Hard Coral

Hard corals grow in colonies and are the architects of coral reefs. Hard corals—including such species as brain coral and elkhorn coral—create skeletons out of calcium carbonate (also known as limestone), a hard substance that eventually becomes rock. Hard corals are hermatypes, or reef-building corals, and need tiny algae called zooxanthellae (pronounced zo-zan-THEL-ee) to survive.