Nav: Home

Corals are becoming more tolerant of rising ocean temperatures

August 07, 2018

The existence and causes of coral bleaching are recognized as an increasing world-wide environmental concern related to climate change. A number of experiments have been conducted since the early 1970s at the Hawai'i Institute of Marine Biology's (HIMB) Coral Reef Ecology Laboratory in Kāne'ohe Bay, Hawai'i and the Mid-Pacific Marine Laboratory (MPL) at Enewetak, Marshall Islands to determine the long-term temperature thresholds inducing coral bleaching. A new study published in PeerJ - the Journal of Life & Environmental Sciences replicates 1970s experiments and provides encouraging evidence to suggest corals today are adapting at an unexpectedly rapid rate. Still, these rates are being outpaced by rising ocean temperatures.

Coral bleaching is a process wherein corals lose their symbiotic algae, zooxanthellae, which provide a significant food source and color to their coral host. The white coral skeleton is then visible through the transparent tissues giving the characteristic "bleached" appearance. Mortality will occur if the coral and symbiont relationship is not reestablished shortly.

The 1970s experiments were the first to simulate elevated temperature stress in a flow-through seawater system, and the results duplicated the effects of high temperature events that have become increasingly wide-spread, frequent and severe in coral habitats worldwide in the last 35 years. Results determined temperature tolerances of corals were very low, only +1-2ºC above the normal maximum temperatures. Although temperatures varied geographically, most coral species began to bleach within this range.

The identical system, methodology, and location of the initial 1970s experiment was replicated in 2017 with one of the original researchers, Dr. Steve Coles. This provided an unique opportunity to evaluate whether coral bleaching thresholds have changed in nearly half a century. Corals were held at ambient and elevated temperatures comparable to the 1970 experiment for a one-month exposure, followed by a 28 days recovery period.

"To better understand coral acclimatization and adaptation, most studies compare corals from different reef locations, whereas this is the first study to compare the same coral species from the same location over time. Re-running a 48-year old experiment using the same coral species, same experimental setup, and same observer allows us to directly test changes in coral temperature tolerance over the last half century." Dr. Keisha Bahr

Differences between the two experiments were dramatic. Results show a substantial increase in temperature tolerance within the tested corals. In the three species of Hawaiian corals retested, bleaching occurred later, with higher survivorship and growth rates than corals in 1970. In 2017, survivorship was significantly higher (60-92%) as compared to 1970 (0-40%). Under elevated temperatures, calcification growth rates were reduced by an average of 26-63%, whereas in 1970, average calcification reductions ranged between 99-173%.

Such dramatic differences in coral bleaching temperature thresholds indicate a capacity for adjustment in temperature tolerance, either by changes in physiological process or shifts in symbiotic zooxanthellae types (acclimatization), or natural selection for the survival of more temperature tolerant corals (adaptation). Until now, it had not been determined how long these processes take or if this change can occur at a pace rapid enough to adjust to the frequency and severity of current elevated temperature events.

"Although these results are encouraging in their indication that acclimatization/adaptation of corals and their symbionts can occur at an unexpectedly rapid rate, increased bleaching tolerance may not be enough for widespread coral survival." Dr. Ku'ulei Rodgers.

"A possible influence on the results may be the substantially higher water quality in the Kāne?ohe Bay in 2017 as compared to 1970 due to nearby secondary treated sewage release at the time. Elevated levels of dissolved nitrogen have been implicated in stimulating coral bleaching. When verified, this will support the importance of reducing land-based source nutrients to assist management in limiting coral bleaching and mortality." Dr. Steve Coles

The slow growth and recruitment of many species of corals, combined with repetitive bleaching events of increasing severity and duration, may lead to catastrophic regional reductions in coral diversity and abundance. To prevent or even mitigate, this will require reduction in use of fossil fuels and lower emissions of CO2 and other greenhouse gases that are increasing air and seawater temperatures worldwide at an alarming rate.

Bleached coral colonies in the Kāne?ohe Bay, Hawaii. Photo credit: Dr. Keisha Bahr

Healthy and bleached brown rice coral (Montipora capitata). Photo credit: Dr. Keisha Bahr

Dr. Keisha Bahr surveying a large bleached coral colony. Photo credit: Ji Hoon Justin Han

Dr. Keisha Bahr surveying a healthy coral colony. Photo credit: Ji Hoon Justin Han

The Coral Reef Ecology Lab located at the Hawai?i Institute of Marine Biology in Kāne?ohe Bay, Hawaii. Photo credit: Claire Lager

Full Media Pack including image:

Link to the Published Version of the article (quote this link in your story - the link will ONLY work after the embargo lifts): your readers will be able to freely access this article at this URL.

Citation to the article: Coles et al. (2018), Evidence of acclimatization or adaptation in Hawaiian corals to higher ocean temperatures. PeerJ 6:e5347; DOI 10.7717/peerj.5347


PeerJ is an Open Access publisher of two peer-reviewed journals and a preprint server. PeerJ's mission is to help the world efficiently publish its knowledge. All works published by PeerJ are Open Access and published using a Creative Commons license (CC-BY 4.0). PeerJ is based in San Diego, CA and the UK and can be accessed at

PeerJ is the peer-reviewed journal for Biology, Medicine and Environmental Sciences. PeerJ has recently added 15 areas in environmental science subject areas, including Natural Resource Management, Climate Change Biology, and Environmental Impacts.

PeerJ has an Editorial Board of over 1,900 respected academics, including 5 Nobel Laureates. PeerJ was the recipient of the 2013 ALPSP Award for Publishing Innovation. PeerJ Media Resources (including logos) can be found at:

Media Contacts

For the authors:

Keisha Bahr -

For PeerJ: email: ,

Note: If you would like to join the PeerJ Press Release list, please register at:


Related Climate Change Articles:

The black forest and climate change
Silver and Douglas firs could replace Norway spruce in the long run due to their greater resistance to droughts.
For some US counties, climate change will be particularly costly
A highly granular assessment of the impacts of climate change on the US economy suggests that each 1°Celsius increase in temperature will cost 1.2 percent of the country's gross domestic product, on average.
Climate change label leads to climate science acceptance
A new Cornell University study finds that labels matter when it comes to acceptance of climate science.
Was that climate change?
A new four-step 'framework' aims to test the contribution of climate change to record-setting extreme weather events.
It's more than just climate change
Accurately modeling climate change and interactive human factors -- including inequality, consumption, and population -- is essential for the effective science-based policies and measures needed to benefit and sustain current and future generations.
Climate change scientists should think more about sex
Climate change can have a different impact on male and female fish, shellfish and other marine animals, with widespread implications for the future of marine life and the production of seafood.
Climate change prompts Alaska fish to change breeding behavior
A new University of Washington study finds that one of Alaska's most abundant freshwater fish species is altering its breeding patterns in response to climate change, which could impact the ecology of northern lakes that already acutely feel the effects of a changing climate.
Uncertainties related to climate engineering limit its use in curbing climate change
Climate engineering refers to the systematic, large-scale modification of the environment using various climate intervention techniques.
Public holds polarized views about climate change and trust in climate scientists
There are gaping divisions in Americans' views across every dimension of the climate debate, including causes and cures for climate change and trust in climate scientists and their research, according to a new Pew Research Center survey.
The psychology behind climate change denial
In a new thesis in psychology, Kirsti Jylhä at Uppsala University has studied the psychology behind climate change denial.

Related Climate Change Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Changing The World
What does it take to change the world for the better? This hour, TED speakers explore ideas on activism—what motivates it, why it matters, and how each of us can make a difference. Guests include civil rights activist Ruby Sales, labor leader and civil rights activist Dolores Huerta, author Jeremy Heimans, "craftivist" Sarah Corbett, and designer and futurist Angela Oguntala.
Now Playing: Science for the People

#521 The Curious Life of Krill
Krill may be one of the most abundant forms of life on our planet... but it turns out we don't know that much about them. For a create that underpins a massive ocean ecosystem and lives in our oceans in massive numbers, they're surprisingly difficult to study. We sit down and shine some light on these underappreciated crustaceans with Stephen Nicol, Adjunct Professor at the University of Tasmania, Scientific Advisor to the Association of Responsible Krill Harvesting Companies, and author of the book "The Curious Life of Krill: A Conservation Story from the Bottom of the World".