Coral reefs are nature's wonderland, some of which are among the world's most diverse and complex ecosystems. The coral reefs themselves create habitats for many other animals, including fish, crustaceans, worms, echinoderms and molluscs. Sea weeds also occur on coral reefs, providing food for many herbivores. Because of the abundance of food, reefs attract larger predators such as sharks and marine mammals. The many plants and animals all live together by sharing the reef, each specialising in a different lifestyle and occupying a different niche. Coral reefs are also very valuable to humans. Coastal communities have, for centuries, relied for their livelihoods on the fish and invertebrates found associated with coral reefs. The enormous coral structures help to protect islands from erosion and storms by acting as natural breakwaters. Reef related eco-tourism is also an increasingly important source of revenue for island communities.
 
Over millions of years, tiny individual coral animals called polyps have built the coral reefs. Each polyp, which looks like a tiny sea anemone, produces a cup-shaped limestone skeleton in which it lives. As they grow, divide and reproduce, so the coral reef grows, forming complex coral colonies made up of millions of polyps, joined together by their limestone skeletons. The polyps have tentacles with stinging cells that paralyse the tiny animals called plankton, on which they feed. Single-celled plants called zooxanthellae live inside the coral tissue, and, through photosynthesis, they use the sun's energy to make food for themselves and for the coral polyps. Because of this plant and animal association, reef-building corals are only found in shallow, clear water, where light can penetrate. These conditions are generally found in the tropics, which is why most coral reefs are found in tropical and sometimes sub-tropical regions.