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.

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In their Review, “Coral reefs under rapid climate change and ocean acidification” (14 December 2007, p. 1737), O. Hoegh-Guldberg et al. present future reef scenarios that range from coral-dominated communities to rapidly eroding rubble banks. Notably, none of their scenarios considers the capacity for corals to adapt. The authors dismiss adaptation because “[r]eef-building corals have relatively long generation times and low genetic diversity, making for slow rates of adaptation [relative to rates of change].” We think the possibility of adaptation deserves a second look.
Many features of coral life histories, such as extended life spans, delayed maturation, and colony fission, do result in long generation times (1) [some between 33 and 37 years (2)]. However, other corals, such as many species of Acropora and Pocillopora, mature early, grow rapidly, and suffer whole-colony mortality, as opposed to colony fission, after mechanical disturbances (3) and thermal stress (4). The life histories of these ecologically important and abundant species suggest an underappreciated capacity to adapt rapidly to changing environments.

Repeated bleaching episodes in the same coral assemblages and the increasing scale and frequency of coral bleaching have been cited as evidence that corals have exhausted their genetic capacity to adapt to rising sea surface temperatures (5). However, comparisons of the rates of mortality within populations among bleaching events are not available. Without these data, it is not possible to assess whether the adaptive response has been exhausted. Indeed, the effects of temperature and acidification on even the most basic vital rates in corals, such as growth, mortality, and fecundity, are largely unknown, as are the physiological trade-offs among these traits. Consequently, the sensitivity of population growth to climate-induced changes in vital rates remains almost completely unexplored [but see (6)]. In the absence of long-term demographic studies to detect temporal trends in life history traits, predicting rates of adaptation, and whether they will be exceeded by rates of environmental change, is pure speculation. Indeed, where such data are available for terrestrial organisms they demonstrate that contemporary evolution in response to climate change is possible (7).

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