Watch Coral Bleaching Happen Before Your Eyes

This video, taken by scientists in a laboratory at Queensland University of Technology in Brisbane, Australia, captures a coral ejecting its resident population of algae. The behavior helps corals survive warmer water in the short term, but extended periods of bleaching can be lethal to corals and the ecosystems they support. (Video courtesy Brett Lewis, QUT)

Corals are dying in ocean reefs worldwide, the victims of abnormally hot waters warmed by El Niño and climate change.

Scientists have long known that higher ocean temperatures set off a biological reaction called bleaching. Heat-stressed corals expel the colorful symbiotic algae that provide them with food, and in doing so, the corals turn bone white and face potential starvation.

Now, Australian scientists have captured these death throes on video—and it’s horrible to behold.

The researchers placed specimens of the coral Heliofungia actiniformis into aquatic tanks that simulate their natural environment, and then they turned up the heat.

The footage reveals that the corals eject the algae through a process called pulsed inflation. They expand their bodies to as much as 340 percent of their normal size before violently contracting and spitting out the tiny organisms over a period of four to eight days. Seen in time-lapse video, the sea creatures turn paler with each convulsion.

In the short term, bleaching does have its benefits.

“The rapid expulsion of the coral's algal symbionts during thermal stress … could very well increase H. actiniformis's chance of survival during abnormally high sea temperatures," says Luke Nothdurft, one of the investigating scientists, in a press release.

That’s because, over time, the heat causes the symbiotic algae to become toxic, and expelling the algae keeps the coral alive. But if the water doesn’t cool enough to allow new algae to quickly recolonize the coral, the bleached reef will die.

Signs of coral stress are already appearing around the globe. In April, a comprehensive new map revealed that up to 93 percent of Australia’s famed Great Barrier Reef is suffering from the effects of bleaching.

Follow Mark Strauss on Twitter.

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