**5. Conclusions**

This study reports for the first time the presence of the OvoA gene, a key player of the ovothiol biosynthetic pathway, in arthropods. By mining the new transcriptomic resources for marine arthropods, we report a single OvoA gene in copepods, decapods, and amphipods. Phylogenetic analysis places all the marine arthropod sequences in two separate branches, suggesting possible events through evolution. Changes in expression of the OvoA gene across development and under stress conditions, suggest that ovothiol may play a role as a defensive compound in *C. finmarchicus*, thus proposing this gene as a new biomarker of stress in holozooplanktonic species. The finding that many copepods have the OvoA gene and are thus capable of producing bioactive compounds opens up further possibilities for both new drug discovery as well as in the marine biotechnology field.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/md19110647/s1, Table S1: List of in publicly available marine arthropod datasets mined for OvoA using the query from *P. lividus*. For each dataset, information on Genus, Species, Phylum, class, and Order are provided. Accession number of the mined transcriptome (TSA database) and the Bioproject are listed. For *Eurytemora affinis* and *Tigriopus californicus* the protein sequence database was mined (blastp algoritm). For *Lepeophtheirus salmonis* the TSA accession number was not provided. For three decapods, transcriptomes were mined using CrustyBase repository. Table S2: List of OvoA protein sequences used for the phylogenetic analysis. The list includes sequences identified in this study (light blue) and sequences publicly available for selected marine metazoan [1]. For each sequence, information on Genus, Species, Phylum, class, and Order are provided. Supplementary File S3: FASTA file of OvoA transcript encoding protein identified in this study for marine arthropods.

**Author Contributions:** Conceptualization, V.R., C.L. and Y.C.; methodology, V.R.; writing—original draft preparation, V.R., C.L. and Y.C.; writing—review and editing, V.R., C.L. and Y.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Bioproject and transcriptome shotgun assembly (TSA) numbers of the National Center for Biotechnology Information (NCBI) database for the datasets examined in the present study are indicated in Supplementary Table S1. In Supplementary File S3, the OvoA transcript encoding protein identified in this study for marine arthropods is indicated.

**Acknowledgments:** We would like to thank P. Lenz from the University of Hawai'i at Manoa and A. Ianora from Stazione Zoologica A. Dohrn for their intellectual contributions.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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