Worldwide analysis of reef surveys sorts coral taxa by associations with recent and past heat stress

Abstract

Coral reefs around the world are under threat due to widespread decline of hard corals caused by anomalous heat waves. Coral taxa of different morphologies are known to have different sensitivities to heat stress. However, little is known about how long-term effects of heat exposure differ between coral taxa, nor in particular how such effects might drive adaptive processes.Here, we combined worldwide reef survey data with remotely sensed thermal anomalies to evaluate how local rates of taxa-specific coral cover were associated with heat stress. We found a negative association between coral cover and heat stress, where associations were weaker for corals with boulder-like morphology (massive corals), intermediate for corals with arborescent morphology (branching corals) and stronger for corals with encrusting or laminar morphology (plating corals). Additionally, we found that the negative association between branching coral cover and recent exposure to heat stress (measured as the year before a survey) was mitigated by the effect of long-term heat stress (measured since 1985), suggesting that adaptive processes occurred. In contrast, this “mitigating effect” of past heat-stress was not observed for either massive or plating corals.We hypothesize that these different responses to recent and past heat stress mirror different life history traits of coral taxa. For instance, branching corals are known to show higher sensitivities to heat stress and faster growth rates, two characteristics that might boost adaptive rates via evolutionary processes, when compared with other taxa.The results of this work suggest that all coral taxa, regardless of their morphology, will be “losers” of coral bleaching, and that conservation strategies should be adjusted to match taxa-specific responses to heat stress.