Cost and behavioural avoidance of trematode cercariae in fathead minnows

xi, 84 leaves : ill. ; 29 cmNatural selection should favour host defenses that reduce a host’s exposure to parasites or reduce their negative effects. One strategy that resolves the substantial costs of host immunity and/or tolerance is to avoid infective stages altogether. For fish, behavioural avoidance is well-known for defense against aquatic predators, but it is poorly known for defense against parasites. I used a model system that is amenable to experimental manipulation to test the behavioural avoidance hypothesis for fathead minnows exposed to the larvae of two of their common flatworm parasites. First, I showed that minnows exposed to a liver encysting trematode, Ornithodiplostomum sp. showed an increase in lipid peroxidation, an indicator of oxidative stress, persisting through worm development. Three lines of evidence provided support for the behavioural avoidance hypothesis. First, shoal area decreased in groups of minnows exposed to O. ptychocheilus cercariae compared to those exposed to cues from other aquatic threats. Second, average worm numbers were 50 % lower in fish confined to artificial shoals compared to non-shoaling minnows, indicating that shoaling reduces risk of exposure. The third experiment showed that minnows within the centre of shoal reduced their risk of infection by 67%. Taken together, these results demonstrate a cost of trematode infection on minnows, that minnows can detect infective larvae within the water column, and that social living reduces a hosts’ risk of exposure