Coral reefs are often called 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 marine species. Reefs provide food and shelter for thousands of kinds of fish, as well as for sea turtles, dolphins, octopuses and countless smaller creatures. They also protect coastlines from storms and bring billions of dollars a year to countries through tourism and fishing.
A coral reef is not a plant or a rock, as many people assume, but a massive living structure built by tiny animals called coral polyps. Each polyp has a soft body and a hard outer skeleton made of calcium carbonate. Over thousands of years, as generations of polyps live and die, their skeletons form the colourful, complex shapes that divers come to admire. Reefs also depend on microscopic algae that live inside the polyps and provide them with most of their food through photosynthesis.
In recent decades, coral reefs have suffered serious damage. Rising sea temperatures cause the algae to leave the polyps, turning the coral white in a process known as bleaching. Pollution from farms and cities adds further stress, and so does the increased acidity of the oceans, which makes it harder for polyps to build their skeletons. Some reefs that were healthy a generation ago are now nearly lifeless.
Scientists and local communities are working to save what remains. Marine protected areas limit fishing and anchoring in sensitive zones, while laboratories experiment with breeding corals that tolerate warmer water. The challenge is enormous, but even small reefs can recover if given the chance.