Thermally tolerant corals still susceptible to bleaching

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Thermally tolerant corals still susceptible to bleaching

December 03, 2015 - 19:33
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Corals adapted to naturally high temperatures, such as those off the north west coast of Australia, are nonetheless highly susceptible to heat stress and bleaching.

Intertidal Acropora corals exposed to air at low tide

With up to 10m tides, the Kimberley region in north Western Australia has the largest tropical tides in the world, creating naturally extreme and highly dynamic coastal habitats that corals from more typical reefs could not survive.

Researchers at the University of Western Australia's Oceans Institute were thus surprised to find that corals from the region are just as sensitive to heat stress and bleaching as their counterparts from less extreme environments elsewhere.

"We found that exceeding their maximum monthly summer temperatures by 1° C for only a few days is enough to induce coral bleaching," says study lead author, Dr Verena Schoepf from the ARC Centre of Excellence for Coral Reef Studies (Coral CoE) "We were surprised because under normal conditions, Kimberley corals can tolerate short-term temperature extremes and regular exposure to air without obvious signs of stress."

These observations suggests that corals living in naturally extreme temperature environments are just as threatened by climate change as corals elsewhere.

Fat helps

A year ago, a different study also led by Dr. Schoepf discovered that fat helps coral survive heat stress over the short term—and now it seems that fat helps coral survive over the long term, too. The team found that the same fat-storing coral species that showed the most resilience in a 2014 bleaching study has recovered more fully in the year since, compared to other species that stored less fat. In fact, evidence that fat is a key component to coral survival has been building in recent years.

In 2014, the researchers exposed three different coral species to two rounds of annual bleaching, then tested them six weeks later to see how well they had recovered. At that time, finger coral (Porites divaricata), the species which kept the largest fat reserves, had fared the best. Boulder coral (Orbicella faveolata), which kept less fat reserves, had recovered to a lesser extent. Mustard hill coral (Porites astreoides), which stored the least fat, had recovered the least.

Now, one year later, the researchers have revisited the corals and discovered that both the finger coral and boulder coral have recovered, while the mustard hill coral has not yet recovered, and likely never will if bleaching frequency remains high.

Surprisingly, all three species appear to be healthy at first glance. The symbiotic algae had returned to their cells, so the corals’ normal color had returned. But further analyses of the corals’ bodies tell a different story.

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