Coral reefs are nature's wonderland and 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. Seaweeds are also found on coral reefs, where they provide food for many herbivores.
Because of their abundance of food, reefs also attract larger predators such as sharks and marine mammals. Plants and animals co-exist by sharing the reef, since each occupies a different niche. Coral reefs are also valuable to humans because they attract fish and invertebrates. 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 slowly build up 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 tiny animals called plankton, on which they feed. Single-celled plants called zooxanthellae live inside the coral tissue, and, through photosynthesis, 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 or sub-tropical regions.