Coral reefs are the undersea equivalent of rainforests, teeming with fish and other sea life. The reefs are built by tiny, sea-anemone-like animals called polyps
Coral polyps live all their lives in just one place, either fixed to a rock or to dead polyps.
Sea anemones look like flowers with petals, but they are actually carnivorous animals with their ring of tentacles.
Sea anemones are tiny, meat-eating animals that look a bit like flowers. They cling to rocks and catch tiny prey with their tentacles (see life on the seashore).
When coral polyps die, their cup-shaped skeletons become hard coral.
Coral reefs are long ridges, mounds, towers and other shapes made from billions of coral polyps and their skeletons.
Fringing reefs are shallow coral reefs that stretch out from the seashore.
Barrier reefs form a long, underwater wall a little way offshore.
The Great Barrier Reef off eastern Australia is the longest reef in the world, stretching over 2,000 km.
Coral atolls are ring-shaped islands that formed from fringing reefs around an old volcano (which has long since sunk beneath the waves).
Coral reefs take millions of years to form – the Great Barrier Reef is about 18 million years old, for example. By drilling a core into ancient corals, and analysing the minerals and growth rate, scientists can read history back for millions of years.
Many fish species live in warm seas around coral reefs. They are often very colourful, which makes them instantly recognizable to their own kind.
Butterfly fish and angelfish have slender, oval bodies and are popular as aquarium fish.
Male triggerfish boost their colour to attract females.
Cuckoo wrasse are all born female, but big females change sex when they are between 7 and 13 years old.
Cleaner fish are the health the lionfish are very poisonous. clinics of the oceans. Larger fish such as groupers queue up for cleaner fish to go over them, nibbling away pests and dead skin.
The banded coral shrimp cleans up pests in the same way as cleaner fish do, from fish such as moray eels.
The sabre-toothed blenny looks so like a cleaner fish that it can nip in close to big fish but then it takes a bite out of them.
Cheilinus is a carnivorous fish of coral reefs which changes colour to mimic harmless plant-eating fish, such as parrotfish and goatfish. It swims alongside them, camouflaged, until it is close to its prey.