the anal sac distinguishes among the 2 species.

Associated organisms
It host commensal species like the shrimp Stegopontonia commensalis. Saron marmoratus and Saron sp. stay close for protection, like many fishes of the families Apogonidae (cardinal fish) and Aeoliscidae (razor fish).