Coral reefs are often described as the rainforests of the sea. Although they…

Coral reefs are often described as the rainforests of the sea. Although they cover less than one percent of the ocean floor, they are home to roughly a quarter of all known marine species. Yet many people are surprised to learn that corals are not plants or rocks but animals.

Each coral is made up of hundreds or thousands of tiny animals called polyps, many no bigger than a grain of rice. Over time, the polyps build hard skeletons of calcium carbonate, and these skeletons, piled up over thousands of years, form the enormous structures we know as reefs.

The secret of a reef's success lies in a partnership. Living inside the tissues of most reef-building corals are microscopic algae. The algae use sunlight to produce food through photosynthesis, and they share much of this food with the coral. In return, the coral provides the algae with a safe home and the materials they need. The algae also give many corals their bright colours.

This partnership, however, is fragile. When the water becomes too warm for too long, the relationship breaks down and the coral pushes the algae out of its tissues. Without them, the coral loses its colour, revealing the white skeleton beneath. This process is known as bleaching. A bleached coral is not dead, and if temperatures return to normal quickly enough, the algae can come back and the coral may recover. But if the heat continues, the coral will eventually starve.

Bleaching has become much more common as ocean temperatures have risen. Australia's Great Barrier Reef, the largest reef system in the world, has suffered several mass bleaching events since 2016, each affecting large parts of the reef. Even corals that survive a bleaching event are often weakened, growing more slowly and becoming more vulnerable to disease.

The consequences reach far beyond the reefs themselves. Hundreds of millions of people depend on reefs for food, income from fishing and tourism, and protection from storms, since reefs break the force of waves before they reach the coast. Scientists are experimenting with ways to help, such as growing heat-tolerant corals in nurseries and planting them on damaged reefs. Most agree, however, that such efforts can only buy time. In the long run, the future of coral reefs depends on limiting the rise in ocean temperatures.