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.