The Common Rock Urchin (Echinometra mathaei) is an Indo-Pacific urchin very similar to the Rock Boring Urchin (Echinometra lucunter) found in the Atlantic Ocean. They are both purplish-brown in overall color and generally grow to around 3-4" in diameter. Some of these urchins may also develop a reddish tint.
Urchins are grazers that will roam about the tank searching for food and algae on the substrate and rockwork. They will not actively eat corals, but some delicate corals may be damaged if the urchin crawls on top of them or knocks them over. Urchins have also been known to eat coralline algae desirable to some aquarists. Urchins may also occasionally feed on coralline algae that some aquarists may be trying to promote. If algae levels in the aquarium are low, urchins can be fed by attaching algae sheets to a piece of rock with a clip or rubber band.
Some urchins are venomous, but most species are harmless to aquarists. The most common injuries from urchins happens when the aquarists bumps against an urchin and gets poked by its spines. Spines can become broken off in the skin, especially with thinner-spined urchins, so always take care around your urchin to avoid injuring it or yourself.