Relationship between the endangered freshwater pearl mussel Margaritifera margaritifera, its salmonid host and co-infectants

The relationship between the freshwater pearl mussel (hereafter FPM), Margaritifera margaritifera, and its salmonid host — pivotal for the conservation of this endangered bivalve — is characterized by a long parasitic stage (up to 11 months) and excessive host specificity. This thesis focuses on this relationship by experimentally studying the effect of FPM infection on the growth and resistance of salmonid fish and how the infection influences vulnerability of the host to other parasites and diseases, as well as how exposure of the host to other parasites affects vulnerability to FPM. Infection with FPM results in (i) reduced growth of the host, brown trout, during the parasitic period. Glochidia infecting fish gills cause a respiratory burden which may explain the negative growth-effect. In addition, (ii) infection induced dose-dependent acquired immunity in repeated exposures. Instead, FPM infection leads to (iii) increased vulnerability of brown trout to eye fluke Diplostomum pseudospathaceum. It maybe the respiratory burden of FPM infection that increases ventilation and exposes the fish to these gill-penetrating eye fluke. FPM infection also causes (iv) increased resistance of the host to Flavobacterium columnare (a pathogen that enters fish through the gills) — possibly due to enhanced nonspecific immunity or altered structure of the fish’s gills. However, (v) pre-infection of trout with Anodonta anatina glochidia did not affect susceptibility to FPM, indicating that co-existing with A. anatina, is not a threat to FPM. Nevertheless, (vi) pre-infection with eye fluke cercariae within a short period (20 h) increased susceptibility of the host to FPM. Possibly, the tissue damage caused by recently penetrated eye fluke cercariae enhances attachment of FPM glochidia. These results shed light on complex, previously unstudied relationships between salmonid, FPM and other parasites/diseases and provide important new information that can be potentially utilized in conservation of this endangered specie