Acclimation to elevated temperatures in Acropora cervicornis: effects of host genotype and symbiont shuffling

Abstract

Climate change is increasing the average surface temperatures of tropical waters, creating unfavorable conditions for corals. Some species of coral can physiologically acclimate to elevated temperatures, but the degree to which genetic variation underlies differences in this ability is currently unknown. Acclimation to elevated temperatures in coral has been hypothesized to be due to either alterations in the symbiont community or to changes in gene expression. The present study investigated the ability of Acropora cervicornis to acclimate to elevated temperatures, estimated the heritability of plasticity in upper thermal tolerance, and tested whether observed acclimation patterns could be explained by symbiont shuffling. Coral fragments from a nursery in the Florida Keys (USA) were acclimated at either ambient (27 ± 1°C) or elevated (30 ± 1°C) temperatures and then exposed to a second heat stress (32 ± 1°C) and monitored for mortality. Fragments acclimated to elevated temperatures showed significantly longer lifespans in the subsequent heat stress than did those acclimated at ambient temperature. The ability to acclimate to elevated temperatures differed significantly among coral genets, yielding low, but significant, estimates of broad-sense heritability. A subsequent experiment revealed no changes in either bacterial or dinoflagellate communities of symbionts as a result of acclimation, suggesting that symbiont shuffling did not account for the differences in lifespan between treatments. While estimates of heritability were low, the results suggest that plasticity in upper thermal tolerance significantly differs among coral genets, and that acclimation is likely a result of alterations in gene expression as opposed to symbiont shuffling.