Coral may expel their zooxanthellae if stressed, such as when water temperature gets too high (though the mechanism for how this expulsion occurs is poorly understood). Without their pigmented algae, the white coral skeleton can be seen through the translucent polyp—leading to the term “coral bleaching.” Bleaching may be an adaptation that allows the coral to take up a different species of zooks, one that can tolerate the warmer temperatures, for instance. Coral may be able to survive for a few months without the zooks but if the polyps don’t eventually take other algae back up, they will die. Multiple warming events or a warming event that persists for several weeks may stress the coral beyond its ability to survive. Pollution and even excess sun exposure are also linked to bleaching events.

Bleached staghorn coral on a Fiji reef

Tim Laman/National Geographic Stock