There are two main types of corals: hard corals and soft corals.
Hard corals, like elkhorn coral and staghorn coral, grow in colonies and are often referred to as “reef-building corals.” Hard corals create skeletons out of calcium carbonate, a hard substance that eventually becomes rock. Overtime, this rock builds up to form the foundation of a coral reef and provides a structure upon which baby corals can settle. Hard corals depend upon tiny algae called zooxanthellae (pronounced zo-zan-THEL-ee) that live inside them. Together, they share a symbiotic relationship—the corals provide the zooxanthellae with shelter, and in return, the zooxanthellae provide the corals with food.