What is a coral reef? How reefs form, live and grow
Coral reefs cover less than 1% of the ocean floor but support around a quarter of all marine species. What corals are, how reefs are built, and why they matter.
A coral reef can look like a garden of rocks and plants, but it is really a city built by animals. Each coral is a colony of tiny creatures called polyps, and over thousands of years their skeletons build up into reefs large enough to be seen from space. Here is how they work.
Is coral an animal or a plant?
Coral is an animal. A coral polyp is a small, soft-bodied creature related to jellyfish and sea anemones, with a ring of stinging tentacles around its mouth. Most reef-building corals live in colonies of thousands of identical polyps, all connected.
The reason corals can look like plants is a partnership. Inside their tissues live microscopic algae called zooxanthellae. The algae photosynthesise, using sunlight to make sugars, and pass most of that food to the coral. In return they get shelter and nutrients. This partnership gives corals much of their colour, and it is why most reefs grow in shallow, clear, sunlit water.
How reefs are built
Hard corals take calcium and carbonate from seawater and build skeletons of limestone (calcium carbonate) beneath their bodies. As colonies grow and die, new corals settle on the old skeletons, and other organisms such as coralline algae cement everything together. Over centuries this builds a reef. Some coral colonies grow only a few millimetres a year; fast-growing branching corals can add 10 cm or more.
Corals reproduce in two ways. Colonies grow by budding new polyps, and broken fragments can regrow. Many corals also spawn: on a few nights a year, often linked to the moon, they release clouds of eggs and sperm into the water, one of nature’s great spectacles.
Types of coral reef
- Fringing reefs grow close to the shore, as in Bonaire and the Red Sea.
- Barrier reefs are separated from the shore by a lagoon, like the Great Barrier Reef and the Belize Barrier Reef.
- Atolls are rings of reef around a lagoon, formed as a volcanic island sinks while the reef keeps growing upwards, as Charles Darwin first explained. Rangiroa and the Maldives are atolls.
- Deep or mesophotic reefs grow in dim light 30–150 metres down, such as Pulley Ridge.
There are also cold-water corals, which live in deep, dark water without algae and build reefs off Norway, Scotland and elsewhere.
Where coral reefs are found
Most tropical reefs lie between about 30° north and 30° south of the equator, in water between roughly 20°C and 30°C. The richest are in the Coral Triangle, between Indonesia, the Philippines, Malaysia, Papua New Guinea, Timor-Leste and the Solomon Islands, home to around three-quarters of all known coral species. See our list of the world’s top 20 coral reefs.
Why coral reefs matter
Reefs cover less than 1% of the ocean floor, yet around a quarter of all marine species depend on them at some point in their lives. They feed hundreds of millions of people, protect coastlines by breaking waves during storms, and support tourism worth many billions of dollars a year. Reef organisms have also given us medicines and materials used in medical research.
What threatens coral reefs
The biggest threat is warming seas. When water stays too hot for too long, corals expel their algae and turn white, a process called coral bleaching. If the heat lasts, they starve and die. Reefs also suffer from pollution and run-off from land, overfishing, destructive fishing, coral diseases, coastal development and ocean acidification, as the sea absorbs carbon dioxide.
How you can help
- Choose reef-safe sunscreen, or cover up with a rash vest.
- Never touch, stand on or collect coral.
- Choose dive and snorkel operators that use moorings and brief their guests.
- Avoid souvenirs made from coral, shells or turtle shell.
- Support reef protection and cut your carbon footprint where you can.
Want to see a reef for yourself? Read our guide to the best snorkelling in the world, and the best places to snorkel from the beach.
Questions people ask
What is a coral reef made of?
Mainly the limestone skeletons of hard corals, cemented together by coralline algae and other organisms.
How old are coral reefs?
Reefs have existed for hundreds of millions of years. Most living reefs began growing in their present form within the last 10,000 years, after the last ice age.
What is the largest coral reef in the world?
The Great Barrier Reef in Australia, about 2,300 km long.
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