Microenvironment of Gyrodactylus derjavini on rainbow trout Oncorhynchus mykiss : association between mucous cell density in skin and site selection

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Abstract
Microhabitat selection of Gyrodactylus derjavini on the body surface of rainbow trout changed markedly during a 6-week experimental infection period. Pectoral fins, pelvic fins and anal fins were the most important sites (expressed in terms of parasite density) during the initial part of the infection. In the later stages of infection, the corneal surface and tail fin became increasingly more heavily infested. Factors responsible for this dynamic site selection were investigated. The density of superficial mucous cells in the epithelium of fins and skin was weakly correlated (r = 0.23) with parasite density in the first part of the infection. This association changed into a significant negative correlation (r = −0.92) as the infection progressed and the parasite population increased. These results strongly indicate that mucous cell contents play a decisive role in gyrodactylid site selection. Lysozyme, protease, immunoglobulin (Ig), complement factor C3, enzymes, lectin-binding carbohydrates and peptides adrenocorticotropic hormone, interleukin (IL-1) and somastatin) were detected in mucus and some of these (Ig, C3, IL-1, carbohydrates) are suggested to influence the infection dynamics. Thus, some molecules in mucus are liable first to attract the gyrodactylids, but subsequently reactive substances present in increasing amounts will counteract the performance of parasites in mucous-cell-rich microhabitats. The mechanisms involved in this process are discussed.

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