Thirty years of coral heat-stress experiments: a review of methods

For over three decades, scientists have conducted heat-stress experiments to predict how coral will respond to ocean warming due to global climate change. However, there are often conflicting results in the literature that are difficult to resolve, which we hypothesize are a result of unintended biases, variation in experimental design, and underreporting of critical methodological information. Here, we reviewed 255 coral heat-stress experiments to (1) document where and when they were conducted and on which species, (2) assess variability in experimental design, and (3) quantify the diversity of response variables measured. First, we found that two-thirds of studies were conducted in only three countries, three coral species were more heavily studied than others, and only 4% of studies focused on earlier life stages. Second, slightly more than half of all heat-stress exposures were less than 8 d in duration, only 17% of experiments fed corals, and experimental conditions varied widely, including the level and rate of temperature increase, light intensity, number of genets used, and the length of acclimation period. In addition, 95%, 55%, and > 35% of studies did not report tank flow conditions, light-dark cycle used, or the date of the experiment, respectively. Finally, we found that 21% of experiments did not measure any bleaching phenotype traits, 77% did not identify the Symbiodiniaceae endosymbiont, and the contribution of the coral host in the physiological response to heat-stress was often not investigated. This review highlights geographic, taxonomic, and heat-stress duration biases in our understanding of coral bleaching, and large variability in the reporting and design of heat-stress experiments that could account for some of the discrepancies in the literature. Development of some best practice recommendations for coral bleaching experiments could improve cross-studies comparisons and increase the efficiency of coral bleaching research at a time when it is needed most.