In this manuscript, Jury and Toonen investigate the heritability of combined temperature and ocean acidification tolerance in 8 coral species. There are very few studies that investigate combined stressors and this manuscript is an important addition to this broad area of research. Using historical data and their heritability estimates, the authors are able to estimate the potential increase in thermal tolerance in the coming century. This is an important and critical area of research that is of high publication importance. There were many aspects of the manuscript that made this a strong experimental design, for example, the use of biologically diverse mesocosms to ensure the environment was as realistic as possible, the inclusion of a diversity of coral species, genetically checking that individuals weren't related to each other, replication of ramets in each treatment, and investigating the effects of an unbalanced sampling design. Overall, I have few minor suggestions. I believe a slight reorganization of some of the introduction and discussion would help improve the interpretation of some of their results with regards to the trade-offs section. Please see my more detailed comments below.

Introduction:
The introduction provides a strong explanation on concepts of multi-stressors, the two different methods/definitions of heritability, and adaptation to future climate change. My only suggestion is to further expand on the concept of trade-offs in the introduction since this is an important topic in the results and is discussed quite a bit in the discussion. Without needing to lengthen the manuscript, I think the authors could move much of their explanation on trade-offs from the discussion section into the introduction section. In addition, I think it would be really useful if the authors provided some insight into what it might mean for the corals if trade-offs existed and if they did not exist. I believe this will help readers with interpreting Figure 3.

Methods:
To help increase the understanding of the manuscript for a broad audience at Proc B, it would be helpful if the authors could provide more reminders for coral specific definitions throughout the manuscript. For example, the word “ramet” was defined once in the introduction and I think it would be useful to once again define on line 120.
L153-154: Are these the correct dates if measurements were taken before and after 8 months of exposure?
L163: It is mentioned that Tukey post hoc tests were performed for calcification rates, but I don’t see the results in the main manuscript or in the supplemental info. Tukey-post doc results would be incredibly helpful if they could be displayed as letters above the box plots in Figure 1.
L180-185: The methods section for the LD50 estimates are lacking. Please provide more information here (and include the results for this somewhere, perhaps in the supplemental section?) or if these methods/results are in another publication, it should be referenced here and throughout the manuscript.

Results:
L259-268: I think it is important to display the correlation co-efficients and the pvalues for each regression in Figure 3, and not just in the supplemental information. Additionally, further explanation on how to interpret the figure in relation to trade-offs (such as the visual aid in the bottom right hand corner), would help with interpreting the main results of this figure.

The authors may be limited by space, but it might be worth considering putting together a table with each row as a coral species and columns characterizing the species (ie evolutionary lineages, life history strategies), as well as major results/conclusions (ie calcification rates, trade-off regressions, heritability). I think this addition would provide a nice comparison for the readers to compare and contrast different species and for speculating future research.

Discussion:
Overall, the authors’ discussion was coherent and thought-provoking. It was a very interesting discussion on the rate of increased tolerance to temperature warming based using estimates of the response to selection and broad-sense heritability. However, I imagine an upper limit would eventually be reached? Were there some corals in the experiment that had the highest growth rates or highest thermal tolerances (based on LD50) that might provide some insight into what the upper temperature tolerance would be and whether this is above or below the predicted rates of change in tolerance and global warming?

I applaud the authors for their hard work incorporating the revisions! I agree with the authors that the changes they have made strengthen the integrity of the paper. I have just a few very minor suggestions for the authors, however, I think that the manuscript could be accepted after inclusion of these minor suggestions without needing additional review.

The line numbers for the comments below reference the tracked-changes document.
L68-71: The inclusion of the trade-offs in the introduction highly strengthens the overall introduction. However, I’m not sure if the authors have used the strongest or clearest example of what a trade-off is. Trade-offs are typically defined within a single individual and while the symbiont’s heat tolerance is “imparted” to the coral, the trade-off isn’t reduced growth in the coral. I would just state a bit more clearly that the trade-off is that high heat tolerant symbiont are less productive. Therefore, the trade-off is technically in the symbiont, but the negative effect of reduced calcification is in the coral. It’s a bit muddy, so if the authors had another example that was a bit simpler and specific to coral that could also work. It may also not be the best example since you’re measuring thermal tolerance in the coral and not symbiont for this paper? In addition, symbiont species are not identified in this study.
L79-91: I appreciate the authors taking my comments into consideration for prepping the readers for interpretating what a trade-off would look like (ie negative correlation). This is a good explanation, but perhaps a bit more detailed than is needed for the introduction. Specifically, I think the authors could remove the word “synergistically” and just leave it as a negative effect or interaction. The response doesn’t necessarily have to be synergistic, it could also be a simple additive response, but both would still constitute a negative interaction/response.
I think a connecting sentence would be helpful between paragraph 3 and 4
L73: “hear” should be “heat”
L551-552: I think this would be an example of synergistic correlation if this authors want to go into further detail about what a synergistic correlation is.