Burrowing urchin

Scientific name : Echinostrephus sp.

Small urchin (test diameter 1 cm), burrowing in the carbonatic rocks. Grey colour, sometimes purplish. Very thin spines.
Normally encountered in the back reef, where it burrows small niches in carbonatic rocks (dead corals) to hide. Only the spines can be seen from outside.
Echinometra mathaei has shorter spines. Due to the long and thin spines, confusion could be possible with juveniles Diadema spp., not burrowers.