IgD+IgM- B cells are prevalent in gills and gut but not spleen, raising the possibility that the IgD gene repertoire exhibits distinct molecular signatures in these microbiota-colonized mucosal organs compared to sterile systemic organs. To test this hypothesis, IgD immune repertoires were analysed in comparison with IgM and IgT immune repertoires using targeted-PCR amplification from cDNA and sequencing using MiSeq (Illumina).