Coral reefs: ecology | WWF
All coral reefs began life as single polyp - a tiny, soft marine animal like a small sea anemone - which attached to a hard surface.
Coral polyps, California, USA.

"Stony" coral species (order Scleractinia) live as colonies and exude calcium carbonate, which forms an external skeleton. As the polyps grow and die, these stony corals create the reefs that we know. There are also other coral and coral-like species, including soft corals (order Alcyonacea) which do not form a skeleton.

Like their jelly fish and sea anemone relatives, coral polyps have stinging tentacles that they use to catch food. During the day, these tentacles are usually tucked away, out of reach of hungry fish and other marine animals. But at night they unfold to catch their prey, mainly plankton - small plants and animals floating in the currents.

Tropical corals additionally get energy from small algae living symbiotically inside the polyp. It is these algae, called zooxanthellae, which give the corals their colour.

There are three basic kinds of coral reefs: fringing reefs, barrier reefs, and atolls. Fringing reefs grow in shallow waters close to the coast. Barrier reefs are separated from land by a lagoon, growing parallel to the coast and forming a large and continuous reef. Atolls are ring-shaped reefs that develop near the sea surface on underwater islands or islands that sink, or subside.

© / WWF