Predation on parasites: Effects of predator type, parasite species, and environmental conditions

Background/Question/Methods Parasites can contribute significantly to biomass flow and network characteristics of aquatic food webs. These effects arise not only from interactions with their hosts, but also through direct predation on free-living infective stages, such as trematode cercariae. However, links between parasites and predators are often inferred based on the dietary size range of the predators. To better understand the interactions between predators and parasites, it is important to consider how specific traits of the parasites, the predators, and the environmental context influence predation risk. Our research objectives were to 1) compare parasite consumption by vertebrate and invertebrate predators that varied in size, 2) quantify the vulnerability of four trematode species that differed across a body size gradient, and 3) evaluate the effect of a light or dark environment on consumption. Because trematode species have variable circadian shedding patterns, they are not continuously available to predators and may vary in predation risk as a function of light conditions, particularly for visual predators. We assessed the consumption of cercariae using short-term laboratory experiments quantifying the number of parasites removed by the predators. We preformed necropsies on a subset of predators to isolate the role of consumption and infection processes. 

Results/Conclusions We observed strong and complex interactions between the factors tested by our laboratory study. First, consumption differed based on the predator species. Damselfly nymphs consumed 30-50% of two parasite species of intermediate size (770-1080 μm) but less than 4% of cercariae smaller (530 μm) or larger (1180 μm). Damselflies also consumed more parasites in the light compared to the dark, but damselfly body length did not influence consumption. Mosquitofish consumed between 10-90% of cercariae. Small mosquitofish (~10 mm) consumed over 80% of each parasite taxa, but their consumption was reduced by more than half in the dark treatments. Larger mosquitofish (~30 mm) showed similar consumption rates in the light and dark treatments, but overall consumption correlated positively with parasite body size. None of the parasite taxa infected the damselflies, but two species Ribeiroia ondatrae and an unidentified echinostome (morphotype: magnacauda) infected the mosquitofish. Collectively, these results show diverse traits can significantly influence parasite consumption with implications for biomass and energy flow in food webs and transmission to downstream hosts if predators cause significant cercariae mortality.  Therefore, it is necessary to incorporate a wider trait-based perspective of parasite-predator interactions.