From the flight deck of a spacecraft, most signs of life on Earth vanish into an anonymous blur of green and brown. One does not. Trailing along the north-east coast of Australia, a pale turquoise seam runs for well over 2,000 kilometres — longer than the distance from London to Warsaw — clearly visible from low orbit. Astronauts have picked it out with the naked eye for decades. It is the Great Barrier Reef, routinely described as the largest structure ever built by living things, and its architects are among the smallest animals in the sea.
The reef is not one reef but a sprawling mosaic: roughly 2,900 individual reefs and some 900 islands and cays, spread across an area of around 344,000 square kilometres off the coast of Queensland. That is larger than Italy. It sits within the Great Barrier Reef Marine Park, one of the most heavily protected marine zones on the planet, and was inscribed as a UNESCO World Heritage Site in 1981. For all that scale, the thing was assembled grain by grain over thousands of years by creatures you could balance on a fingertip.
The animals that build islands
The builders are coral polyps — soft, tentacled animals related to sea anemones and jellyfish, most only a few millimetres across. Each polyp draws dissolved calcium and carbonate from seawater and secretes a cup of limestone around its lower body. Millions live together in colonies, and as generations die their stony skeletons remain, cemented into ever-larger ramparts on which the next generation grows. The living veneer of a reef is thin; beneath it lies the accumulated masonry of ancestors. The present reef structure is geologically young, most of it having grown since the end of the last Ice Age, when rising seas flooded the coastal plain and gave the corals fresh ground to colonise.
What gives reef-building corals their vivid colour, and their extraordinary productivity, is a partnership. Inside the polyps' tissues live microscopic algae called zooxanthellae. The algae photosynthesise, converting sunlight into sugars, and hand the great majority of that energy to their coral hosts; in return they get shelter and a steady supply of the polyps' waste nutrients. This symbiosis is so efficient that coral reefs flourish in the clear, nutrient-poor tropical waters where little else can, which is why they are often called the rainforests of the sea. Reefs cover well under one per cent of the ocean floor yet support roughly a quarter of all marine species.
Why the reef turns white
The partnership is also the reef's weakness. The algae are exquisitely sensitive to temperature. When the surrounding water grows too warm for too long — often just a degree or so above the usual summer maximum, sustained over weeks — the relationship breaks down and the coral expels the algae from its tissues. Stripped of the pigmented algae, the transparent polyp tissue reveals the white limestone skeleton beneath, and the coral turns a ghostly, luminous white. This is coral bleaching. It is worth being precise about a common misconception: a bleached coral is not yet a dead coral. The polyp is still alive but starving, having lost the partner that supplied most of its food. If cooler conditions return quickly, the algae can recolonise and the coral may recover; if the heat persists, the coral dies and is soon overgrown.
The other great confusion is the reef's own status as an inanimate landmark. The Great Barrier Reef is not a rock formation with some coral on it; the visible structure is, layer upon layer, the collective skeleton of the animals that made it — a piece of geography that is also, uniquely, biography. Mass bleaching events, once rare, have struck the reef repeatedly in recent decades during unusually warm summers, and they remain the subject of intense monitoring and research. Alongside heat, the reef contends with cyclones, coastal runoff and periodic population explosions of the coral-eating crown-of-thorns starfish, a spiny predator that can strip a reef of living tissue.
Stand on a boat above it on a calm day and the water is glassy and unremarkable; the drama is all below and, improbably, all above, in the one work of animal engineering that a human being can see from space. The corals that raised it will never know how large their city grew.
Quiz nuggets
- The Great Barrier Reef stretches for over 2,000 kilometres off Queensland and covers around 344,000 square kilometres — larger than Italy.
- It is the largest structure built by living organisms and was made a UNESCO World Heritage Site in 1981.
- Reefs are built by coral polyps, tiny animals related to jellyfish and anemones, which secrete limestone skeletons.
- Coral colour and food come from symbiotic algae called zooxanthellae living inside the polyps' tissues.
- Bleaching happens when warm water makes corals expel their algae, exposing the white skeleton — a bleached coral is stressed, not necessarily dead.