PREreview of Rise of the Cybercrabs: how digital cloning in an integrated taxonomic framework can support deep-sea exploration

In the manuscript entitled “Rise of the Cybercrabs: how digital cloning in an integrated taxonomic framework can support deep-sea exploration” Reynaud et al. use an inexpensive 3D digitization method to argue that this technique can be useful for species assignation, for mapping morphological variation, and for sharing of morphological data by digital means across the world. The authors use as case study specimens of deep-sea crabs of the genus Segonzacia. They focus on these animals for two reasons: first, to emphasize how digitization can enhance the value of hard-to-get specimens such as those living in deep-sea environments.  And second, to show how 3D digitization can be used in combination with molecular data to assign species and map morphological variation in populations with sparse taxonomical and ecological information. The authors use a photogrammetry method to digitize in 3D four specimens of Segonzacia, and provide COI sequencing data for a fifth specimen collected in parallel. By analysing these data, Reynaud et al. conclude that the Segonzacia genus may be more diverse than previously known, implying that this genus may actually be composed of more than one species. The authors convincingly argue In this work for the multiple benefits of 3D digitization and provide valuable morphological and molecular data for the study of deep-sea animals, as well as a relatively well-detailed pipeline for 3D digitization. We consider that improvements to both the structure of the manuscript and the depth and breadth of the analysis could further improve and strengthen the conclusions of this work.

List of major issues and feedback

This study builds upon a very real, very important problem in today’s taxonomy and ecology: Emphasized by more or less strict lockdowns and the extremely limited option to travel (especially internationally) during the recent pandemic it became clear, that one of the main limitations in taxonomy of especially remote systems such as the deep sea is the availability of material - either because only few samples exist, or because they are deposited in only few research institutions. Since funding to travel to these museums or research labs is scarce, especially early career researchers often have a hard time to analyze and revise their taxa in enough detail. Digitization of collection material is a main goal in almost all museums worldwide by now, especially after e.g. massive losses such as through the fire in the museum in Brazil, but the workflow is normally time consuming and costly, which is why the author’s approach to use photogrammetry – an approach that requires very little specific setup, but detailed instructions in order to replicate the approach to a certain extent – is so interesting and their example of the deep sea crab will help the field to modify the workflow according to their needs. Thus said, the presented study introduces a lot of different approaches and ideas, yet fails to convincingly put them to a test in the end, which is why we feel it would benefit from focusing on only a few topics (e.g., the taxonomic finding that DNA and morphology reveal similar results, the use of the 3D models or “clones'' for morphometric studies (which could potentially be integrated in existing interactive keys?), and the introduction of the photogrammetry-workflow including the pipeline to construct the 3D models) and keeping less testable propositions such as the digitization-for-taxonomy idea for the outlook and perspectives – at least in the presented paper it could not be shown convincingly that the species was identified solely based on the created 3D models. For this it might also help to separate the results from the discussion more clearly than it is done right now, thereby creating more of a red thread to follow through instead of what feels right now like a spider’s net of ideas, approaches, results and conclusions without clear connections between them.

The authors picked an interesting case with a monotypic genus of deep sea crabs – we couldn’t help wondering whether maybe a larger sample size, the inclusion of more outgroups as well as potentially more genes (e.g., 18S, 16S, 28S, H3, .. as more conserved markers than COI) could have revealed cryptic speciation within this genus? Ideally, a larger sample size could also help to include species delineation tests in order to test for the existence of one, two, or several species. With all these methods though, we felt that the study would gain more support if the DNA-samples were taken from the same samples that were later on investigated morphologically. Especially with crustaceans, who form the rigid external cuticle covering the entire body it is possible to extract DNA and preserve the outer “shell” for smaller crustaceans, so maybe small incisions in the joints of the legs will help to extract enough material without damaging the overall morphology?

We also think it is important to highlight that a photogrammetric approach such as described in this study is working really well on specimens with a hard (or at least stable) external surface, but that it will not be equally useful for specimens such as worms, jellyfish or sea cucumbers, whose body shape and -integrity is very hard to preserve especially when taking them from extreme environments, and which will not maintain their shape similar to the crabs due to fluid loss.

Regardless, the approach the authors present is very straightforward, yet some key information such as the model number of the software packages, which of the packages is freeware and which is not, how long each step and the entire process takes, which might be the essential steps and which steps are “nice to have” are missing. One of our suggestions would be to rephrase this as some sort of a method paper, which would give the authors the opportunity to explain each step in more detail, and therefore facilitate the implementation of this workflow in new labs and projects.  

List of minor issues and feedback

• There appears to be several typos throughout the text, e.g. in the caption of Figure 6, for example the repetition of the term specimen. We suggest to have another look specifically at the spelling and grammar to  help with the readability of the manuscript.  

• There is some information that could be better presented as a figure: for example the addition of a map demonstrating the location of the sampled specimen would offer greater information and understanding of the geographical location of the sampled population. 

• Some figures could be improved with more information added to their respective legends, for example Figure 2c, where a figure legend explaining the meaning of the different colours would increase the understanding of the photogrammetry  set-up.

• Some figure labels are quite small. We recommend increasing the size of the writing next to the colour scale in Figure 6c to improve the clarity of that figure. As this would help with the overall clarity of the paper. 

• Some sentences are sometimes hard to understand. For example in this sentence “Some of the key elements in the development of this framework included imaging of fine morphological details, accurate 3D model quantification of physical, 3D anatomical reconstructions of soft and hard tissues, complementation of imaging techniques and the creation of online databases with worldwide access” It is unclear what the phrase  “3D model quantification of physical” is referring to precisely. We would recommend rewording the phrase potentially to something akin to ” quantification of physical 3D models” or something along these lines.

Final remarks

We thank the authors for sharing their work as a preprint. We hope our feedback above will be helpful as they consider any revisions to the manuscript or future lines of work.

Competing interests

The author declares that they have no competing interests.