What gives many corals much of their colour?

A coral reef looks like rock, but it is alive. Corals are tiny animals, and many of them share their bodies with even tinier algae. The algae live inside the coral's tissues, help feed it, and give it much of its colour.

This partnership is delicate. When corals are stressed by a change in their surroundings, most often water that is too warm, they push the algae out. Without them the coral loses its colour and turns completely white. This is called coral bleaching.

A bleached coral is not dead. If the stress is mild and does not last, corals can recover. But if the water stays too warm for too long and the algae do not return, the coral grows weaker and eventually dies.

The damage can be enormous. In 2005, a mass bleaching event in the Caribbean killed about half of the coral reefs belonging to the United States in that region in a single year. Warm water is not the only danger, either: in January 2010, unusually cold water in Florida also caused bleaching and killed some coral.