Diadem urchin

Scientific name : Diadema setosum

Variable colour, often black, can have white stripes, blue radial lines. Test up to 9 cm in diameter, spines long to 30 cm. The anal opening is surrounded by a red or orange ring.
A common species in bay and lagoon, on sandy bottoms, among seagrasses, sometimes common where the back reef is damaged. Social, it form dense aggregations. It can be confused with Diadema savignyi, that has banded spines, and with Echinothrix diadema, that has dark light spotted anal sac.

Associated organisms
With seagrasses. 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), Aeoliscidae (razor fish) and Diademichthys lineatus.