Coral Reefs Explained — What They Are, Where They Are, and Why a Warming Sea Threatens Them

A reef is a structure built by living animals. The builders are coral polyps — tiny soft-bodied creatures, each a close relative of the sea anemone and the jellyfish, that secrete a hard skeleton of limestone around themselves. Generation after generation builds on the skeletons of the last, and over thousands of years those skeletons accumulate into the vast limestone ramparts we call a reef. The living coral is only a thin skin on the surface; everything beneath is the rock its ancestors left behind. So a reef is at once a colony of animals, a geological formation, and a habitat — all the same thing.

Reefs cover less than one percent of the ocean floor, yet roughly a quarter of all marine species spend part of their lives on one. The reason is that the reef’s three-dimensional structure — all those nooks, ledges and branches — creates an enormous number of places to hide, hunt, and breed, the way a forest does on land. That is why reefs are called the rainforests of the sea: like a rainforest, a small area packs in a staggering variety of life. They also feed and protect people — reefs shelter coastlines from storm waves and support the fisheries and tourism that hundreds of millions of people depend on.

The single richest reef region is the Coral Triangle — the warm seas around Indonesia, the Philippines, Malaysia, Papua New Guinea, Timor-Leste and the Solomon Islands. More species of reef-building coral live there than anywhere else on Earth, by a wide margin (over three-quarters of all known coral species), which is why it is considered the global centre of marine biodiversity. The other great reef regions are the Great Barrier Reef off Australia, the Caribbean and the Mesoamerican Reef in the Atlantic, the Red Sea, and the atolls of the Indian and Pacific Oceans.

The Great Barrier Reef, off the north-east coast of Australia, is the largest reef system in the world — more than two thousand kilometres long, made up of thousands of individual reefs and hundreds of islands, and large enough to be seen from space. It is the most famous reef on the planet and one of the most biologically rich, home to thousands of species of fish, coral, molluscs and other life. On the map it is the densest single stretch of reef.

Coral gets most of its food, and its colour, from microscopic algae that live inside its tissues — a partnership that only works within a narrow band of temperature. When the water gets too warm for too long, that partnership breaks down: the coral expels the algae, loses its colour, and turns bone-white. That is bleaching. A bleached coral is not yet dead — it is starving, because it has lost the algae that fed it — and if the heat passes quickly it can recover. But if the warmth persists, the coral dies. Because bleaching is driven by heat, reefs are among the first ecosystems to show the strain of a warming ocean, and mass bleaching events have followed the marine heatwaves of recent years.

An atoll is a ring of coral reef enclosing a lagoon, with little or no land in the middle — the classic desert-island shape. It forms in a remarkable way, first worked out by Charles Darwin: a fringing reef grows around a volcanic island, and then, over millions of years, the volcano slowly sinks while the coral keeps growing upward toward the light. Eventually the volcano disappears beneath the surface entirely, leaving only the ring of reef that once surrounded it, encircling a lagoon where the island used to be. The Maldives and many of the islands of the Pacific are atolls.

The reef structure is mostly the limestone skeletons of corals that died long ago — so most of the bulk is not living. But the surface is very much alive: a thin, growing skin of coral polyps, covered in algae, fish, invertebrates and countless other organisms. A healthy reef is one of the most intensely living places on Earth. When people say a reef is ‘dying,’ they mean that living surface layer is bleaching and failing to regrow, leaving bare, algae-covered rock behind.