Selective breeding enhances coral heat tolerance even over small spatial scales

Coral reefs globally are experiencing escalating mass bleaching and mortality. Reefs along the western Indian Ocean have been relatively unimpacted. We assessed heat tolerance baselines in two widespread reef-building Acropora species and used selective breeding from two thermally distinct (present day and stress histories) northern (Mean Monthly Maximum 27.9 °C) and southern (26.6 °C) reefs along the Ningaloo World Heritage Reef. Fitness responses were measured in control and heat stress temperatures (adults = 31.0 °C, larvae = 35.5 °C), including survival, tissue necrosis, bleaching, and photosynthesis. Larvae with one parent from the warmer population exhibited >2.2-fold higher survival under heat stress, while those with both parents from the warmer population survived 1.6-fold better (compared to control larvae with two parents from the cooler population). Photosynthesis was maintained in both species and both populations, suggesting heat responses were host driven. Adults from both populations from one species (A. tenuis) exhibited similar responses to heat, while the other (A. millepora) was more variable. These findings are the first to demonstrate that selective breeding can provide heat tolerance enhancement for corals in the Indian Ocean and will be critical to preparing for future marine heatwaves.