They expel greenish clouds of beneficial algae, leaving them depleted and vulnerable to death.

A diver surveys a graveyard of bleached corals in May near Jarvis Island in the Pacific Remote Island Marine National Monument. (NOAA/Bernardo Vargas-Angel/AP)

The world’s oceans are in the midst of their third-recorded, and largest known, mass bleaching event, a phenomenon in which extreme temperatures make corals turn chalky white and eventually die.

Every significant reef region on the planet has been touched by the necrotic hand of bleaching, which is partly caused by climate change; nearly a quarter of Australia’s Great Barrier Reef alone has perished. So what’s going on under the waves? Researchers from the Queensland University of Technology have answered that with weirdly fascinating footage of hot corals undergoing “pulsed inflation”—which, to anyone who’s puked to the point of dry heaves, will be distressingly familiar.

The scientists placed a tentacled coral known as Heliofungia actiniformis in an aquarium and cranked the heat from 78 to 90 degrees Fahrenheit. In a short period the coral started showing signs of stress, belching out the symbiotic algae that give it the nutrients it needs to thrive. This is one such forced expulsion, filmed in time-lapse:


“Our H. actiniformis used a pulsed inflation to expel Symbiodinium [algae] over time (seen as greenish plumes in the video)—inflating their bodies to as much as 340 per cent of their normal size before suddenly and violently contracting and ejecting Symbiodinium through their oral openings over the four to to eight day duration of the experiments,” researcher Luke Nothdurft says in a press release. Here’s more:

“What’s really interesting is just how quickly and violently the coral forcefully evicted its resident symbionts,” said [Brett] Lewis, from QUT’s Science and Engineering Faculty.

“The H. actiniformis began ejecting the symbionts within the first two hours of us raising the water temperature of the system.”...

While scientists have known for some time that coral bleaching occurs when the relationship between the coral and their Symbiodinium breaks down as ocean temperatures rise, the QUT team’s time-lapse videos show for the first time how this coral removes the algae.

The vomiting leaves coral depleted and the color of bone but, if water conditions return to normal, the researchers say it sometimes can regain its algae and health. If the algae don’t recolonize fast enough, however, the coral can perish. (Read more about this experiment and bleaching in the most recent Coral Reefs.)

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