All corals are animals, but like lichens on land, which are part fungi and part algae, they have a symbiotic relationship with algae. Most corals contain symbiotic algae called zooxanthellae (note: pronounced zo-zan-thel-ee), within their tissues. The coral provides the algae with protection and the compounds it needs to carry out photosynthesis. In return, the zooxanthellae are the corals’ ‘solar panels,’ providing the reef building corals with enough energy to extract calcium from the seawater to build their skeletons quickly. The zooxanthellae use sunlight for photosynthesis and produce oxygen and sugars which the corals can use. This symbiosis is so successful that fast-growing species of corals can grow up to 20 cm (8 inches) per year.

Many other coral reef animals, including sea anemones, soft corals, and giant clams, also have symbiotic relationships with these algae and with each other.