With their study on 'Reefal ostracod assemblages from the Zanzibar Archipelago (Tanzania)' the authors add important knowledge on a largely understudied component of the reef ecosystem. They provide a first baseline (although already impacted by anthropogenic influence) of this functional group along the shores of eastern Tanzania. Overall the study is very thoroughly conducted, the conclusions are valid, and the results presented comprehensively. This is valuable information and should be published.

In general the manuscript is written nicely, but at some parts it becomes lengthy. This is also where my main suggestion for improvement lies. While I think the manuscript is pretty much publishable in its current form, I would like to suggest to streamline it a bit. There are many figures and tables in the manuscript - are all of them necessary or could you put some in SI or combine them? For instance Figure one and table 1 feature in large parts redundant information, maybe combine them or put table 1 in supplements?

Figure 3 - the authors show three panels that feature different approaches but the resulting patterns are not dramatically different. Maybe they could just focus on one parameter for the figure and present the caparison (which is still valid to do) in the supplements? Then it could still be discussed in the text but the figure would be easier to read and less crowded.

Figure 4 and figure 5 are somehow redundant and have a very similar take home massage if I am correct? Maybe just feature one and put the other in SI?

Figure 6-8: I don't understand the figure caption here, but maybe this is just my ignorance...Each figure caption states 'Scanning electron microscopy images of the top 10 ostracod species of highest % mean relative abundance for Biofacies 1-4 based on Horn dissimilarity.' But then you get more than 10 species and different species in each figure... please clarify this for me and maybe others that are not experts in the field.

Minor comments:

Line 27: what is the difference between reefal and algal here? Are algae growing on a reef not considered reefal? Or do you just include calcifying organisms in reefal?

Line 45-49: While the manuscript is generally well written this sentence is too long and convoluted. Please rephrase.

The manuscript under review presents a well-executed study in a field poorly known so far - ostracods in tropical reefs. Considering the high importance of ostracods in geosciences, we urgently need baseline studies on reef associations for understanding past and present diversity and distribution patterns. This need is even more pressing for environmental management tasks because of quickly changing conditions for endangered reefal communities in a time of climate change and other anthorpogenic threats. The paper presented contributes to this field in an excellent way.

There are only a few remarks from my side.  Should be easy to comment on this.

1) The authors state covering all major types of benthic habitats within their study. What is not clear, however, if typical microhabitats beside bioclastic sands and gravels are covered, e.g. muds, phytal habitats, invertebrate colonies. Also I missed information if brackish water conditions have been encountered, despite Perissocytheridea is mentioned as indicator of brackish water conditions. The discussion of habitats and micro-habitats sampled is significant for discussing biodiversity values.

2) What is the sample volume or sample area? Abundance data should be given in a standardised way.

3) Table 1 lists species richness and abundance of ostracods per sample. Abundance refers to which area of the bottom or volume of the sample? (see 2) The number of counted individuals would be helpful to interpret species richness.

4) Fig. 2 shows a legend with sites attributed to colours. This is probably not needed because sites are also given in (B).

5) Multivariate analysis are highly sophisticated and contribute significantly to interpretations. I missed, however, statements about pretreatment of data. Did you exclude sites from analysis because of low number of specimens or other reasons? Did you exclude taxa from analysis because of very low proportions? Did you exclude highly correlating taxa from analysis?

6) Fig. 4 and associated discussion: There are two types of habitats covered only one time by the study: sand flats and mangroves. This limits interpretation of these habitats a little. Would be good to addressing this issue with one or two sentences.

7) Line 299: Write B. elongata

8) Plates with ostracods: Please, indicate samples for figured specimens. Perissocytheridea sp. 1 is probably P. estuaria Benson & Maddocks, 1964.

9) Line 327; Write Bosasella elongata

10) Fig. 8/7 looks quite small compared to fig. 8/8. Are you sure?

11) Lines 424-425: Make clear distribution is based on your study and not necessarily reflecting all habitat types and regions of the study area.

12) Line 472: add space in Figs. 9-10

13) Lines 493-498: Maybe because ostracods are using different microhabitats and food sources than forams?

14) Line 554: Which experiments are mentioned here?

15) References: There are quite a number of spelling mistakes in the references Hartmann 1974, Hsieh et al. 2016, Jellinek 1993

The authors contribute significant insights into a relatively neglected aspect of the reef ecosystem with their investigation titled 'Reefal ostracod assemblages from the Zanzibar Archipelago (Tanzania)'. Their research establishes a foundational understanding, albeit one influenced by human activity, of this functional group's presence along the eastern Tanzanian coastline. The manuscript contributes to assess the diversity of the endangered reefal assemblages, providing valuable information.

Since sampling with a plastic bag underwater implies the loss of the finer particles due to suspension, at deep sites such as Mapenduzi wall and the Haramu passage for instance (below 30m of depth), I think that a better characterization of the sampling sites would be important. More detailed description of the sampling/processing would also be helpful (standard volume/ weight?).

The authors state that “Pemba reefs are likely in pristine conditions with the highest coverage of live hard corals, while Zanzibar reefs are often dominated by dead corals intermingled with algae and seagrass habitats”. I think a better characterization of the sampling sites would be very helpful also for comparisons in future studies.

There is one mangrove site (Menai Bay) and one sand flat site (Kizimkazi Beach) and they are characterized by biofacies 3 as Ocean Paradise, a back reef site whereas Chole Bay 1, another back reef site, is characterized by biofacies 4. This is not discussed, at all.

I am impressed by the statistical analysis. But I think the presentation of the statistical analyses could be better organized. For instance, Fig. 3 is never cited in the text…then is it necessary or could it be included in the supplementary material? Or figs 3 and 5 could be merged figuring only the most significant D and dissimilarity index.

The plates are really nice (Figs 6-7-8) but there is no reference to them in the text. Add a reference or put them in supplementary. Specify to which sample belongs each figured specimen.

I have few remarks, listed below:

Line 124

Please indicate during when (month and year) the sampling took place (to be added in table 1).

Line 135

Small species can form a large part of the assemblage. See for instance Aiello et al. 2024 https://doi.org/10.1016/j.revmic.2023.100755

I agree that you studied time-averaged assemblages but at least an indication on the living assemblage (how many specimens with soft parts in each sample) would be helpful. Moreover, what is the average volume of sediment you processed? 

Line 136

“sediment rich samples” means? Which kind of sediment? Coral sand, coral gravel and dead coral fragments (devoid of algal covering, with algal covering?). Were the samples different in this aspect?

A better characterization of the sampling sites would also help understanding the discussion and the gradient of diversity of the ostracod assemblage from coral reefs to algae turfs.

Reference list: there are many spelling errors, in the german literature especially, please check them.