Peer review of: Extreme environmental conditions reduce coral reef fish biodiversity and productivity (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
This study makes comparisons of fish communities between two gulfs and finds 
them to be different. It then inappropriately attributes these differences to 
differences in temperature regimes between the two locations. Two 
communities can differ for any number of reasons, temperature is just one. To 
conclude, on the basis of comparing just two sites, that temperature is the 
principal driver (from the Abstract: "Rather, impoverished body conditions of 35 
populations in the Arabian Gulf point toward an increased energetic costs of 
growth and 36 homeostasis at higher temperatures). The scale at which the 
factor of interest varies here is "Gulf", consequently the unit of replication to 
make valid inferences is also "Gulf". The study does not replicate at that scale 
and so none of the inferences are supported. I recognise that replicating at that 
scale would be challenging but consider that that the statement that the authors 
is making is substantial - that temperature is the most important driver of 
differences among gulf - to make a substantive statement requires substantive 
evidence. 
There is vast literature on making comparisons across large spatial scales such 
as the one proposed here, particularly protected area studies on both land and 
sea. Likewise, there is a large body of work on local adaptation of physiology 
(Conover in fish is a good exemplar). This work does not approach this problem 
at the appropriate scale and does no provide sufficient support for any of the 
key inferences. 
I'm sorry I cannot be more positive but for work to be published in a high 
impact journal, best practice in terms of experimental design is essential. 
 
 
 
Reviewer #2: 
Remarks to the Author: 
This study compares cryptobenthic reef fish communities and their ecological 
processes from the Gulf of Oman and the Arabian Gulf. The Arabian Gulf is an 
extreme environment and this study investigating how this influences these fish 
in the context of future climate change. Through standardized collections and 
aquaria trials, the authors explored differences in, critical thermal maximum 
and minimum tolerances, gut content using DNA barcoding, and length weight 
relationships for a subset of species. They also modeled growth, mortality, and 
biomass turnover for each region at the community level. Through these 

analysis, the authors conclude that there are ecological differences between the 
fish in the Arabian Gulf and Gulf of Oman reefs driven by extreme temperature 
after coral cover and composition are dismissed, with the extreme temperatures 
in the Arabian Gulf having a detrimental effect on fish communities and 
function. 
 
Firstly, I would like to acknowledge the significant among of field, laboratory, 
and aquaria work to address these questions. There is a considerable amount of 
effort gone into to executing this study in a region that is greatly understudied 
and challenging to work in. This study is excellently communicated, analyses, 
and presented. Furthermore, the majority of this data is collected from the 
natural environment in natural conditions which is rare, However, this does 
reduce the ability to account and control for other variables. They have done a 
great job comparing the two regions, however I caution the strong narrative 
that focuses on temperature as the main driver. 
 
The authors propose that the higher temperatures in the Arabian Gulf is the 
main driving difference between the two regions after the benthos is excluded. 
This is a focus of the manuscript and made clear in the title and throughout the 
manuscript (L130-140, Fig 1). While temperature profiles are clearly different 
and a driver of organism response, there appears to be no inclusion and little 
discussion around other environmental variables that differ greatly between 
regions that could factor into these findings (e.g., salinity and productivity). 
 
The authors do test and dismiss benthic community composition and coral cover 
after finding no significant differences between regions based on the sample 
sites. Given the broad resolution of the benthic composition data, these 
differences might be difficult to detect. Resolution (benthic species, caves, 
complexity) could be important give the close relationship between fish and 
benthos for shelter, feeding and reproduction. Benthic profiling (5 x 20x20cm 
quadrates) over a ~4.6m2 area with broad groups might not captured enough 
of the variation to detect differences in benthic composition and coral cover in 
addition to the assumption of the reef surface area (L657), and therefore 
dismiss the benthos as an additional driver. Given the extreme environment in 
the Arabian Gulf and the bleaching events in the previous years (L448) it is 
surprising that they are similar (see Burt et al. 2019 CR). 
 
Sampling and collections were conducted at the end of April and May, the end of 
the cooler season. If the model species in this study only live weeks to months 
(Lines 99 onwards: 7.4 generations per year, high mortality) and you collected 

them at the end of the winter, would it not be the winter temperatures that are 
influencing body mass per unit length, growth rates, and biomass production? 
 
Minor comments 
 
L130 
Here you are presenting a maximum and minimum sea surface temperature 
from a 10 year period (2008 - 2018). While this presents the extremes for each 
region from a decade, some additional information about the two regions would 
be useful. Either here, or in the methods (L475), can you provide some 
additional details around the thermal regimes: i.e., how long do these max. and 
min. temps persist for, are these temps observed most years, and what is the 
average temperature for each of the two regions? 
 
L154 
It would be informative to include some values for species richness and density 
within this section. Also, the graph has “richness”, the caption has “density”, 
and the text has “diversity”. 
 
L198 
Please provide the values for each region or the difference in heat tolerance for 
E. pulcher 
 
L327 
This is a strong statement considering only one species increased max thermal 
tolerance and one species increased min thermal tolerance. 
 
L324 
While statistically significant, how ecologically relevant are the differences in 
body condition for E. ventermaculus (very small weights) and C. anomolus (for 
the given size comparisons)? 
 
L330 – 333 
Poor body condition suggests a response to temperature and other 
environmental variables, - what about the recorded differences in diet 
composition (as a result of the temperature and other environmental variables) 
also producing poor body condition? 
 
L338 – 342 and L423 - 426 
How does the difference in produced biomass between regions related to the 

observed standing biomass that was similar between the two regions? 
 
L350, L352 
If there is significant self-recruitment back to these thermal hotspots. 
 
L382 
Maybe add that DNA metabarcoding cannot accurately assess quantity of prey 
types in the diet. 
 
L408 
Your length body rate relationships are potential from winter months (if their life 
span is ~3 months). Therefore the response could be to cooler water not 
warmer water, or the temperature envelope if the they live longer than a year. 
Would you expect these differences to be even more pronounced if they were 
collected at the end of the summer period? What is the average life span of 
these 3 species that you tested? 
 
L485 
What was the local water temperatures for each region at the time of the 
collections? 
 
L517-520 
Please provide sample size for each species 
 
L554 - 557 
Please include what the temperature and salinity for the holding tanks and trial 
chambers, and was this the same for species from each region? 
 
L565 
Can you please provide the specific replication for each species from each 
region for each thermal trial (max and min). 
 
L657 
“assumption that outcrops are hemispherical… “ This seem like a big 
assumption, and assumes that all outcrops were of similar shape (and height?). 
Given the collection tarpaulin was standardized and used in this equation, it 
would appear that surface area would be similar among sites. This can obviously 
influence benthic groups % cover, in addition to calculating abundance, 
diversity, and biomass of fish based on the area of this calculation. 
 

L671 
Please include if the data was transformed for any of the ordination analysis? 
 
L680 
Please include if this is proportion or percentage. 
 
L682 
Please define if these “3 records” were per quadrate, outcrop, or region? 
 
L694 
Did you account for the effect of individual size? 
 
Fig 1 and 2 
It is difficult to distinguish among symbols. 
 
Fig 2 
Should the % coral cover in the bottom right be proportion? 0.2% to 0.6% 
seem extremely low 
 
Fig 3 
Check that 11.9oC is correct in the caption. 
 
 
 
Reviewer #3: 
Remarks to the Author: 
The manuscript titled ‘Organismal responses to extreme temperature reduce 
coral reef biodiversity and functioning’ is study on differences in cryptobenthic 
fish communities among two reefs with different environmental conditions. The 
aim here is to understand how and why a community might look like in the light 
of increasing temperature due to climate change. 
General comments: 
What a lovely manuscript to review. The authors have done a great job at 
finding a good ‘natural laboratory’ to answer some of these really important 
questions. The study has been thoroughly performed and addresses many 
aspects of why fish might be different among both sites. 
I only have one more general comment, which I can see in the discussion that 
the authors are already aware of. Although benthic communities might be 
similar, much other than temperature will vary among the ‘two’ locations. 
Especially the introduction is heavily focused on temperature. I think it is worth 

already at the start to acknowledge that there is more than just temperature 
that differs between these sites. It is obviously impossible to find two sites with 
only one variable of change of course, but here temperature is the only variable 
presented. 
Specific comments: 
As mentioned to the editor, some of your study does not fall into my field of 
expertise, nevertheless it would be good to clarify some of these questions I 
stumbled across. 
Line 187-190: I had to read this over and over again. It’s a nicely compact 
sentence, but hard to follow, especially at this point it is the first time the 
reader hears of CTmax 
In the next paragraphs it is not 100% clear if these species are found in both 
sites or just at one or the other (unless stated in brackets). 
The issue with having a methods part after the results – according to your 
figure 3 – you measured two species that occur in both locations, one that is AG 
only and three GoO only? Maybe clarify this in the text (as it took me a while to 
figure out) and the ‘all species’ in line 191 might be misleading – it is all species 
which you tested (correct?). 
Line 284: Is the fact that you only found large individuals of C. anomalus in the 
AG a collection bias or are there no large sized fish? 
Line 335: Is there a chance that the fish at AG feed on restricted food – not just 
because there is less available - but possibly the food they choose to eat is of 
higher ‘quality’(and hence less diverse)? (ok, saw that you did discuss this a few 
paragraphs later) 
Line 329: very cool result. 
Line 352: what about rapid evolution? Kind of curious to see if these are still the 
same species or there is some population structuring and local adaptation 
happening (but of course out of the scope of this paper). 
 
Figure 1: Would it be possible to zoom into sites ( as in the end the whole AG 
temp profile isn’t really that important here)? 
Figure 2: It is hard to identify the sites as the symbols are too small.