Coral Reefs and Why They Matter

What is a coral reef?

A coral reef looks like an underwater garden of colourful rocks, but it is actually built by living animals. Each piece of coral is a colony of thousands of tiny creatures called polyps. A polyp is a soft, sack-shaped animal — a close relative of the jellyfish and the sea anemone — with a ring of stinging tentacles around its mouth.

Each polyp pulls minerals from seawater to build a hard cup of limestone (calcium carbonate) around its body. When a polyp dies, its skeleton stays behind, and new polyps grow on top. Over hundreds and thousands of years, these layers of skeleton stack up into the huge structures we call reefs. The Great Barrier Reef off Australia is so large — over 2,300 km long — that it can be seen from space.

A partnership that builds reefs

Reef-building corals have a remarkable secret. Living inside their tissues are microscopic algae called zooxanthellae (say "zoh-zan-THELL-ee"). These algae use sunlight to make food through photosynthesis, and they share that food with the coral. In return, the coral gives the algae a safe home and the chemicals they need.

This partnership is called symbiosis — two different living things helping each other. It is also why corals are so fussy about where they live. They need:

• Warm water, usually between 23 °C and 29 °C.
• Clear, shallow water, so sunlight can reach the algae.
• Clean water, free of too much mud or pollution.

The algae also give corals their bright colours. A coral without its algae is pale and white.

Why reefs matter so much

Coral reefs cover less than 1% of the ocean floor, yet they are home to roughly a quarter of all marine species. Fish, sea turtles, octopuses, sharks, crabs, sponges and countless others rely on the reef for food and shelter. That is why reefs are nicknamed the "rainforests of the sea." To see how this fits into the wider ocean, explore oceans and sea life.

Reefs matter to people too:

• Coastal protection. Reefs break the power of waves before they hit the shore, shielding beaches and towns from storms and erosion.
• Food. Hundreds of millions of people get protein from reef fish.
• Jobs and tourism. Diving and snorkelling bring income to many countries.
• Medicine. Scientists have found chemicals in reef creatures used to develop treatments for pain, cancer and infections.

The big threat: coral bleaching

When the sea gets too warm — even by just 1 or 2 degrees for a few weeks — corals become stressed and push out their algae. This is called coral bleaching, because the coral turns ghostly white. Without its algae, the coral loses its main food supply. If the water cools quickly, the algae can return. But if the heat lasts, the coral starves and dies.

Bleaching is becoming more common as the oceans warm, which is closely linked to the greenhouse effect. Other dangers include pollution, overfishing, and ocean acidification, where carbon dioxide dissolving into the sea makes it harder for corals to build their skeletons.

What is being done

People are working to save reefs in many ways: creating marine protected areas where fishing is limited, reducing pollution that runs off the land, and even "gardening" — growing coral fragments in underwater nurseries and replanting them on damaged reefs. The most important step of all is slowing climate change by cutting greenhouse gas emissions.

Try it yourself: model a coral polyp's home

You can model why corals need clean, clear water.

1. Fill two clear glasses with water and place a small green leaf (standing in for the algae) at the bottom of each.
2. Leave one glass clear. Stir a spoonful of mud or cocoa powder into the other.
3. Shine a torch down into both glasses, as if it were sunlight.
4. Notice how much light reaches the leaf in each glass.

The muddy water blocks the light, just as polluted or sediment-filled seawater stops sunlight reaching the algae that keep corals alive. This is one reason keeping rivers and coasts clean helps reefs survive.