Original Research · Volume 12, Issue 4 · December 2025
1 Marine Biology Research Centre, James Cook University, Townsville, Australia
2 Department of Oceanography, University of Sri Jayewardenepura, Sri Lanka
3 Smithsonian Tropical Research Institute, Panama
Coral reef ecosystems face unprecedented thermal stress due to rising ocean temperatures, with mass bleaching events increasing in frequency and severity. While many reefs suffer catastrophic mortality during heat waves, certain reef systems in the Indo-Pacific region demonstrate remarkable thermal resilience. Understanding the mechanisms underlying this resilience is critical for conservation strategies and reef restoration efforts.
We conducted field surveys and experimental heat-stress trials across 12 reef sites spanning the Indo-Pacific region, from the Coral Triangle to the central Pacific. We measured bleaching thresholds, recovery rates, and Symbiodiniaceae community composition using ITS2 metabarcoding. Host coral genotypes were assessed via 2bRAD sequencing, and environmental parameters (temperature variability, light regimes, nutrient levels) were recorded over 18 months.
Thermally resilient reefs exhibited three key features: (1) high proportions of heat-tolerant Symbiodiniaceae clades (Durusdinium, formerly clade D) comprising >40% of symbiont communities; (2) elevated expression of heat-shock proteins and antioxidant enzymes in host corals (HSP70 upregulated 3.8-fold, p<0.001); and (3) historical exposure to moderate thermal variability (2–4°C seasonal range). Reefs with all three features showed 73% lower bleaching severity during the 2024 marine heat wave compared to non-resilient counterparts.
Thermal resilience in Indo-Pacific coral reefs is driven by a combination of symbiont flexibility, host genetic adaptation, and environmental preconditioning. These findings inform the selection of thermally resilient genotypes for coral restoration and highlight the importance of protecting reefs with natural thermal variability as climate refugia.