Heat tolerance varies considerably within a reef-building coral species on the Great Barrier Reef

Reef-building coral populations face unprecedented threats from climate warming. Standing variation in heat tolerance is crucial for evolutionary processes necessary for corals to persist. Yet, the spatial distribution of heat-tolerant corals and the underlying factors that determine heat tolerance are poorly understood from individual to ecosystem scales. Here, we show extensive variation in the heat tolerance of a foundational coral species complex across the Great Barrier Reef. Thermal thresholds of 569 individuals differed by up to 7.3 degrees C across scales from meters to >1250 km. Variation in thresholds among reefs was consistent with local adaptation and acclimatization to historical and recent thermal history. However, variation within reefs was sometimes greater than among reefs and largely unexplained by environmental predictors, putative host species, or symbiont communities. This indicates that within-reef heat tolerance differences may be informed primarily by other factors, such as adaptive genomic variation. We anticipate our findings will inform conservation and restoration actions, including targeting individuals for selective breeding of enhanced heat tolerance.