This manuscript deals with an interesting topic: bivalve calcification processes in relation to ocean acidification. However, in a present for, it is not suitable for publication.

The introduction section is too long and should be significantly reduced. Parts of it are hard to follow. The Material and Methods are not sufficiently clearly presented, as relevant information is missing. The choice of target species is not clearly presented; it is not clear why the authors chose to analyse slow-growing Arctica islandica rather than some faster-growing species with an aragonitic shell. Shell sizes used, as well as the number of shells used in the research, are not clearly presented.   

Overall, the manuscript appears more as a draft version rather than a complete document. There are serious issues with the organisation of text, Figures and Tables. Numerous mistakes in text and references are questioning the systematic approach of authors in data analysis. Statistical analysis used is not fully credible. The number of Figures is too large. The discussion as well as Conclusions sections are too long. There is a certain level of repetition between the Discussion and Introduction sections, which is redundant and needs to be revised. In the Discussion section, authors need to start by clearly identifying their most important findings and explaining them in a clear and focused way in relation to previous research.

Specific comments:

Line 38 – insert “e.g.,” in the beginning of brackets. Use this approach in other parts of the text where you are just presenting selected references for a certain statement.

Line 43 – replace “e.g..” with “e.g.,”

Line 43 & 50 & 137 – not sure what you mean by “reviewed”, why not just citing the reference

Line 60 – place relevant references after mentioning individual species names and not at the end of the sentence.

Line 77 – did you mean Zhao et al. 2018? There is no publication from 2016 in reference list. Needs to be checked in detail.

Line 81 – a reference to the Figure should be before the Figure, move the Figure below this text

Figure 1 – consider increasing a bit font site on the bottom of the right figure part

Line 88 – remove extra dot

Line 89 – check the presentation of isotope names and formatting of numbers

Lines 88-90 Species names should be presented in italic font

Table 1 – the title should be presented above the table. Extra horizontal and vertical lines should be omitted from the Table for clarity. This table should not be presented in the Introduction section – it actually contains the Results of this study. It is not sufficiently clearly explained why parameters marked with n.d. were not measured.

Line 96 – it is not clear why the authors refer here to Table 1 and Zhao et al. 2018 paper – and to which of them – as two are listed in the reference section. Did you want to refer to Figure 1? There you cite Zhao et al. 2016 paper.

Line 109 – At the end there is only a reference to Crenshaw (1972), so check “et al.”

Line 111 – year missing after Crenshaw

Line 143- 145 place references after corresponding taxa, not at the end of the sentence

Lines 150-160 & other parts of the text. There is no need to go in so much detail in relation to other taxa. You text is too wide and too descriptive. You need to focus on target taxa and main questions.

Line 164 Full species name is mentioned before in text, so abbreviations should be used here-

Line 176 Authors need to provide a short description of collection and culturing in this manuscript, it is not sufficient to refer to earlier manuscript.  There is no sufficient indication of year research was conducted.

Line 178 & other parts of the text – remove “psu” 

Line 182 you need to clearly specify size of bivalves used in your research as well as sample number

Line 183 – check the formatting of geo coordinates

Line 190 what do you ean by “see” here?

Line 191 – here you refer to Table a that is presented several pages before – this is not appropriate approach to organisation of the manuscript

Line 194 you need to define “dry weight”. What did you measure & with what? What was the precission?

Line 195 You need to define “buoyant weight”

Line 207 Arctica islandica is a slow growing species – you need to explain why you choose 14 days as period here. 

Line 212 You need to present Figure/diagram clearly illustrating how sampling of shell carbonate material was conducted.

Line 232 A sentence should not start with a number, revise

Line 289 You refer here to Table 3 – and it is presented much later in the manuscript. There are serious issues with the organisation of your text, Figures and Tables.

Line 294 & other parts of text - Species names should be presented in italic font.

Line 304 – check the formatting of Fig – “a” vs. “A”

Line 320 – statistical analysis used should be clearly presented in Material and Methods section. Did you test your data for homogeneity of variance (requirement of t test)? See for example Fig 3f – variances are not clearly not homogeneous according to your data presentation. 

Careful formatting of references is needed. Journal names are not consistently presented, as some are referred to by full names and others by their abbreviations. I did not check all in detail, and the following are just some observations: Lines 644-647 – check for use of capital letters in publication titles. Line 646 replace “mollusc” with “Mollusc”. Line 652 – place “Porites” in italics, also check other references and present all species scientific names in italics font.  Line 670 – year should be placed at the end of the reference for consistency.  Line 677 – doi missing. Line 681 – capital letters are used, needs to be corrected.  

The temperatures for A. islandica are worryingly warm, near the upper range of this clam’s thermal tolerance (lines 179/180; Seawater  was  maintained  at  a  pH  of  7.93  ±  0.09,  temperature  of  18.2  ±  1  о  C,  and  salinity  of  35  psu  for  the  aragonitic clam  A. islandica  in the control conditions  (Downey-Wall et al., 2020). The authors need to provide this as a caveat to the results. In other words, are the results scalable for all temperature ranges?? The authors should consider the results of Liu et al. (2015) Environmental controls on the boron and strontium isotopic composition of aragonite shell material of cultured Arctica islandica, Biogeosciences, 12, 3351-3368, doi:10.5194/bg-12-3351-2015, whereby there seemed to be a potential influence of warmer temperatures on boron isotopes.

Add the length of time for the experimental calibration for both species in line 180 at the end.

What are the ages and shell heights for the A. islandica shells? They grow very slowly, thus this is important to have these metrics in this study- (not just citing Downey-Wall  et  al.  (2020))

“2.2 Calcification rate measurements  Net  calcification  rate  was  calculated  using  the  dry  weight  at  the  start  and  end  of  the  experiment.  Initial  dry  weight  was  measured  at  the  start  of  exposure,  on  day  33  or  34,  after  the  acclimation  period  (Downy-Wall  et  al.,  2020).  The  buoyant  weight  was  measured  on  either  day  50  or  80  and  the  final  dry  weight  was  derived  using  a  linear  relationship  between  oyster  dry weight and oyster buoyant weight (Ries et al., 2009).”

This may be suitable for juvenile mollusks but not for adults, especially A. islandica. What are the uncertainties in such measurements for large adult clams?

How are the authors confident that they sampled ONLY calcium carbonate reflecting the experiment? Did they stain the shells with calcein? Did they measure linear growth? This is most relevant for A. islandica because of relatively slow growth rates (i.e., see Liu et al., (2023) Resistant calcification responses of Arctica islandica clams under ocean acidification conditions, Journal of Experimental Marine Biology and Ecology, https://doi.org/10.1016/j.jembe.2022.151855.)

Shell sampling – the organic matrix in shells contain about a magnitude more boron than in the shell, and this likely has a very different isotopic composition (value). Are the authors confident all organics were removed?

Why are the authors explain how they sampled the oyster shells but not the clam shells? The methods should have a parallel structure.

Why haven’t the authors reported calcification rates for A. islandica. This is a central variable that needs to be considered (like Fig. 2a for oysters).

Very interesting result in Figure 3- showing different chemical composition of EPF compared to ambient seawater. Important finding that lots of folks have been suggesting but without the EPF evidence. And when you go to the shells even less Mg than seawater, and EPF. Thus the mollusks must be regulating calcifying fluids.

I really think the authors are missing an opportunity by not exploring changes in the shell geochemistry from both species here with growth rates, shell height, age, etc. The applicability/scalability of the study is far less without the inclusion of such metrics. Why not include these data?

Ok- now some praise for the authors:

This is an important study with important implications. We learned that oysters (C.  virginica) and clams (A. islandica) incorporate some elements and boron isotopes differently. The boron isotopic composition of the EPF for both species is different than seawater. The breakthrough of being able to sample the EPF chemistry/pH is a major advance in biomineralogy. Thus, a mechanistic model for biomineralization can be advance. Also, the mollusks evaluated here are not simple pH meters, and the shell d11B value is a mixture of the seawater d11B value and physiology. These results are consistent with in prep work that I am aware of now. Despite some issues with the description of the experiment and other concerns noted above, this is a major advancement.