THE INFLUENCE OF GYRODACTYLUS-SALARIS MALMBERG, 1957 (MONOGENEA) ON THE EPIDERMIS OF ATLANTIC SALMON, SALMO-SALAR L, AND BROOK TROUT, SALVELINUS-FONTINALIS (MITCHILL) - EXPERIMENTAL STUDIES

The effect of Gyrodactylus salaris on the epidermal structure of hatch ery-reared brook trout parr and Norwegian Atlantic salmon parr was eva luated. Both species were initially susceptible to this parasite, but whereas populations on salmon increased until the host died, brook tro ut responded to, and eventually eliminated, their infections. Salmon s kin samples taken 14 days p.i. showed a reduced mucous cell concentrat ion (less than 1500 cells mm(-2) on the head compared with 2000 cells mm(-2) in controls; the same trend was also seen in other sites) and t he epidermis was thinner (48 mu m compared with GO mu m on pectoral fi ns; similar trend seen in other sites) than in uninfected controls kep t: under identical conditions. Brook trout skin samples were taken 50 days p.i., when the hosts had responded to, and almost eliminated, the ir infections. No change was then observed in mucous cell density, whi le the number of epidermal cell layers and the epidermal thickness of brook trout had increased slightly following infection. These results are related to the nature of the host response, and the thinning of th e epidermis and loss of mucous cells may in some way he related to the inability of Norwegian salmon parr to respond to the parasite.