Behavioural mechanisms underlying 'specific' host manipulation by a trophically transmitted parasite

Background: The larvae of the eye fluke, Diplostomum pseudospathaceum, emerge into the water from a snail and then infect a fish. Once in a fish, they travel to its eye lenses. The parasite infects its definitive host, a fish-eating bird, if the fish is eaten by the bird. Parasitized fish behave in ways that make them more susceptible to bird predation. Previous studies have suggested that this host manipulation is ‘specific’, as parasites do not increase the susceptibility of fish to non-host predators such as piscivorous fish. This suggests that eye flukes evolved to manipulate host fish as a strategy to enhance transmission. Question: Do eye flukes change the behavioural traits of fish important in determining their vulnerability to non-host fish predators? Hypotheses: Given the evidence of specific host manipulation in this system, the parasites (1) should not alter such traits, or (2) alterations in them should have opposite effects in determining the susceptibility of fish to piscine predation. Organisms: The trematode parasite, D. pseudospathaceum, and its rainbow trout, Oncorhynchus mykiss, intermediate host. Methods: A laboratory experiment to determine the effect of eye flukes on behavioural traits of fish that are part of their general anti-predatory tactics or specifically determine their susceptibility to piscine predation. We measured the following traits: activity, use of shelter, and escape response to underwater predator attacks. Results: Parasites did not affect any of the behavioural traits examined. Conclusion: Diplostomum pseudospathaceum manipulates only traits that specifically predispose fish to bird predators, not the traits important in determining their susceptibility to non-hosts.