The majority of corals do not possess a coloration of their own, their vibrant colors provided by the photosynthesized pigments of their symbiotic zooxanthellae. Yellowish brown, green or red are the most common colors. When the sea temperature exceeds thirty degrees celsius, the zooxanthellae start to produce a toxic substance and are, as a direct consequence, expelled by their host. the coral then loses its color. as the coral tissue is transparent, the coral skeleton, which is white, becomes visible and lends its name to the phenomenon known as coral bleaching. if the temperature does not go back to its normal levels in few weeks, allowing the coral to retrieve its zooxanthellae, they will eventually die. one of the biggest threats to the coral reef future is global warming.