Coral reefs are most well-known for their vibrant colors and vast biodiversity. However, not many people know that corals owe their bright rainbow of colors to symbiotic, single-cell algae that live inside their tissues. Reef corals have mainly colorless polyps and white skeletons—it is the tiny zooxanthellae living in their cells that gives the corals their color. They also manufacture organic matter that the coral uses as food. If stressed by disease or high temperatures, corals expel their zooxanthellae, in a process called coral bleaching, and may die of starvation.