We certainly hope that Baird and Maynard are right and that in the coming years corals will exhibit an adaptive capability that they have not yet exhibited in situ or in the laboratory. At this point, however, it appears unlikely.
As Baird and Maynard point out, the coral genera Acropora and Pocillopora have generation times that are short (several years) relative to the generation times of other corals. The majority of coral generation times, however, are still long (decades) relative to the accelerating pace of climate change, throwing doubt on the scope of most coral species for rapid adaptation (1).

Corals, like other organisms, can also modify the risk of coral bleaching over the short term through physiological acclimation (2). Acclimation, however, as with any phenotypic change, is limited. In the same vein, corals that form symbioses with more than one variety of dinoflagellate can shift their populations so that they are dominated by their more thermally tolerant dinoflagellate genotypes during thermal stress. Unfortunately, these short-lived changes have not yet resulted in the novel host-symbiont combinations that will be required for survival in the challenging temperatures and acidities of future oceans under rising atmospheric carbon dioxide.

It is important not to confuse genetic adaptation with the increased average thermal tolerance observed for some coral communities over the past 25 years, which has occurred largely because thermally sensitive species have died out, leaving robust species behind (3). Equally important is the lack of evidence that corals have the capacity to either acclimate or adapt to falling aragonite saturation states. It seems unlikely that genetic adaptation will solve the problems of global change facing corals. Indeed, paleontological evidence indicates that calcifying marine organisms including corals suffered a protracted period of absence after large and rapid changes in atmospheric carbon dioxide associated with the Permian-riassic extinction event (4, 5). It took millions of years for these organisms and ecosystems to recover.

O. Hoegh-Guldberg
Centre for Marine Studies
The University of Queensland
St Lucia, QLD 4072, Australia

*To whom correspondence should be addressed. E-mail: oveh@uq.edu.au
P. J. Mumby
Marine Spatial Ecology Laboratory
School of Biological and Chemical Sciences
University of Exeter
Exeter EX4 4PS, UK

A. J. Hooten
AJH Environmental Services
4900 Auburn Avenue, Suite 201
Bethesda, MD 20814, USA

R. S. Steneck
School of Marine Sciences
Darling Marine Center
University of Maine
Walpole, ME 04573, USA

P. Greenfield
The Chancellery
University of Queensland
St Lucia, QLD 4072, Australia

E. Gomez
Marine Science Institute
University of the Philippines
Diliman, Quezon City, Philippines

D. R. Harvell
Department of Ecology and Evolutionary Biology
E321 Corson Hall
Cornell University
Ithaca, NY 14853, USA

P. F. Sale
International Network on Water, Environment and Health
United Nations University
Hamilton, ON L8N 1E9, Canada

A. J. Edwards
School of Biology
Ridley Building
University of Newcastle
Newcastle upon Tyne NE1 7RU, UK

K. Caldeira
Department of Global Ecology
Carnegie Institution of Washington
Stanford, CA 94305, USA

N. Knowlton
National Museum of Natural History
Smithsonian Institution
Washington, DC 20013, USA

C. M. Eakin
NOAA Coral Reef Watch
NOAA E/RA31
Silver Spring, MD 20910-3226, USA

R. Iglesias-Prieto
Unidad Académica Puerto Morelos
Instituto de Ciencias del Mary Limnología
Universidad Nacional Autónoma de México
Cancún, 77500 QR, México

N. Muthiga
Wildlife Conservation Society
Bronx, NY 10460, USA

R. H. Bradbury
Resource Management in Asia-Pacific Program
Australian National University
Canberra ACT 0200, Australia

A. Dubi
Institute of Marine Sciences
University of Dares Salaam
Tanzania

M. E. Hatziolos
Environment Department
MC5-512
The World Bank
Washington, DC 20433, USA

References

D. K. Skelly et al., Conserv. Biol. 21, 1353 (2006).
S. L. Coles, B. E. Brown, Mar. Biol. 46, 183 (2003).
P. W. Glynn, J. L. Maté, A. C. Baker, M. O. Calderón, Bull. Mar. Sci. 69, 79 (2001).
G. D. Stanley Jr., Earth-Sci. Rev. 60, 195 (2003).
R. S. Steneck, Paleobiology 9, 44 (1983).