Abstract
Trematodes of the genus Diplostomum (Diplostomidae) are widely distributed and significant fish pathogens known for causing a range of negative effects. Any mechanism that protects the host from the parasite thus represents an evolutionary advantage. In this study, we observed an unusual localisation of Diplostomum metacercariae, which were displaced into the sac protruding from the lens, in North-American bullhead catfish species of the genus Ameiurus. The occurrence of the sac was found in all sampling sites (Czech Republic) where A. nebulosus and A. melas were infected with Diplostomum, regardless of the parasite species, as both D. pseudospathaceum and D. spathaceum induced sac formation. The probability of sac formation increased with the intensity of Diplostomum infection. Experimental infection of juvenile A. melas showed that the first signs of sac formation appeared shortly after the infection. While visual observation under the light microscope suggested the first signs at 7 days post-infection, histological examination documented the start of epithelial proliferation by the third day post-infection. Our study suggests that the process of sac formation, along with parasite displacement, might present a host reaction to mitigate the negative effects of infection and the impaired vision caused by the parasite.