A coral reef looks like rock dressed in color, but the living part is a thin skin of animals. Corals are colonies of tiny polyps, relatives of jellyfish, that secrete calcium carbonate cups. Many reef-building species host algae called zooxanthellae inside their tissues. The algae photosynthesize and share sugars. The coral offers shelter and nutrients. That partnership lets reefs grow in clear, nutrient-poor tropical water where other structures would starve. The skeleton they leave behind becomes the maze of ridges, caves, and flats that fish, crabs, and turtles use as home.

Reefs need warmth, light, and water that is not too fresh or too muddy. They grow slowly. Storms, crown-of-thorns starfish, and bleaching can undo decades in a season. Bleaching happens when stress, often heat, makes corals expel their algae and turn white. They can recover if conditions ease, but repeated heat leaves dead limestone.

Though they occupy a tiny fraction of the ocean floor, reefs support a huge share of marine species and protect coasts by breaking wave energy. People harvest fish, tourists come to look, and limestone from ancient reefs is now building stone in cities far from any lagoon. A reef is an animal city built of its own bones, powered by sunlight trapped in borrowed plants.
