Corals are pretty and colorful and fluorescent. They produce their vibrant colors because they don’t live alone, which is also what keeps them alive.

Over billions of years they’ve worked out a special arrangement with algae: Corals give them shelter and algae convert light into food for the corals. Corals do other things for the algae, too. Deep inside their tissue are little proteins that take the sun’s ultraviolet light and turn it into a glowing green sunscreen, shielding from the sun these corals that live just below the water’s surface.