In the face of fear: the success of encounters between digenean cercariae and an intermediate target host under predation pressure.

Abstract

Predation is one of the most potent factors shaping relationships between organisms with different trophic levels. The interaction is an evolutionary arms race, where early detection of the other side often holds the key to success. Preying on the free-living parasite larvae is a valid and underestimated factor affecting parasite infection dynamics. Our study aimed to investigate whether often dominant invertebrates in aquatic ecosystems, gammarids, induce a significant reduction in host-parasite encounters, taking into account the influence of the presence of additional chemical signals from top predators (fish) and the parasite prey "age" on the possible dilution effect. The study is based on a model involving representatives of native (Gammarus jazdzewskii) and invasive (Pontogammarus robustoides) gammarids as consumers and two species of free-living parasite larvae, echinostome cercariae (Echinoparyphium aconiatum and Molinella anceps), as prey. The invasive gammarid species modified E. aconiatum success more significantly than the native one. However, a dilution effect was detected exclusively for P. robustoides, utilising freshly released cercariae and the absence of additional stressors (fish kairomones in the water). In contrast, the presence of both gammarid species usually significantly reduced the success of parasite transmission. The presence of fish cues or the differences in the post-emergence "age" of cercariae only affected treatments with the invasive P. robustoides. Our results suggest that the strength of predation's impact on the cercarial dilution effect can be modelled depending on the attractiveness of the prey. Additionally, this study potentially provides the first evidence of the influence of "enemy scent" on the strategy adopted by free-living larval trematodes, where a trade-off between cercarial age and anti-predator strategy was observed. Moreover, the study discusses how investigations conducted solely using freshly released cercariae may provide an incomplete or distorted picture of what is happening in the environment.