Evolution, climate change and coral

Increased carbon dioxide in the atmosphere poses several problems for organisms living in the marine environment. Increases in temperature and ocean acidification are the two best known and most worrying. In order to predict how climate change and ocean acidification will affect marine species, we need to know how they respond to these conditions. The effect of climate change on corals has attracted a lot of attention because of their importance for biodiversity.

We can't just expose corals to predicted conditions because corals of the future won't be naive to these environments and are likely to have evolved. We know that evolution can be extremely rapid, often within decades. Ignoring the potential for evolution to influence the effects of climate change on marine organisms could lead to inaccurate projections of the effects of climate change on extinction risk. Yet many authors are ignoring the effects of evolution and acclimation in making their predictions. 

The three-spine stickleback, Gasterosteus aculeatus has been documented adapting to freshwater conditions from saltwater ancestors in just 13 generations (photo Wikipedia)

In their 2007 paper, Hoegh-Guldberg, et al. dismiss the importance of evolution because "reef-building corals have relatively long generation times and low genetic diversity, making for slow rates of adaptation". But, long generation times are not present in all coral species and the response of corals to climate change is going to depend partly on their algal symbionts, which have short generation times. 

Unfortunately, the rates of evolution in corals and their symbionts are extremely poorly known. In terrestrial systems though, genetic variation for traits related to thermal performance is common and evolutionary responses to changing climate are typical. For instance, changes in allele frequencies consistent with responses to global warming have been documented in a number of insects, such as fruit flies and mosquitoes (e.g. Bradshaw & Holzapfel 2001, Umina et al. 2005).

Acclimation, or phenotypic plasticity, will also affect the way that corals respond to climate change. Plastic responses to the environment can occur within generations and across them. For instance, Donelson et al. (2011) looked at the tropical damselfish, Acanthochromis polyacanthus, and found that their offspring could completely compensate for the negative effects of higher temperatures. But, this only occurred when both they and their parents where reared at the same temperature.

The tropical damselfish, Acanthochromis polycanthus (photo Wikipedia)

There are indications that some acclimation is occurring in corals too. Under stress, corals expel their algal symbionts, which gives them the appearance of having been bleached. Coral reefs that experience greater variability in sea surface temperature and those that have recently been subjected to bleaching are less susceptible to bleaching. This greater resilience suggests that some acclimation to climate change is possible within short time-frames. 

A bleached coral in the foreground with an unbleached coral of the same species behind (photo Wikipedia)

We need a better understanding of how evolution and acclimation may influence the response of corals to climate change so that our predictions are accurate. But, we already know which direction things are probably going to go. John Pandolfi's work has shown that under historical climate change, diversity on corals reefs has declined and populations have moved to higher latitudes (e.g. Pandolfi et al. 2011, Kiessling et al 2012). 

Climate change is currently more rapid than previous episodes and this will limit the amount of adaptation that can occur. Corals are also already under significant pressure from other anthropogenic sources of stress that have resulted in substantial declines and changes in population composition. These pressures, too, will decrease the ability of corals to cope with the effects of climate change. By removing these pressures, we will give corals the best chance possible to adapt to a warmer and more acidic ocean.

Hoegh-Guldberg, O., Mumby, P., Hooten, A., Steneck, R., Greenfield, P., Gomez, E., Harvell, C., Sale, P., Edwards, A., Caldeira, K., Knowlton, N., Eakin, C., Iglesias-Prieto, R., Muthiga, N., Bradbury, R., Dubi, A., & Hatziolos, M. (2007). Coral Reefs Under Rapid Climate Change and Ocean Acidification Science, 318 (5857), 1737-1742 DOI: 10.1126/science.1152509  

Bradshaw, W., & Holzapfel, C. (2001). Genetic shift in photoperiodic response correlated with global warming Proceedings of the National Academy of Sciences, 98 (25), 14509-14511 DOI: 10.1073/pnas.241391498

Umina, P., Weeks, A. R., Kearney, M. R., McKechnie, S. W., & Hoffmann, A. A. (2005). A Rapid Shift in a Classic Clinal Pattern in Drosophila Reflecting Climate Change Science, 308 (5722), 691-693 DOI: 10.1126/science.1109523

Donelson, J., Munday, P., McCormick, M., & Nilsson, G. (2011). Acclimation to predicted ocean warming through developmental plasticity in a tropical reef fish Global Change Biology, 17 (4), 1712-1719 DOI: 10.1111/j.1365-2486.2010.02339.x

Pandolfi, J., Connolly, S., Marshall, D., & Cohen, A. (2011). Projecting Coral Reef Futures Under Global Warming and Ocean Acidification Science, 333 (6041), 418-422 DOI: 10.1126/science.1204794 

Kiessling, W., Simpson, C., Beck, B., Mewis, H., & Pandolfi, J. (2012). Equatorial decline of reef corals during the last Pleistocene interglacial Proceedings of the National Academy of Sciences, 109 (52), 21378-21383 DOI: 10.1073/pnas.1214037110