Cities built by animals

From above, a coral reef can look like a garden of colourful rocks. Look closer and the rocks turn out to be alive. A reef is built by animals — tiny coral polyps — over many generations, and it becomes a city for thousands of other species.

Small area, huge importance

NOAA’s National Ocean Service puts the importance of reefs into two numbers. Coral reefs cover only about 1 percent of the world’s oceans, yet they provide habitat for at least 25 percent of the world’s marine life. Reefs are home to more than 4,000 species of fish, corals and other marine life.

That concentration of life is why reefs are so often compared to rainforests. A single reef offers shelter, hunting grounds, nurseries and cleaning stations for animals of every size.

An animal with a plant inside

Corals are animals, related to jellyfish and sea anemones. But many reef-building corals have a secret partner. Inside their tissues live microscopic algae called zooxanthellae.

NOAA explains that these algae are the coral’s primary food source and give the coral its colour. The algae use sunlight to make food, and pass much of it on to the coral. In return, they get a sheltered place to live. This partnership is the engine of the reef — and the reason reef-building corals grow in clear, sunlit, shallow water.

When the partnership breaks

Corals are sensitive to their surroundings. When they are stressed, NOAA explains, they expel the algae living in their tissues and turn completely white. This is called coral bleaching.

Several things can cause it: pollution, very strong sunlight and extremely low tides. But NOAA names the leading cause plainly: increased ocean temperature caused by climate change.

A bleached coral is not dead — but it is in trouble. Without its algae it has lost its main food source. If the stress is not severe, corals have been known to recover. If the loss of algae lasts and the stress continues, the coral eventually dies. A reef that suffers repeated bleaching may not get enough time to recover in between.

What reefs do for people

Reefs are not only important for marine life. NOAA describes coral reefs as a natural barrier against waves, storms and floods, helping to protect coastlines. Reef fisheries feed people and support jobs: NOAA estimates the commercial value of U.S. fisheries from coral reefs at more than 100 million dollars. And many drugs are now being developed from compounds discovered in coral reef animals and plants.

What helps reefs

Reducing the warming of the oceans is a global task. But reefs also benefit from local care: cleaner water with less pollution and run-off, less physical damage from anchors and careless diving, and healthy fish populations that keep the reef in balance. Snorkellers and divers can help by not touching or standing on coral and by choosing operators that respect the reef.

Why it matters

A coral reef shows how much life a small place can hold when conditions are right. It also shows how fragile such a place can be. The partnership between coral and algae has built some of the largest structures made by living things. Keeping it intact is one of the clearest tests of how well we look after the ocean.