A selective breeding design based on parental rapid heat stress thresholds did not produce more heat‐tolerant coral larvae

Abstract Climate change threatens coral reef ecosystems, motivating the development of restoration strategies such as selective breeding to enhance coral heat tolerance. Rapid heat stress assays offer promise for high‐throughput phenotyping in selective breeding programmes, but their effectiveness in detecting coral‐based heritable variation in heat tolerance remains unvalidated. Further, the existence of trade‐offs with heat tolerance are poorly studied. We estimated thermal thresholds for photochemical efficiency and chlorophyll retention in gravid colonies of Acropora tersa and Acropora spathulata on the Great Barrier Reef and assessed a possible trade‐off with fecundity, because reduced fecundity of heat‐tolerant parental colonies could impact restoration stock production. Rankings of photochemical efficiency thermal thresholds were then used to select A . spathulata broodstock and predict offspring heat tolerance. Fecundity (number of eggs per polyp) positively correlated with chlorophyll retention under rapid heat stress in Acropora spathulata but did not correlate with either trait in Acropora tersa . We found that offspring of colonies ranking in the top 25th percentile did not show significantly greater survival under 27.5°C or 35.5°C than offspring of colonies ranking in the bottom 25th percentile or offspring produced by gametes from all spawning colonies, regardless of their heat tolerance. Rankings of parental photochemical efficiency thermal thresholds therefore failed to predict larval heat tolerance, likely due to weak correlations between adult photophysiology and larval thermal tolerance traits and/or confounding maternal effects. Practical implication . Rapid heat stress assays measuring broodstock photochemical efficiency are inadequate for informing the production of heat‐tolerant A. spathulata larvae, necessitating development of alternative high‐throughput broodstock screening methods that better capture heritable thermal tolerance.