Thermal refugia support the long-term survival and growth of giant coral colonies

Coral reefs face global threats, primarily from ocean warming, acidification, diseases and pollution. Mass coral mortality events due to severe thermal stress are becoming increasingly frequent, intense, and widespread. Models predict a loss of over 90 % of coral cover by the century's end, while historical records document widespread coral mortality in recent decades. Despite this, some coral colonies have survived and grown for centuries, reaching remarkable sizes. Focusing on 85 large coral colonies with sizes varying between 5 and > 20 m in their maximum dimension, we investigated whether their extended survival is due to intrinsic resilience to thermal stress or to their occurrence in thermal refugia characterized by reduced exposure to heatwaves. For that, we used historical data to assess the exposure of giant coral colonies to bleaching-triggering conditions in the last 4 decades. Most colonies (85 %) never experienced such conditions within the observational time window. However, 13 giant colonies were exposed to severe heat stress multiple times (experiencing between 2 and 22 events, with a median of 13), and six of them significantly more often than in control localities. These results support both hypotheses. On the one hand, environmental protection emerges as a major factor favoring the survival of giant corals. On the other hand, the few giant coral colonies that survived through high thermal stress might have exceptional features worth further, in-depth investigation. However, future climate projections indicate that most giant corals might soon face unprecedented thermal stress, which begs the question of whether giant corals will withstand the looming “coralgeddon” by the end of the century.