Old Taxonomy Masks the Phenotypic Diversity of Pleurobrachia Fleming (Ctenophora: Tentaculata): An Analysis of the Morphological Variation in the Genus from Mexican Coasts

Response to Reviewer 1 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. We really appreciate that you liked our proposal on the morphological attributes and techniques used for the analysis of these organisms. Please find the detailed responses below and the corresponding revisions/corrections highlighted in the re-submitted files.

2. Questions for General Evaluation

3. Point-by-point response to Comments and Suggestions for Authors

Pleurobrachia taxonomy

Ctenophores are still highly enigmatic species, and their taxonomy is in a state, requiring substantial revision.

Thus, this is an essential paper that provides a detailed morphological description of dozens of characters for Pacific and Atlantic species of Pleurobrachia, one of the best-studied ctenophore genera.

The authors provided an extensive list of morphological criteria suggesting the existence of novel, yet formally undescribed species of the genus Pleurobrachia. As a result, this is a valuable contribution to the literature. However, some suggestions can improve the value and impact of the manuscript.

-We really appreciate the time you took to review our manuscript, and we also thank you for your comments. They helped us clarify some aspects and refine others, and we have followed all your suggestions.

Comments 1: The most updated survey includes 8 species as cited in the most recent published taxonomy (doi.org/10.1007/978-1-0716-3642-8_2). This paper should be cited in the manuscript.

1. Species Pleurobrachia bachei A. Agassiz, in L. Agassiz, 1860

2. Species Pleurobrachia brunnea Mayer, 1912

3. Species Pleurobrachia cyanea (Chun, 1889)

4. Species Pleurobrachia globosa Moser, 1903

5. Species Pleurobrachia pigmentata Moser, 1903

6. Species Pleurobrachia pileus (O. F. Müller, 1776)

7. Species Pleurobrachia rhodopis Chun, 1879

8. Species Pleurobrachia striata Moser, 1908

The species reported in the manuscript Pleurobrachia australis (Benham, 1907) is now accepted as Hormiphora australis (Benham, 1907) and should be removed from their list or referred to as Hormiphora.

Response 1: Thank you for the suggested clarification in the proposed literature; we have already incorporated it. Lines 78-86.

Comments 2: Line 47 – it is better to cite the most recent review, consider all aspects of ctenophore biology and phylogeny (doi.org/10.1007/978-1-0716-3642-8_1)

Response 2: We agree and have added the citation from the suggested literature. Line 47.

Comments 3: Line 49 – cite the most recent taxonomy of Pleurobrachia (doi.org/10.1007/978-1-0716-3642-8_2)

Response 3: We agree and have added the citation from the suggested literature. Line 49.

Comments 4: Line 72 – incorrect citation about the species. Probably, the citations about the sequenced genome of Pleurobrachia would be appropriate (doi.org/10.1007/978-1-0716-3642-8_1) as well as some work on developmental biology or neuroscience, or feeding biology of this species should be added as the most recent

Response 4: We have added the suggested citations, considering the most recent ones. Line 72.

Comments 5: Line 78 – 8 Pleurobrachia species have been recognized (doi.org/10.1007/978-1-0716-3642-8_2). Please cite this reference

Response 5: Done. Line 78.

Comments 6: Line 85 – P. cyanea and P. globosa should be added as recognized species to the authors’ list (doi.org/10.1007/978-1-0716-3642-8_2).

Response 6: Done. Lines 83-86.

Comments 7: Lines 526–528 - Again, it is also important to refer to the most recent published survey of all extant ctenophores, which summarizes 185 valid ctenophore species names (with illustrations of most ctenophore genera) as of the end of 2024 (doi.org/10.1007/978-1-0716-3642-8_2).

Response 7: The clarification and suggested literature have been added. Lines 561, 562.

Comments 8: I would suggest that authors include higher-resolution photographs and possibly a summary anatomical diagram illustrating the differences for the proposed, yet not formally described, species or subspecies, as mentioned in the discussion. It would be important, as this paper may serve as a practical taxonomic guide to Atlantic and Pacific ctenophores.

Response 8: Thank you for pointing this out. We have already updated the resolution of the figures and diagrams in this latest version of the manuscript (see Figures 1, 4, 5 and 6). Regarding the suggestion of anatomical diagrams, these are already presented in Figure 1 and supported by the photographs in Figures 2 and 3 (also properly described in the body of the manuscript, lines 379-393). In addition, general diagrams of both species P. pileus and P. bachei are also presented in Figure 6 as a visual reference of the morphological differences between the species we propose and the previous ones.

Comments 9: I also suggest adding a subsection to the discussion that provides a comparison of distinct anatomical features among already described species of ctenophores or at least include some comments on a few other species of Pleurobrachia and Hormiphora, as they are closely related genera.

Response 9: We agreed with you, so we have compared the main attributes that distinguish the species previously described and the new ones proposed in this paper, considering the descriptions and updates of those species. Lines 519-533.

We decided not to consider the comparison with the genus Hormiphora, because the morphological attributes of that genus are perfectly distinguishable among all Pleurobrachia specimens, so we preferred to make comparisons and discussions only with sister species.

Comments 10: Finally, in the method section, I suggest adding a more detailed description of locations and associated factors, including depth, temperature, and the presence of other ctenophores or planktonic organisms at the same time of collection, as well as notes about collection seasons.

Response 10: Thank you very much for your suggestions on this point. We have added the requested information regarding the locations and ecological aspects of the sites. Lines 144-161.

Response to Reviewer 2 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. We really appreciate that you liked our proposal on this work. Please find the detailed responses on your comments below and the corresponding revisions/corrections highlighted in the re-submitted files.

2. Questions for General Evaluation

3. Point-by-point response to Comments and Suggestions for Authors

Dear authors, I greatly enjoyed working with the submitted manuscript and applaud you for the fine work! I left a considerable number of edits and comments that should be considered, incorporated, or addressed before acceptance by the editors. I must say, however, that I have no doubts about a sound revision based on the initial file I read. Excellent work!

- Dear Reviewer,

Thank you for your exceptionally positive and encouraging feedback. We are thrilled that you enjoyed the manuscript. We have carefully reviewed your edits and comments, and have addressed all of them, so you can see a new and improved version of the manuscript.

Thank you again for your time and positive review.

Comments 1: Line 70. I suggest to cite also this very recent paper: https://www.sciencedirect.com/science/article/pii/S0141113625005173.

Response 1: Thank you for the suggested literature, we have already incorporated it. Line 72

Comments 2: Line 74. I suggest to cite this recent paper as well: https://cdnsciencepub.com/doi/full/10.1139/cjz-2024-0171.

Response 2: Thank you for the suggested literature, we have already incorporated it. Line 74

Comments 3: Line 85. What about Pleurobrachia globosa Moser, 1903?

Response 3: Thank you for your comment. We have updated the list of species recently accepted for the genus, adding P. globosa and others that you can find in this section. Lines 79-86.

Comments 4: Line 106. What about social media and diver-generated pictures of Pleurobrachia? From these, it is difficult to get enough information on the determining characteristics, too.

Response 4: We agree and have added these notes regarding photographs on the internet and by photographers who are not experts in this group. Line 107.

Comments 5: Line 114. What about regional genetic studies pinpointing either this or that species?

Response 5: Indeed, as you mention, there is genetic information that recognizes species of the genus (although inaccurately) on the coasts of Mexico, so we have incorporated the citation. Lines 116-118.

Comments 6: Line 125. You have to go a bit into the argument of integrative taxonomy and why you didn't complement you analyses with genetics. See https://academic.oup.com/plankt/article-abstract/36/1/64/1520590.

Response 6: We understand your suggestion. In fact, in the Discussion section, we consider the importance of an integrative approach to improve knowledge of the boundaries between species. Lines 565-568. However, this work represents a preliminary and novel morphological approach to distinguish species of the genus distributed along the coasts of Mexico, which in turn could be useful worldwide. In addition, it will serve as a baseline for future work (including genomic studies such as the one we are currently working on).

Comments 7: Line 128. “series of specimens” What does this mean?

Response 7: We mean that a specific and equal number of organisms was used to evaluate the morphology between the analyzed populations. Anyway, in order to improve clarity, we substituted it for “among a comparable number of specimens”. Line 131.

Comments 8: Line 131. I don't see this below. Could you also develop a dichotomous ID key that can be widely applied? Would elevate the paper a lot!

Response 8: Thank you for your recommendation. We understand the importance of a dichotomous key for species ID. However, as discussed above, there is little up-to-date information (photos, diagrams, distribution, descriptions, etc.) on the other species of the genus to do so. Therefore, we have decided for this moment to only consider the discrete characters that distinguish the morphological species found in this study and compare them with those that had previously been recognized. Lines 379-393, 519-533. Nevertheless, we expect to develop such a key for the genus (perhaps family) soon, by implementing the techniques proposed in this study.

Comments 9: Line 139. Why the imbalanced study design?

Response 9: Currently, we are focusing our assessment of organisms on a latitudinal gradient in the Gulf of Mexico and considering a location in the Pacific Ocean. In the future, we want to continue assessing the diversity of these species present on the other Mexican coasts.

Comments 10: Lines 146-147. “before fixation by taking photographs” ???

Response 10: We mean that genus identification was developed in the field, through direct observation of specimens or by taking photographs of live specimens.

Comments 11: Line 150. Include in the table caption what these various keys mean and what the acronym stands for.

Response 11: Thank you for your comment. The table caption explaining how the key for each location was constructed has been added. The meaning of each key has not been unnecessarily repeated, as it is described in the table. Lines 170-172.

Comments 12: Line 484. It is not explained what the various colours represent. Does this tree mean that the Mexican specimens are WIDELY different from reference P. pileus and P. bachei?

Response 12: Thank you for your comment. The changes have been made in the figure caption. Regarding the cluster analysis, yes, with a morphological approach, the nominal species known as P. pileus and P. bachei differ substantially from the morphological species proposed here. The anatomical differences are discussed in lines 519-533.

Comments 13: Lines 503-507. This is, indeed, super interesting. However, what would genetic sequence clustering say? If you don't have this kind of information, it would be, in light of Holst and Laakmann, highly advised to loop this thought in.

Response 13: We really appreciate your comment on this issue. You are absolutely right, it is essential to add the molecular aspect. However, even with new studies proposing specific primers for Ctenophora (See https://onlinelibrary.wiley.com/doi/full/10.1111/1755-0998.13459), it has been very difficult to obtain adequate results for molecular analyses. We as a group expect to make some progress very soon, in order to fully describe the boundaries of species and define them definitively, covering aspects of integrative taxonomy and making them reproducible worldwide.

Finally, we are very grateful for your comments, observations, and corrections to the manuscript. The rest of your observations that do not appear in this letter were reviewed, accepted, and incorporated into the new version of the manuscript, including figures and tables captions, edition in figures, clarifications, suggested literature, grammatical corrections, and your suggestions for the references.