The study presented here aims to interbreed broodstock corals selected for high heritable tolerance, as determined by two high throughput and rapid metrics (photochemical efficiency (Fv/Fm) and chlorophyll retention) to generate offspring, destined for restoration of coral reefs, with enhanced fitness under elevated temperatures.
This is a critically important and timely topic given the current state of coral reefs due to the high levels of coral cover loss over the last few decades, particularly on the Great Barrier Reef, where repeated episodes of thermal-stress induced coral bleaching has occurred. The manuscript is clearly written and easy to follow. The methodology and experimental design are well thought-out.
The data seem to indicate that the metrics chosen were not reliable or efficient in predicting heat tolerance in larvae that were selectively bred and suggest that either adult and larval heat tolerance traits do not correlate strongly and/or maternal effects confound larval tolerance. I disagree, because in my opinion the premise behind the chosen parental heat stress thresholds metrics (photochemical efficiency (Fv/Fm) and chlorophyll retention) is flawed. These two parameters are not metrics of the parent coral but rather of the photosynthetic symbionts that live within the coral. Given that the larvae of both Acropora species studied here do not inherit symbionts directly from the parent colony but rather take up symbionts from the environment (horizontal transmission) once they settle, then the heat tolerance of the symbionts will not correlate with heat stress tolerance of the parental colony. The symbionts that were taken up from the environment then may or may not have higher heat tolerance, which would explain the lack of a clear pattern in heat tolerance as well as the very high variability in heat tolerance in these larvae as they take up whatever symbionts are available from the environment.
This is a very different situation to that of Stylophora pistillata, where the symbionts are acquired from the parental colony (vertical transmission) in the study done by Voolstra et al (2020) that is cited in the discussion of this manuscript as a comparison of results with the current study. When the symbionts of S. pistillata are inherited it would make sense that parental colonies with symbionts that have higher heat stress thresholds would pass on the symbionts to the larvae, which are brooded within the parental colony and therefore directly inherit these more tolerant symbionts.