Parasitic mussels induce upstream movement in their fish hosts: early evidence of extended phenotype

Parasites often have a large impact on their hosts and can alter host phenotype to increase their own fitness, a phenomenon known as extended phenotype. Studies demonstrating extended phenotype for non-trophically transmitted parasites are scarce. Unionid mussels have a parasitic life stage adapted to parasitize fish, which can affect host behaviour, habitat use and growth rates, raising the question of whether parasitic freshwater mussels can also manipulate their host fish to compensate for downstream dispersal and to reach habitats favourable for newly excysted juvenile mussels. Wild-caught, parasite-naïve juvenile brown trout (Salmo trutta) were PIT-tagged, half infested with parasitic larvae from the freshwater pearl mussel (Margaritifera margaritifera), and then returned to their home stream. During the following year, trout were tracked to investigate movement and habitat use, and also periodically recaptured to measure growth and condition factor. The infested trout showed significantly higher upstream movement than non-infested trout and were more often recaptured in stream sections with slow-moving shallow water, particularly during the parasite excystment period (270 days post-infestation). These data suggest that the juvenile mussels were successfully transported an average of 170 m upstream from the host trout release points to stream sections favourable for adult mussels. Infested trout survived as well as the non-infested, but had a significantly lower specific growth rate than non-infested trout. These results indicate a first example of extended phenotype in unionid mussels and highlight the importance of understanding glochidia-induced changes to host fish behavioural ecology.