Heat waves are roasting reefs, but some corals may be resilient

It’s no secret that warming ocean waters have devastated many of the world’s coral reefs. For instance, a 2016 marine heat wave killed 30 percent of coral in the Great Barrier Reef, a study published online April 18 in Nature reports. But some coral species may be able to adapt and survive in warmer waters for another century, or even two, a second team reports April 19 in PLOS Genetics. And that offers a glimmer of hope for future ocean biodiversity.

“What we’ve just experienced [in the Great Barrier Reef] is one hell of a natural selection experiment,” says coral reef expert Terry Hughes of James Cook University in Townsville, Australia. In total, about 50 percent of the reef’s corals have died since 2016, he says. A bright side, maybe: “The ones that are left are tougher.”

While the marine heat wave particularly damaged staghorn corals, at least one staghorn species, Acropora millepora, may prove to be resilient, Mikhail Matz, a biologist at the University of Texas at Austin, and his colleagues report in PLOS Genetics. A new analysis shows the branching, fast-growing coral — a key reef builder — is genetically diverse enough to survive for another 100 to 250 years, depending on how quickly the planet warms. Other studies have suggested coral reefs may not last this century.

What happens to coral reefs affects vast underwater ecosystems, and the hundreds of millions of people who depend on those ecosystems for fishing, tourism and more. So scientists want to understand how corals might fare as climate change brings longer and stronger marine heat waves (SN: 4/10/18, p. 5).

These heat waves can cause coral bleaching (SN: 02/03/18, p. 16) — corals eject the symbiotic algae known as zooxanthellae that provide corals with both nutrients and color. If the algae don’t return, the corals eventually turn bone white and die.

In extreme cases, marine heat waves kill corals directly by essentially roasting them alive. That’s one of the things that happened in the Great Barrier Reef, Hughes and his colleagues found. Using satellite images and ocean surveys, the team found that it took less heat stress — how hot the water was, and how long the water was hot — to kill off corals than expected.

Staghorn corals proved to be particularly vulnerable to those marine heat waves. Yet Matz’s study suggests that, extreme events aside, the corals show signs of adapting quickly enough to keep pace with warming waters — at least for now.

A. millepora corals live throughout the Great Barrier Reef, though waters in the northern portion can be more than 5 degrees Celsius warmer than in the south. That means “there are already genetic variants out there which make corals locally adapted,” he says. As an area gets too hot to survive, these heat-adapted corals could migrate, sending out larvae that settle in slightly cooler spots. This reproduction strategy lets corals spread rapidly to more suitable habitats without needing to wait for genetic mutations that make it hardier in the face of change.

If corals need to move to adapt, humans should focus on connecting reefs so migration can more easily happen, says William Cheung, a marine biologist at the University of British Columbia at Vancouver who was not involved in either study. That may be easier said than done, given widespread damage Hughes’ team documented in the Great Barrier Reef.

Even in light this recent destruction, Matz says his team’s study suggests that all is not yet lost. “There is still time for us to act before corals actually start going extinct.”    

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Editor’s note: This story was updated on April 20, 2018, to correct the location of Magnetic Island and on April 25, 2018, to clarify that not all staghorn corals belong to the species Acropora millepora.