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The long-term response of coral reefs to climate change depends on the ability of reef-building coral symbioses to adapt or acclimatize to warmer temperatures, but there has been no direct evidence that such a response can occur. Here we show that corals containing unusual algal symbionts that are thermally tolerant and commonly associated with high-temperature environments are much more abundant on reefs that have been severely affected by recent climate change. This adaptive shift in symbiont communities indicates that these devastated reefs could be more resistant to future thermal stress, resulting in significantly longer extinction times for surviving corals than had been previously assumed.

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Coral reef bleaching, the temporary or permanent loss of photosynthetic microalgae (zooxanthellae) and/or their pigments by a variety of reef taxa, is a stress response usually associated with anthropogenic and natural disturbances. Degrees of bleaching, within and among coral colonies and across reef communities, are highly variable and difficult to quantify, thus complicating comparisons of different bleaching events. Small‐scale bleaching events can often be correlated with specific disturbances (e.g. extreme low/high temperatures, low/high solar irradiance, subaerial exposure, sedimentation, freshwater dilution, contaminants, and diseases), whereas large scale (mass) bleaching occurs over 100s to 1000s of km2, which is more difficult to explain. Debilitating effects of bleaching include reduced/no skeletal growth and reproductive activity, and a lowered capacity to shed sediments, resist invasion of competing species and diseases. Severe and prolonged bleaching can cause partial to total colony death, resulting in diminished reef growth, the transformation of reef‐building communities to alternate, non‐reef building community types, bioerosion and ultimately the disappearance of reef structures. Present evidence suggests that the leading factors responsible for large‐scale coral reef bleaching are elevated sea temperatures and high solar irradiance (especially ultraviolet wavelengths), which may frequently act jointly.

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Ocean acidification describes the progressive, global reduction in seawater pH that is currently underway because of the accelerating oceanic uptake of atmospheric CO 2. Acidification is expected to reduce coral reef calcification and increase reef dissolution. Inorganic cementation in reefs describes the precipitation of CaCO3 that acts to bind framework components and occlude porosity. Little is known about the effects of ocean acidification on reef cementation and whether changes in cementation rates will affect reef resistance to erosion. Coral reefs of the eastern tropical Pacific (ETP) are poorly developed and subject to rapid bioerosion. Upwelling processes mix cool, subthermocline waters with elevated pCO 2 (the partial pressure of CO 2) and nutrients into the surface layers throughout the ETP. Concerns about ocean acidification have led to the suggestion that this region of naturally low pH waters may serve as a model of coral reef development in a high-CO 2 world. We analyzed seawater chemistry and reef framework samples from multiple reef sites in the ETP and found that a low carbonate saturation state (⍀) and trace abundances of cement are characteristic of these reefs. These low cement abundances may be a factor in the high bioerosion rates previously reported for ETP reefs, although elevated nutrients in upwelled waters may also be limiting cementation and/or stimulating bioerosion. ETP reefs represent a real-world example of coral reef growth in low-⍀ waters that provide insights into how the biological-geological interface of coral reef ecosystems will change in a high-CO 2 world. coral reef persistence ͉ inorganic cementation ͉ ocean acidification ͉ climate change A tmospheric CO 2 is increasing exponentially because of the unregulated combustion of fossil fuels (1). Approximately one-third of all of the CO 2 released into the atmosphere since the industrial revolution has been absorbed by the oceans (2). This ongoing uptake of atmospheric CO 2 is causing a drop in seawater pH at the global scale, causing an acidification of the oceans (3-5).Ocean acidification results in a decrease in seawater [CO 3 2Ϫ ] and, consequently, a decrease in the saturation state (⍀) of carbonate minerals {⍀ ϭ [Ca 2ϩ ][CO 3 2Ϫ ]/KЈ sp , where KЈ sp is the apparent solubility product of a carbonate mineral (e.g., aragonite, calcite)}. Acidification is expected to reduce coral reef calcification and increase reef dissolution, and the relative rates of change will likely be related to the partial pressure of CO 2 (pCO 2 ) in surface seawater,

Various physical and biological factors affecting coral community structure were investigated by direct observation and periodic censusing (supplemented with laboratory observations and experiments) on three coral reefs off the Pacific coast of Panama from 1970 to 1975. The physical environment has a strong control over coral growth at shallow depth; physical factors are also important subtidally (light, sediment transport). However, paralleling the pattern on temperate shores, biological processes (competition, predation, bioturbation, mutualism) assume an increasing influence on community structure in deeper and more diverse reef assemblages. Coral zonation is marked on these biologically simple and small reefs; the following assemblages are recognized: drying reef flat–live coral cover moderate, species diversity low; reef crest and upper reef slope–highest cover, lowest diversity; lower reef slope and reef base–cover moderate to low, diversity highest. Coral populations in the different zones, though spatially close, are affected by unique sets of conditions. Recurrent extreme tidal exposures devastate reef flat corals (Pocillopora mortality = 40%—60%). The mortality rate of pocilloporid corals is higher than for other corals; this has a diversifying effect on the reef flat assemblage. Acanthaster normally feeds in deep reef zones and numerically its major prey are the predominant pocilloporid corals (most often small colonies and broken branches). Electivity indices and prey choice experiments indicate that less abundant, nonbranching corals are preferred over Pocillopora. Large, branching pocilloporid colonies harbor crustacean symbionts (Trapezia and Alpheus) which can repulse Acanthaster and therefore protect this group of corals. Experimental removal of the symbionts results in a shift of prey preference from nonbranching corals towards the branching pocilloporids. Crustacean symbionts were present in all large Pocillopora colonies sampled, but the density of Trapezia in colonies on the reef flat was about twice that in colonies from deep zones where Acanthaster forages. Further, small pocilloporid colonies and fragments contained relatively few (and a high proportion of juvenile) symbionts. The variety of preferred coral prey present along the seaward reef flanks and the relatively low abundance of Pocillopora in this habitat are considered important factors affecting the distribution of Acanthaster. In addition, a continuous live cover of pocilloporid corals, which Acanthaster avoids, can protect reef zones (e.g., the reef flat) or preferred prey species from attack. The selective destruction of nonpocilloporid corals by Acanthaster tends to lower both live coral cover and species diversity (H'). This trend is evident on the Uva Island study reef where a significant decline in coral cover (47%—18%) and H' (1.06—0.58) occurred over a 4—mo period.