Tropical coral polyps have small algae, or zooxanthellae, growing inside them. This is a cooperative, or symbiotic, venture. The algae gets shelter and food (in the form of nutrients from captured plankton) from the polyp, while the polyp also gets some food in return from the algae via photosynthesis (turning light energy from the sun into food). This photosynthesis means algae need sunlight to live, and this is why tropical corals only grow where the sea is shallow and clear.
The algae also give corals their colour. If the algae become stressed, such as if the water temperature becomes too high, they leave the polyp. This exposes the white, calcium carbonate skeletons of the coral and is what we call coral bleaching.
Tropical coral reefs form the basis of complex ecosystems. Their soft polyps provide a ready food source for many other sea creatures, while reef caves and crevices are first-rate locations for breeding or hiding from predators. Up to 4,000 individual species can co-exist on a single reef at a density 100x greater than the average for the open ocean. These include the coral species themselves as well as algae, fish, dugongs, marine turtles, sea snakes, worms, crustaceans, mollusks, starfish, and many, many more.
View the interactive coral reef map from ReefBase