• Energy Research

Digenean trematodes are common and abundant in aquatic habitats and their free-living larvae, the cercariae, have recently been recognized as important components of ecosystems in terms of comprising a significant proportion of biomass and in having a potentially strong influence on food web dynamics. One strategy to enhance their transmission success is to produce high numbers of cercariae which are available during the activity peak of the next host. In laboratory experiments with 13 Lymnaea stagnalis snails infected with Trichobilharzia szidati the average daily emergence rate per snail was determined as 2,621 cercariae, with a maximum of 29,560. During a snail's lifetime this summed up to a mass equivalent of or even exceeding the snail's own body mass. Extrapolated for the eutrophic pond where the snails were collected, annual T. szidati biomass may reach 4.65 tons, a value equivalent to a large Asian elephant. Emission peaks were observed after the onset of illumination, indicating emission synchronizing with the high morning activities of the definitive hosts, ducks. However, high cercarial emission is possible throughout the day under favorable lightning conditions. Therefore, although bird schistosomes, such as T. szidati constitute only a fraction of the diverse trematode communities in the studied aquatic ecosystem, their cercariae can still pose a considerable risk for humans of getting cercarial dermatitis (swimmer's itch) due to the high number of cercariae emitted from infected snails.

Blue mussels (Mytilus edulis) are important ecosystem engineers along Atlantic coastlines, where they are regularly subjected to rapid changes in temperature during the transition between tides. Global climate change and more frequent extreme weather events are expected to intensify this thermal stress even further. These increases in temperatures will not only affect intertidal mussels directly but also increase transmission dynamics of their parasites. Together, the effects of rises in temperature and parasitism will likely result in higher pressure on M. edulis and their ability to perform vital ecosystem services. In a set of experiments, we tested the effects of infections with the trematode Himasthla elongata and high temperatures during low tide air-exposure. Overall, we hypothesised that temperature and parasite infection intensity would each have significant negative effects on M. edulis survival, and that both stressors together would have a synergistic detrimental impact. Overall, high temperature levels had a strong negative effect on mussel survival. However, our results revealed a surprisingly more complex picture in infected individuals. While moderate parasite loads and increased temperature showed additive negative effects on mussel survival, high parasite infection intensities appeared to nullify the detrimental effects of temperature stress on mussels. Under climate warming, these benefits of parasites might actually outweigh the costs of infection and prove beneficial. Overall, these results suggest that the interactions between host-parasite systems and their changing environment are much more complex than a simple additive effect of multiple stressors.