Screening of gill samples with standard histology was performed on samples fixed in 10% neutral buffered formalin, followed by dehydration in an ascending alcohol series ending in xylol and afterwards embedded in paraffin. Paraffin blocks were cut in 2–3 μm thin sections, mounted on glass slides and stained using a routine protocol for haematoxylin and eosin (HE) staining.
For electron microscopy, formalin-fixed gill tissues were post-fixed in a mixed solution of 1% paraformaldehyde and 2.5% glutaraldehyde in 0.1 M sodium phosphate buffer, pH 7.5 at 4°C overnight. Samples were prepared for TEM by embedding into epoxy resin according to standard procedures (Seth-Smith et al., 2016 (link)). Epoxy resin blocks were screened for epitheliocystis lesions using semithin sections (1 μm) which were stained with toluidine blue (Sigma-Aldrich). Ultrathin sections (80 nm) were mounted on copper grids (Merck Eurolab AG, Dietlikon, Switzerland), contrasted with uranyl acetate dihydrate (Sigma-Aldrich), and lead citrate (Merck Eurolab AG) and investigated using a Philips CM10 transmission electron microscope. Images were processed with Imaris 7.6.1 (Bitplane, Oxford Instruments) and assembled into panels for publication and annotated using Photoshop (Adobe).
For electron microscopy, formalin-fixed gill tissues were post-fixed in a mixed solution of 1% paraformaldehyde and 2.5% glutaraldehyde in 0.1 M sodium phosphate buffer, pH 7.5 at 4°C overnight. Samples were prepared for TEM by embedding into epoxy resin according to standard procedures (Seth-Smith et al., 2016 (link)). Epoxy resin blocks were screened for epitheliocystis lesions using semithin sections (1 μm) which were stained with toluidine blue (Sigma-Aldrich). Ultrathin sections (80 nm) were mounted on copper grids (Merck Eurolab AG, Dietlikon, Switzerland), contrasted with uranyl acetate dihydrate (Sigma-Aldrich), and lead citrate (Merck Eurolab AG) and investigated using a Philips CM10 transmission electron microscope. Images were processed with Imaris 7.6.1 (Bitplane, Oxford Instruments) and assembled into panels for publication and annotated using Photoshop (Adobe).
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