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RESEARCH PRODUCT

Impaired crypsis of fish infected with a trophically transmitted parasite

subject

description

Trophically transmitted parasites may enhance their transmission by altering the phenotype of infected hosts to increase their vulnerability to predation by the next hosts in the life cycle. In an experimental study, we investigated whether the parasite Diplostomum spathaceum (Trematoda) alters cryptic coloration and cryptic behaviour of infected rainbow trout, Oncorhynchus mykiss, so that they would be more conspicuous to avian predators. The parasite reduces the vision of fish by lodging in the eyes and inducing cataract formation, which gives it an opportunity to affect fish crypsis. We examined the effect of the parasite on the ability of fish to adjust their coloration to the environment and on the preference of fish for different backgrounds. When maintained against a black substrate coloration, the colour of both infected and control fish closely matched that of the environment. However, in lighter environments, the contrast between fish and background coloration increased, especially in infected fish, which were darker than control fish. Furthermore, when given a choice between black and white substrates, the preference of infected fish for the dark background, where the fish were least conspicuous, was reduced compared to that of controls. These results indicate that D. spathaceum eye flukes impair crypsis of fish which is one potential mechanism predisposing them to predation by bird hosts.