Corals are invertebrates that are in symbiosis with an algae or a dinoflagellate. Coral bleaching occurs when the corals lose their zooxanthellae (the algae or dinoflagellate is what causes the beautiful colours of coral). The result of losing its algae causes the coral to bleach (become completely white). When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white.
Scleractinian corals diversified during the Mesozoic and Caenozoic eras and are at the height of their development today.