Enlarge / This southern reef escaped bleaching, but other areas were not so lucky. (credit: Rick Stuart-Smith, Reef Life Survey)

In a heat wave, humans sweat and try to avoid heat exhaustion. But in a coral reef, a wave of warm ocean temperatures can trigger a very different response from ours. Corals host single-celled, photosynthetic symbiotes, called zooxanthellae, that provide food in exchange for shelter. During a heat wave, the corals are forced to kick the zooxanthellae out (when stressed, the zooxanthellae secrete toxins) and thus lose their source of food as well as the majority of their color—hence the name “coral bleaching”.

If the water stays warm too long, the corals starve and leave behind lifeless carbonate skeletons. Young corals may repopulate the area in time, although algae will often claim the abandoned structures in the meanwhile.

The last few years have hit Australia’s Great Barrier Reef pretty hard, with a massive bleaching event in 2016 and persistently elevated temperatures giving no respite—part of a trend in a warming climate. A team led by the University of Tasmania’s Rick Stuart-Smith hit the water almost a year after the bleaching to get a first look at the recovery process.