The banded sea urchin or double spined urchin, Echinothrix calamaris, is a sea urchin with a test (shell) 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. This gives it a significant advantage.
It is found in the Indo-Pacific region, on back reefs. It is active at night, hiding in crevices or under rocks at night.
Many juvenile fish of the family Apogonidae (cardinal fish) may hide in the spines for protection.
The differences between Echinothrix calamaris and Echinothrix diadema are: the spines of E. diadema are not banded, there is only one layer, and they are darker. The more similar species is Diadema savignyi (which has banded spines) - the distinguishing feature being that the banded spines of D. savignyi are closed at their ends.
In Hawaii, E. calamaris is often hosts a symbiotic crab, Echinoecus pentagonus.