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Stressed Corals-- what happens inside?

When corals are stressed, they expel their colorful—and life-sustaining—algae.

Dive into a coral polyp to see both how and why.

With the help of new tools, including high-powered underwater microscopes and photogrammetry equipment, scientists are gaining unprecedented insights into a process that is changing both the face and function of our planet’s coral reefs: coral bleaching. These new glimpses into an animal’s powerful physiological response to stress are both surprisingly beautiful and unsurprisingly sobering. When corals are under duress, they expel their resident algae—tenants that provide the animals with both their color and up to 90 percent of their energy. Without their in-house food source, the corals slowly begin to starve. This reaction has been occurring for millennia in response to a wide range of stressors such as fluctuating water temperatures and sedimentation. Until recently, however, coral bleaching happened only in a localized, ephemeral way.

In 1998, scientists documented the first mass coral bleaching event—a response to the warm waters of an El Niño year that killed 16 percent of the world’s corals. Since then, two more global bleaching events have occurred: one in 2010 and another, by far the worst yet, that lasted from 2014 to 2017 and impacted 70 percent of the world’s coral reefs. While all three events have been associated with El Niño conditions, these notorious climatic cycles aren’t solely to blame for the bleaching. Before 1998, El Niño years didn’t raise ocean temperatures enough to trigger large-scale bleaching. However, as global climate change increases base ocean temperatures—and as reef systems are weakened by local threats like overfishing, pollution, and sedimentation—El Niño events are taking an increasingly heavy toll.

Many scientists have found reason to hope that we can still save our planet’s coral reefs. As bioGraphic reported in the feature story "Picture of Health," healthy reefs that have been protected from overfishing, pollution, and coastal development are more resilient in the face of global climate change and bleaching events. And new approaches, like dosing reefs with probiotics, may help to fortify weakened reefs. But for now, the suite of threats impacting the world’s coral reefs mean that bleaching events will likely continue to occur with increasing frequency and severity. The more scientists understand about the mechanics of this response, the better able they’ll be to prevent or treat it in the future. This animation takes you inside the tissues of a stressed coral for a close-up look at what we know today about the process of coral bleaching.

Story by Stephanie Stone