Can corals adapt to warmer temperatures?

For corals to survive the increases in seawater temperatures predicted for this century, they would need to adjust to the higher temperatures. There are several ways that they might do this.

It is unknown whether corals can adapt to warmer seas or will continue to bleach. Photo by GBRMPA.

Firstly, corals could alter their physiology in a process known as acclimatisation. Corals have several internal mechanisms to cope with stresses such as increased temperature and sunlight. However, these mechanisms are usually only effective for short periods and up to defined limits. They are unlikely to prepare corals for a future bleaching event. There is evidence that many corals have bleached two or three times during successive bleaching events, suggesting that acclimatisation may not be a reliable way for corals to adjust to climate change. If global temperatures rise in the predicted way, water temperatures in 100 years will be much greater than those that trigger bleaching now, so corals would need to acclimatise continually to survive. Most research indicates that acclimatisation is limited and unlikely to allow corals to adapt to the predicted water temperatures.

A second process by which coral populations could adapt to new conditions is by natural selection. This results in a gradual change in the temperature-tolerance of the population through the elimination of the coral colonies that cannot tolerate higher temperatures. Different colonies of the same coral species may respond to thermal stress differently. If only the most temperature-tolerant corals survive a bleaching episode, the offspring from those corals might be on average more temperature-tolerant than the previous generation. Again, there might be limits in how high the temperature can rise before corals reach the limits of adaptation. Such adaptations are thought to occur slowly, over several generations (with most corals having generation times of at least 5-10 years), but potential rates of adaptation have never been estimated.

A third process is one in which larvae from warm-adapted coral populations may disperse to cooler areas as they warm, thereby changing the distribution of species. However, this is likely to be a slow process.

Corals can have several types of zooxanthellae in their tissues. Some scientists have suggested that corals may adapt to warmer conditions by changing the dominant type of zooxanthellae within their tissues. Corals with a certain type of zooxanthellae can tolerate temperatures of 1-1.5°C higher than corals of the same species without that type. Scientists do not yet know how many of the 400 or so species of hard corals can change the type of dominant zooxanthellae or, indeed, what stimulates the change. Increased temperature tolerance may also come at a cost. For example, juvenile corals with heat-tolerant zooxanthellae grow up to three times slower than those with a different zooxanthellae type. The type of zooxanthellae that occur in the corals may also affect other aspects of coral fitness.

It is possible that a combination of natural selection and switching of zooxanthellae may help corals cope with climate change over the next few decades. Faster rates of change are possible for zooxanthellae than for the coral animal because the algae have shorter generation times. However, adaptation of corals and zooxanthellae to high temperatures has not yet been studied. Research is underway to investigate these different adaptation possibilities.

CRC Reef Research Centre Ltd was established and is supported under the Australian Government’s Cooperative Research Centres Program. Its mission is to plan, fund and manage world-leading science for the sustainable use of the Great Barrier Reef World Heritage Area. CRC Reef is a joint venture between Association of Marine Park Tourism Operators, Australian Institute of Marine Science, Great Barrier Reef Marine Park Authority, Great Barrier Reef Research Foundation, James Cook University, Queensland Department of Primary Industries and Fisheries, Queensland Seafood Industry Association, Sunfish Queensland Inc. The University of Queensland is an associate member.