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High-Temperature Tolerance in Corals
Volume 11, Number 39: 24 September 2008

In an important paper recently published in Marine Biology, Maynard et al. (2008) describe how they analyzed the bleaching severity of three genera of corals (Acropora, Pocillopora and Porites) via underwater video surveys of five sites in the central section of Australia's Great Barrier Reef in late February and March of 1998 and 2002, while contemporary sea surface temperatures were acquired from satellite-based Advanced Very High Resolution Radiometer data that were calibrated to local ship- and drift buoy-obtained measurements, and surface irradiance data were obtained "using an approach modified from that of Pinker and Laszlo (1991)."

With respect to temperature, the four researchers report that "the amount of accumulated thermal stress (as degree heating days) in 2002 was more than double that in 1998 at four of the five sites," and that "average surface irradiance during the 2002 thermal anomaly was 15.6-18.9% higher than during the 1998 anomaly." Nevertheless, they found that "in 2002, bleaching severity was 30-100% lower than predicted from the relationship between severity and thermal stress in 1998, despite higher solar irradiances during the 2002 thermal event." In addition, they found that the "coral genera most susceptible to thermal stress (Pocillopora and Acropora) showed the greatest increase in tolerance."

In discussing their findings, Maynard et al. write that they are "consistent with previous studies documenting an increase in thermal tolerance between bleaching events (1982-1983 vs. 1997-1998) in the Galapagos Islands (Podesta and Glynn, 2001), the Gulf of Chiriqi, the Gulf of Panama (Glynn et al., 2001), and on Costa Rican reefs (Jimenez et al., 2001)," and they say that "Dunne and Brown (2001) found similar results to [theirs] in the Andaman Sea, in that bleaching severity was far reduced in 1998 compared to 1995 despite sea-temperature and light conditions being more conducive to widespread bleaching in 1998."

As for the significance of these and other observations, the Australian scientists say that "the range in bleaching tolerances among corals inhabiting different thermal realms suggests that at least some coral symbioses have the ability to adapt to much higher temperatures than they currently experience in the central Great Barrier Reef," citing the work of Coles and Brown (2003) and Riegl (1999, 2002). In addition, they note that "even within reefs there is a significant variability in bleaching susceptibility for many species (Edmunds, 1994; Marshall and Baird, 2000), suggesting some potential for a shift in thermal tolerance based on selective mortality (Glynn et al., 2001; Jimenez et al., 2001) and local population growth alone." Above and beyond that, however, they say that their results additionally suggest "a capacity for acclimatization or adaptation."

In concluding their paper, Maynard et al. say "there is emerging evidence of high genetic structure within coral species (Ayre and Hughes, 2004)," which suggests, in their words, that "the capacity for adaptation could be greater than is currently recognized." Indeed, as we note in our Editorial of 20 February 2008, quoting Skelly et al. (2007), "on the basis of the present knowledge of genetic variation in performance traits and species' capacity for evolutionary response, it can be concluded that evolutionary change will often occur concomitantly with changes in climate as well as other environmental changes."

Consequently, it can be appreciated that if global warming were to start up again (it has been in abeyance for about the last decade), it need not spell the end for earth's highly adaptable corals.

Sherwood, Keith and Craig Idso

Ayre, D.J. and Hughes, T.P. 2004. Climate change, genotypic diversity and gene flow in reef-building corals. Ecology Letters 7: 273-278.

Coles, S.L. and Brown, B.E. 2003. Coral bleaching-capacity for acclimatization and adaptation. Advances in Marine Biology 46: 183-223.

Dunne, R.P. and Brown, B.E. 2001. The influence of solar radiation on bleaching of shallow water reef corals in the Andaman Sea, 1993-1998. Coral Reefs 20: 201-210.

Edmunds, P.J. 1994. Evidence that reef-wide patterns of coral bleaching may be the result of the distribution of bleaching susceptible clones. Marine Biology (Berlin) 121: 137-142.

Glynn, P.W., Mate, J.L., Baker, A.C. and Calderon, M.O. 2001. Coral bleaching and mortality in Panama and Equador during the 1997-1998 El Nino Southern Oscillation event: spatial/temporal patterns and comparisons with the 1982-1983 event. Bulletin of Marine Science 69: 79-109.

Jimenez, C., Cortes, J., Leon, A. and Ruiz, E. 2001. Coral bleaching and mortality associated with the 1997-1998 El Nino in an upwelling environment in the eastern Pacific (Gulf of Papagyo, Costa Rica). Bulletin of Marine Science 69: 151-169.

Marshall, P.A. and Baird, A.H. 2000. Bleaching of corals on the Great Barrier Reef: differential susceptibilities among taxa. Coral Reefs 19: 155-163.

Maynard, J.A., Anthony, K.R.N., Marshall, P.A. and Masiri, I. 2008. Major bleaching events can lead to increased thermal tolerance in corals. Marine Biology (Berlin) 155: 173-182.

Pinker, R.T. and Laszlo, I. 1991. Modeling surface solar irradiance for satellite applications on a global scale. Journal of Applied Meteorology 31: 194-211.

Podesta, G.P. and Glynn, P.W. 2001. The 1997-98 El Nino event in Panama and Galapagos: an update of thermal stress indices relative to coral bleaching. Bulletin of Marine Science 69: 43-59.

Riegl, B. 1999. Corals in a non-reef setting in the southern Arabian Gulf (Dubai, UAE): fauna and community structure in response to recurring mass mortality. Coral Reefs 18: 63-73.

Riegl, B. 2002. Effects of the 1996 and 1998 positive sea-surface temperature anomalies on corals, coral diseases and fish in the Arabian Gulf (Dubai, UAE). Marine Biology (Berlin) 140: 29-40.

Skelly, D.K., Joseph, L.N., Possingham, H.P., Freidenburg, L.K., Farrugia, T.J., Kinnison, M.T. and Hendry, A.P. 2007. Evolutionary responses to climate change. Conservation Biology 21: 1353-1355.