Abstract
The freshwater pearl mussel (Margaritifera margaritifera) is a highly host-specific parasite, with an obligate parasitic stage on salmonid fish. Atlantic salmon (Salmo salar) and brown trout (Salmo trutta f. trutta and Salmo trutta f. fario) are the only hosts in their European distribution. Some M. margaritifera populations exclusively infest either Atlantic salmon or brown trout, while others infest both hosts with one salmonid species typically being the principal host and the other a less suitable host. Glochidial abundance, prevalence and growth are often used as parameters to measure host suitability, with the most suitable host species displaying the highest parameters. However, it is not known if the degree of host specialisation will negatively influence host fitness (virulence) among different host species. In this study we examined the hypothesis that glochidial infestation would result in differential virulence in two salmonid host species and that lower virulence would be observed on the most suitable host. Atlantic salmon and brown trout were infested with glochidia from two M. margaritifera populations that use Atlantic salmon as their principal host, and the difference in host mortality among infested and control (sham infested) fish was examined. Higher mortality was observed in infested brown trout (the less suitable host) groups, compared to the other test groups. Genetic assignment was used to identify offspring from individual mother mussels. We found that glochidia from individual mothers can infest both the salmonid hosts; however, some mothers displayed a bias towards either salmon or trout. We believe that the differences in host-dependent virulence and the host bias displayed by individual mothers were a result of genotype × genotype interactions between the glochidia and their hosts, indicating that there is an underlying genetic component for this parasite-host interaction.
Highlights
Parasites are typically classified as either being host specialists or host generalists depending on their host range; i.e. the former attain high fitness on very few host species compared to the latter, which attain it on many (Veiga et al 1998; Poulin 2007; Leggett et al 2013; Lievens et al 2018)
Brown trout exposed to glochidia, from both the freshwater pearl mussel (FPM) populations, displayed a higher mortality compared to Atlantic salmon as well as control fish (Table 1)
The odds of a brown trout exposed to glochidia dying was 10.69 times that of a trout dying in the control group (Fisher’s test: pvalue < 0.001, odds ratio = 10.69; Fig. 2)
Summary
Parasites are typically classified as either being host specialists or host generalists depending on their host range; i.e. the former attain high fitness on very few host species compared to the latter, which attain it on many (Veiga et al 1998; Poulin 2007; Leggett et al 2013; Lievens et al 2018). The freshwater pearl mussel (FPM), Margaritifera margaritifera, is an example of a long-lived specialist parasite with a reproductive lifespan that is 30 times longer than its host (Geist and Kuehn 2008), and it is a good model to examine the influence of high host specificity on parasite fitness, in particular virulence and infectivity (host bias). Infective glochidia are not selective during attachment and are able to attach to all objects (e.g. wood, plastic or paper) (Kat 1984; Dodd et al 2005), but in order to be encysted by gill epithelial cells of the host, they must induce an immune response (Nezlin et al 1994; Jansen et al 2001). After a parasitic period lasting between 9 and 11 months, free-living juvenile mussels excyst and spend the 5 years buried in the river substratum (Smith 1976; Bauer 1987)
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