How do coral reefs grow?

Each coral polyp lives in a symbiotic relationship with microscopic algae called zooxanthellae, which provide much of the coral’s energy through photosynthesis. The polyps deposit calcium carbonate skeletons beneath them, and new generations grow on top of old ones. This slow process can create immense reefs such as the Great Barrier Reef, the largest reef system on Earth. Reefs need warm, clear, shallow water and are highly sensitive to heat stress and ocean acidification. Because they support huge biodiversity, reefs are often called the rainforests of the sea.