High coral thermal tolerance at a potentially climate-resistant reef

Ecosystems globally are under increasing pressure from warming caused by climate change. However, some locations appear resistant to impacts. Although rare, these locations might be able to better resist damage from thermal stress. In marine environments, coral bleaching is widely viewed as one of the most pervasive stress responses threatening reef health, structure, and function. Yet we still have a poor understanding of the drivers of spatial and temporal variation in the scale and severity of coral bleaching. Here, we characterize a prolonged, intense, and widespread marine heatwave event and uncover a conspicuous lack of bleaching at the southernmost coral reef in the Indian Ocean (Houtman-Abrolhos Islands), despite predictions of catastrophic mortality. We then provide additional empirical evidence of resistance in a subset of coral species using data collected during a simulated heatwave experiment. Overall, compared with currently accepted thresholds, survival rates were twice as high (∼50%), bleaching resistance was 3.7 times higher and lasted 3.8 times longer, and photo-physiological performance was 22 times higher. Taken together, these results suggest that some common species in the HAI are remarkably resistant to heat stress, where high survival rates contrast with the downward trajectory reported for corals in other reefs globally. Our findings also highlight the capacity of multiple, functionally diverse coral species to resist bleaching at very high heat stress levels. Although hopeful, historical data from this region suggest that these reefs are neither a refuge from bleaching nor resilient to bleaching indefinitely if ocean warming continues to increase.