Discover the busy underwater communities sometimes called the rainforests of the sea.
Coral reefs are underwater ecosystems built mainly by colonies of tiny animals called coral polyps. Although reefs cover only a small part of the ocean floor, they provide food and shelter for an amazing variety of marine life.
Learn why coral is an animal rather than a rock or underwater plant.
Learn MoreDiscover how generations of corals slowly create enormous reef structures.
Learn MoreExplore fringing reefs, barrier reefs, and ring-shaped atolls.
Learn MoreMeet some of the fish, turtles, sharks, and invertebrates that depend on reefs.
Learn MoreSee how corals cooperate with microscopic algae to obtain energy.
Learn MoreLearn what happens when stressed corals lose the algae living inside them.
Learn MoreFind out how people can help protect these important ocean habitats.
Learn MoreA coral colony is made of many tiny animals called polyps. Each polyp has a soft body and a mouth surrounded by tentacles that can capture tiny particles of food. Many reef-building corals also make hard calcium carbonate skeletons beneath their bodies.
Reefs can grow when new coral polyps build their skeletons on top of older coral skeletons. This process happens slowly over many generations. Over long periods of time, the accumulated skeletons can form large underwater structures that are visible from boats, aircraft, and even satellites.
Fringing reefs grow close to shore, while barrier reefs are separated from land by deeper water or a lagoon. Atolls are ring-shaped reefs that often surround a central lagoon. Different reef shapes form because of geology, sea level, coral growth, and the changing shape of islands and coastlines.
Coral reefs provide hiding places, feeding grounds, and nursery areas for thousands of species. Reef residents can include clownfish, butterflyfish, sea turtles, crabs, sea stars, octopuses, eels, and reef sharks. Every species plays a part in the reef's complicated food web.
Many corals have microscopic algae living inside their tissues. The algae use sunlight to make food through photosynthesis and share some of that energy with the coral. In return, the coral gives the algae a protected place to live and materials they need for photosynthesis.
When corals experience severe stress, especially unusually warm water, they may lose the colorful algae living in their tissues. The pale skeleton beneath the coral then becomes visible, creating a condition called coral bleaching. Bleached coral is still alive at first, but long-lasting stress can make survival difficult.
Healthy reefs benefit both wildlife and people by supporting fisheries, tourism, biodiversity, and coastal protection. Reducing pollution, protecting marine habitats, avoiding damage to living coral, and addressing climate change can all help reefs. Scientists also study coral restoration methods that may help damaged reefs recover.