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 established heat tolerance baselines and selective breeding efforts for two widespread reef-building Acropora species within the Ningaloo World Heritage Area. To accomplish these goals, we included corals from two thermally distinct southern and northern reefs (mean monthly maximum 27.9°C and 26.6°C, respectively), which reflect both present-day and stress histories. 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 with 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 of one species (Acropora tenuis) exhibited similar responses to heat, while the other (Acropora 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.