A fish parasite that spends part of its life inside fish-eating birds has been shown to manipulate the behaviour of host fish to make them more likely to be consumed by birds.
The trematode eye fluke Diplostomum spathaceum has been shown to induce the formation of cataracts in the eyes of infected fish, which makes the fish more susceptible to avian predators and reduces their ability to escape.
According to a study by Seppala, Karvonen and Tellervo of the University of Jyvskyl in Finland, the catchability of fish was higher when they had a parasite-induced cataract, and the eye flukes only manipulated their hosts when they had fully developed inside the fish eye.
By waiting until they were fully-formed, and ready to enter the next stage of their life cycle within the gut of a piscvirous bird, the parasites were able to manipulate the host when they were at their most infective.
The paper suggests that selection may also play a part in the occurence: "Intensity of infection, however, did not affect vulnerability of fish to capture by dip-net.
"These findings suggest that the ability of the parasite to manipulate fish behaviour by impairing its vision may have resulted from selection preferring parasite genotypes with higher transmission efficiency."
For more information see the journal paper: Seppala O, Karvonen A and E Tellervo Valtonen (2005) - Manipulation of fish next term host by eye flukes in relation to cataract formation and parasite infectivity. Animal Behaviour. Volume 70, Issue 4, October 2005, Pages 889-894.