Abstract
The freshwater pearl mussel (Margaritifera margaritifera) is an endangered bivalve with an obligate parasitic stage on salmonids. Host suitability studies have shown that glochidial growth and load vary significantly between host strains as well as among individuals of a suitable strain. Variation in host suitability has been linked to environmental conditions, host age and/or size, genetic composition of the host and parasite, or a combination of these factors. In our study, we wanted to investigate if brown trout (Salmo trutta) displayed an age-dependent response to glochidial infestation. We hypothesised that 1+ naive brown trout hosts tolerate glochidial infestation better than 0+ hosts. In order to test our hypothesis, we infested 0+ and 1+ hatchery reared brown trout with glochidia from closely related mothers and kept them under common garden conditions. This allowed us to observe a pure age dependent host response to infestation, as we eliminated the confounding effect of genotype-specific host interactions. We analysed the interaction between glochidial load and host condition, weight and length, and observed a significant age-dependent relationship. Glochidial load was negatively correlated to host condition in 0+ fish hosts and positively correlated in 1+ hosts. These contradictory findings can be explained by a change in host response strategy, from resistance in young to a higher tolerance in older fish. In addition, we also examined the relationship between glochidial load and haematocrit values in the 1+ hosts and observed that haematocrit values were significantly higher in heavily infested hosts. Our results have important conservation implications for the management of wild pearl mussel populations, as well as for captive breeding programmes.
Highlights
Host-parasite systems have been extensively studied to understand coevolutionary processes
The results of this study suggest that the duration of the parasitic phase of freshwater pearl mussel (FPM) larvae on their fish hosts has positive effects on their subsequent size and growth rates
Our results strongly indicate that the duration of the parasitic phase of FPM has a significant effect on their post excystment performance
Summary
Host-parasite systems have been extensively studied to understand coevolutionary processes. The traditional view is that parasites have a greater evolutionary potential and adaptive plasticity resulting from larger population sizes, higher mutation rates, and shorter generation times compared to their hosts (Ebert, 1994; Kaltz & Shykoff, 1998; Gandon & Michalakis, 2002) In addition to these circumstances, a narrow host range and larger migration rates would most likely result in the parasite being locally adapted to its hosts (Dawkins & Krebs, 1979; Lajeunesse & Forbes, 2002; Kawecki & Ebert, 2004; Morgan et al, 2005). Infective glochidia, released by gravid mothers, passively attach to a suitable fish host and become encysted on gills (Young & Williams 1984b; Wächtler et al 2001; Taeubert et al 2010; Taeubert et al 2013) as parasites that depend on nutrient transfer from the host (Denic et al 2015) They reduce host swimming performance (Taeubert & Geist 2013). After 9–11 months (Larsen 2005), juvenile mussels excyst (May to June) and spend the five years buried in the river sediment, after which they rise up to the substratum surface and develop into adults (Young & Williams 1984b)
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