*2.1. Identification of OvoA Encoding Transcripts in Marine Arthropods*

In this study we report for the first time that marine arthropods also possess OvoA, a key player gene of ovothiol biosynthesis. By mining the publicly available transcriptome database (TSA) limiting to Arthropoda, we identified a single transcript encoding OvoA in 19 copepods and nine malacostracans (Table 1, Table S1). Within the copepod subphylum, OvoA was identified in 11 Calanoids, three Cyclopoids, three Harpacticoids and two Siphonostomatoids. The Calanoida order, with the highest number, included mostly individuals from the Calanidae family such as *Calanus finmarchicus*, *C. helgolandicus* and *Neocalanus flemingeri*. Among the malacrustracans, OvoA were found in seven decapods and two amphipods (e.g., *Gammarus pulex*, *G. fossarum*) (Table 1, Table S1). Reciprocal blast confirmed that all transcripts were annotated as protein OvoA with 70% encoding for full length proteins. Structural domain analysis confirmed the presence of the three expected functional domains, the DNA damage-inducible (DinB), the formylglycine-generating sulfatase (FGE-sulfatase), and the S-adenosylmethionine (SAM) methyltransferase) domains, as shown in the copepods *C. finmarchicus* and *C. helgolandicus* (Figure 2).


**Figure 2.** OvoA alignment in the copepods *Calanus finmarchicus* and *C. helgolandicus*. Protein alignment of the deduced OvoA sequenecs for the copepod *C. finmarhcicus* and *C. helgolandius*; "\*" beneath the alignment indicates residues that are identical in the two sequences while ":" and "." indicate conservatively substituted aminoacids shared between the protein pair. The three strunctural functional domains identified by the Pfam database are highlighted as follows: DNA damage-inducible (DinB) (light blue), formylglycine-generating sulfatase (FGE-sulfatase) (magenta), and S-adenosylmethionine (SAM) methyltransferase (green).

**Table 1.** OvoA in marine arthropods. The list includes the organisms for which a single OvoA transcript was found as result of the in silico transcriptome mining that included a reciprocal blast and structural domain analysis. For each organism, phylum, subphylum, subclass, and order are listed. For each sequence, detailed information on the mined database and National Center for Biotechnology Information (NCBI) accession number are provided in Table S1.

