Coral reefs

Corals look like plants or rocks, but they are animals related to jellyfish and sea anemones. A reef-building coral colony is made of thousands of tiny polyps, each a soft tube with a ring of stinging tentacles. Polyps build hard skeletons of calcium carbonate beneath themselves.

Over thousands of years, generations of corals build on the skeletons of those before them. The Great Barrier Reef off northeastern Australia, the largest reef system on Earth, stretches for about 2,300 kilometres.

Reef-building corals depend on microscopic algae called zooxanthellae that live inside their tissues. The algae photosynthesise and pass most of the sugars they produce to the coral. In return, they gain a protected home and nutrients from the coral's waste.

This partnership is why reef corals need clear, shallow, sunlit water, and why they grow in warm tropical seas where nutrients are otherwise scarce. The algae also give many corals their colour.

Coral reefs cover well under one percent of the ocean floor but support around a quarter of all known marine species. The reef's three-dimensional structure offers countless hiding places, and its productivity feeds an enormous variety of fish, crustaceans, molluscs, sea turtles and sharks.

Reefs matter directly to people. Hundreds of millions depend on reef fisheries for food and income, reefs attract tourism, and they protect coastlines by breaking up waves that would otherwise erode beaches and damage coastal communities during storms.

• Fringing reefs grow close to shore along coastlines.
• Barrier reefs lie farther offshore, separated from the land by a lagoon.
• Atolls are rings of reef around a lagoon, formed when a volcanic island sinks while its fringing reef keeps growing upward, as Charles Darwin proposed.
• Deep, cold-water corals also exist, growing without symbiotic algae in dark waters.

When water becomes too warm for too long, the partnership breaks down. Stressed corals expel their algae, lose their colour and turn white, which is called bleaching. Bleached corals are not dead, and they can recover if temperatures return to normal, but if the stress lasts they starve and die.

Mass bleaching events linked to ocean warming have become more frequent and severe. The Great Barrier Reef has suffered several mass bleaching events since 2016, and global events have affected reefs across all major ocean basins.

Beyond warming, reefs face ocean acidification, which makes it harder for corals to build skeletons; pollution and sediment running off farmland; overfishing, which removes grazing fish that keep algae in check; destructive fishing with dynamite or poison; and outbreaks of coral-eating crown-of-thorns starfish.

Local protection helps reefs recover from damage, and scientists are studying heat-tolerant corals and coral restoration methods. But scientists broadly agree that the long-term survival of most reefs depends on limiting global warming.