A coral reef can look like a rock or a strange plant, but it is really the work of millions of tiny animals called coral polyps. Each polyp is only a few millimetres wide, yet together they create some of the largest living structures on the planet.
Every polyp takes minerals from the seawater and builds a hard skeleton of limestone around itself. When a polyp dies, its skeleton stays behind, and new polyps grow on top. Repeated over thousands of years, this slow process forms enormous reefs. Most polyps also share their home with tiny algae, which use sunlight to make food and give the coral both energy and colour.
Reefs are among the richest habitats on Earth. Although they cover only a small part of the ocean floor, they provide food and shelter for roughly a quarter of all marine species. Unfortunately, reefs are fragile. When the sea becomes too warm, the coral loses its algae, turns white and may die — a process called bleaching. Protecting the oceans from heat and pollution is therefore essential to keep these living cities alive.
