Great Barrier Reef

Coral looks like a plant or a rock, but it is made by animals.

Tiny coral animals build hard homes. They grow on top of old ones, again and again. Over a very long time, they make a whole reef.

Coral may look like rock or a plant, but corals are animals. A coral is made of hundreds to thousands of tiny creatures called polyps, most no thicker than a nickel. Each soft polyp builds a hard skeleton of limestone underneath itself. When polyps die, new ones grow on top, and slowly the limestone piles up into a reef.

Most corals have tiny plant-like algae living inside them. The algae use sunlight to make food and share it with the coral. UNESCO says today's reefs have grown over the past 15,000 years on the continental shelf, the shallow sea floor next to Australia.

NOAA explains that stony corals are colonies of polyps, each secreting a skeleton of calcium carbonate onto rock or the skeletons of earlier polyps; as generations grow and die, they lay down the limestone framework of a reef. Most corals host zooxanthellae, microscopic algae living in their tissues that photosynthesize using the coral's waste products. Reefs begin when coral larvae settle on hard surfaces near coasts; barrier reefs, like this one, border a shoreline at a distance.

UNESCO describes the Great Barrier Reef as shaped by at least four glacial and interglacial cycles. When glacial sea levels fell, the reefs were exposed as flat-topped hills of eroded limestone, with rivers winding between them; when seas rose again, continental islands and coral cays formed and new coral growth began. Over the past 15,000 years reefs have grown on the continental shelf, and old massive corals preserve this history in their cores.

Sometimes the sea gets too warm.

Then some corals turn white. Corals can get better if the water cools down soon. Many people work to keep the reef healthy.

Corals are sensitive to heat. When the water is too warm, corals push out the tiny algae living inside them and turn completely white. This is called coral bleaching. A bleached coral is not dead. If the stress is not too severe, it can recover, but if it goes on too long, the coral can die.

In 2024 the Great Barrier Reef had its fifth mass bleaching since 2016, during a bleaching event around the world. People are working to help. Parts of the marine park are no-take zones, where nothing may be caught, reef fishing stops at times when reef fish spawn, and plans aim to keep the water that flows from the land clean. UNESCO says cleaner water and healthier coasts give the reef its best chance to recover.

NOAA explains bleaching: when corals are stressed by changes in temperature, light or nutrients, they expel their zooxanthellae and turn white; they can survive, but if the stress continues they eventually die. Smithsonian Magazine reported in April 2024 that NOAA's Coral Reef Watch had confirmed the fourth global coral bleaching event on record, and that for the Great Barrier Reef Marine Park it was the fifth mass bleaching since 2016, with all three of the park's areas bleaching heavily at the same time for the first time.

UNESCO's summary of the 2009 Outlook Report says the reef's long-term challenges are dominated by climate change, alongside declining water quality from land-based sources, loss of coastal habitats and some impacts of fishing; cyclones and crown-of-thorns starfish outbreaks are natural pressures. Responses include a five-yearly Outlook Report, a Reef Water Quality Protection Plan, a Climate Change Action Plan, 115,000 km² of no-take and no-entry zones, and a ban on reef fishing during the moon phases when reef fish spawn. UNESCO notes that most habitats and species groups can recover from disturbance, and that building resilience gives the reef its best chance against a changing climate.

Look at the coral, but never touch it or stand on it.

Never chase turtles or fish. Take all your rubbish home with you. Then the reef stays beautiful for the fish.

Queensland's parks service and the Reef Authority ask boat crews to use public moorings, blue cone-shaped buoys, whenever they can. Dropping an anchor on coral can damage or destroy it in seconds, and the coral may take years to grow back, or never fully recover. Where there is no mooring, boats anchor in sand or mud, away from coral, and never inside the white pyramid-shaped buoys that mark no-anchoring areas.

In the water, never touch, stand or rest on coral. NOAA's Florida Keys marine sanctuary explains that even a brush with a hand or fin can damage the tiny coral animals. Don't touch, feed or chase turtles, fish or other sea animals. Take all your rubbish with you and put it in bins on shore.

A 2019 guide from the Queensland Parks and Wildlife Service and the Reef Authority sets out how to protect coral. Use public moorings, blue cone-shaped buoys whose colored band shows the vessel size and wind limits; never anchor inside the white pyramid-shaped reef protection markers that mark no-anchoring areas; and otherwise anchor in sand or mud away from coral. An anchor dropped on coral can damage or destroy it in seconds; even under ideal circumstances it may take years to rebuild, and some coral never returns to its former state. Damaging coral is an offense in the marine parks. The guide also asks visitors to check zoning maps, avoid taking plant-eating fish such as parrotfish, which remove seaweed and make space for new corals, report suspected illegal fishing, never touch or stand on corals, and take all rubbish ashore.

General reef guidance from NOAA's Florida Keys National Marine Sanctuary adds that even a minor brush with hands, fins or gear can damage coral, and that touching, handling, feeding or chasing marine life can stress animals and disrupt their feeding and mating.