Is acclimation beneficial to scleractinian corals, Porites spp.?
With coral reefs impacted by climate change, attention is turning to the extent to which scleractinian corals can acclimatize to new physical conditions. The implicit assumption that acclimatization is beneficial has not been tested for scleractinians, although it has been investigated in other systems and has been referred to as the beneficial acclimation hypothesis (BAH). This study tests this hypothesis for scleractinians in experiments on massive Porites spp. from Moorea, French Polynesia (17° 28.564S, 149° 49.018W). Corals were acclimated for 15–21 d to three temperatures within the range experienced in the collection habitat, then transferred to each of the same temperatures for a treatment period of 14–15 d. The response of the holobiont was measured as growth, the response of the Symbiodinium populations as maximum photochemical efficiency of open reaction centers II (RCIIs, Fv/Fm). An ANOVA with polynomial contrasts was used to distinguish among the BAH, three alternative hypotheses and a null hypothesis describing the consequences of acclimation. In the first experiment (2009), massive Porites spp. were unresponsive to temperature. In the second experiment (2013), the BAH was not supported, but growth of the holobiont conformed to the “hotter is better” (HIB) hypothesis; the response of Symbiodinium populations conformed to developmental buffering (DB). These results suggest that acclimation by massive Porites spp. to temperatures experienced routinely in the natural environment does not have clear beneficial value for growth or photochemical efficiency (i.e., BAH was not supported), but they reveal that acclimation to elevated temperature can have value in responding to a variety of subsequent temperatures (i.e., support for HIB).