Can coral reefs adapt?

The temperature tolerance limits of corals

Maximum summer sea temperatures that are just 2 – 3°C above normal values can kill corals. The upper temperature limit of corals depends on where they are found. Corals that usually live in cooler conditions, for example, where summer maximum temperatures are 28°C, will be bleached at lower temperatures than corals that usually live in hotter parts of the reef where summer temperatures reach 31°C.

The extent of bleaching also depends on the length of time that the water temperature is raised. A coral which normally grows in summer temperatures of 29°C may have little reaction if exposed to temperatures of 32°C for a few hours, but could bleach if temperatures reach 31°C for a week. It is a combination of the length of time, and the amount by which water temperature exceeds normal maximum values, that is critical in predicting the extent of bleaching.

Can corals adapt to warmer temperatures?

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

Firstly, corals could alter their physiology in a process known as acclimatisation. Corals found in warmer waters are more tolerant of high temperatures than corals found in cooler waters, so some changes in coral physiology seem possible. There is no evidence that corals have acclimatised to resist bleaching in the last 20 years, because many corals have bleached two or three times. If global temperatures rise in the predicted way, water temperatures in a hundred years will be much greater than those that trigger bleaching now, so corals would need to continue to acclimatise to survive.

The 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. 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 go before corals reach the limits of adaptation. Such adaptations could only occur slowly, over several generations, with most corals having generation times of at least five to ten years. Alternatively, already warm-adapted coral populations may disperse to cooler areas, thereby short-circuiting the normally lengthy process of adaptation.

Adaptation of zooxanthellae is also a possibility, with natural selection that favours more temperature-resistant strains of algae. Faster rates of change are possible for zooxanthellae than for the coral animal because the algae have shorter generation times. Adaptation of zooxanthellae to high temperatures has not yet been observed. Research is underway to investigate these different adaptation possibilities.