The Diadema Urchin (Echinothrix diadema) 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.
It hosts commensal species like the shrimp Stegopontonia commensalis. Saron marmoratus stays close for protection, like many fish of the families Apogonidae (cardinal fish) and Aeoliscidae (razor fish).
The difference with Echinothrix calamaris is that spines are not banded. The more similar species is Diadema setosum, having longer spines, the distinguishing feature being the colour of the anal sac.