Crown-of-Thorns Seastar (Acanthaster planci) feeding on coral. Starfish are marine invertebrates. This is a poisonous starfish that lives on coral reefs in the Indo-Pacific region. It has 12 to 19 arms and long spines on its upper surface. The epithelial cells covering the spines contain glands that release poison into the skin of an attacker. Its mouth is on its lower side, feeding on the polyps that build a coral reef. It can reach 45 centimeters across. The increase in numbers of this starfish, resulting from man's decimation of its chief predator, poses a serious threat to coral reefs.