The Purple Urchin, Echinometra viridis, is an avid algae-eater than possesses sharp spines across its body. They also have several smal, sticky visible feet across its body allowing it to move around the tank. In addition to algae, they will feed on small inverts around the tank. As nocturnal animals, the Purple Urchin will be relatively sessile during the day. They should not be housed with crabs, sunflower stars, and snails as they can harm the urchin. This species is very sensitive to water quality and can actually be an indicator species as its spines will fall out if the water quality is bad.