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.