Cities Beneath the Waves

A coral reef looks like colorful stone, but it is actually a city of tiny animals called polyps. Each polyp builds a hard cup of limestone around its soft body, and millions of these cups, stacked over centuries, form the reef. The polyps have partners: microscopic algae live inside their tissues, turning sunlight into food that the polyps share. The algae also give coral its brilliant colors. When ocean water grows too warm, the stressed polyps expel their algae. The reef turns bone white — an event called bleaching — and the polyps begin to starve. If the water cools in time, algae return and the city recovers; if not, the reef dies, and the thousands of fish species that shelter there lose their home.