Deterioration of basic components of the anti-predator behavior in fish harboring eye fluke larvae

Abstract

Parasites can manipulate their host’s behavior in order to increase their own fitness. When a parasite is trophically transmitted, it can alter the host’s anti-predatory behavior to make it more susceptible to the next host in the lifecycle. We experimentally infected young-of-the-year rainbow trout, Oncorhynchus mykiss, with realistic, naturally occurring numbers of the common eye fluke, Diplostomum pseudospathaceum, to investigate whether these parasites alter fish activity, depth preference, and activity resumption latency following a simulated avian predation attack. Behavioral tests for the first two common anti-predatory behavioral traits, which are closely related to the host’s conspicuousness to the fish-eating bird (the final host of the parasite), were performed after the parasites had attained maturity (>4 weeks post-infection). In activity latency, we also studied potential conflict between mature and immature parasites. The fish harboring mature metacercariae increased their activity, preferred to stay closer to the water surface, and spent less time immobile after the simulated avian predator attack compared to the control fish. We did not find evidence of intraspecific conflict between mature and immature eye fluke metacercariae. Interestingly, these behavioral changes did not correlate with infection intensity. Our results suggest that the D. pseudospathaceum metacercariae can change rainbow trout’s behavior predisposing them to avian predation. Since eye flukes are common freshwater fish parasites, the resulting behavioral changes caused by these parasites likely play an important role in freshwater food webs.

Significance statement

By sabotaging the intermediate host’s anti-predatory behavioral traits, a parasite can predispose the host to predation by the final host. We experimentally studied whether the parasitic eye fluke, Diplostomum pseudospathaceum, alters rainbow trout’s anti-predatory behavior. Infected fish were more active, preferred upper water layers, and recovered quickly from the simulated avian predator attack compared to control fish. Our results suggest that the eye fluke changes its host’s behavior in order to make it more vulnerable to the final host. Most importantly, the observed behavioral changes arose, when the infection intensity was similar to rates found in natural conditions. This implies that, in natural conditions, eye flukes can substantially alter host anti-predatory defenses and affect predator–prey interactions.