The concept of ‘ecology of fear’ recognises the importance of non-consumptive effects of predators inspiring fear in their prey, but has subsequently been broadened to include the non-consumptive effects of parasites on their hosts. Here, we experimentally tested whether blue mussels (Mytilus edulis) exhibit species-specific avoidance behaviour in response to infective transmission stages (cercariae) of three trematode species: Himasthla elongata and Renicola roscovita, both of which use blue mussels as hosts, and Cryptocotyle lingua, which parasitises fish. Furthermore, we investigated whether avoidance behaviour could be triggered solely by chemical cues. The results show that mussels reduced clearance rates by 34% in the presence of H. elongata alone and of 51% in the presence of H. elongata and R. roscovita in sympatry, which was comparable to previously reported direct consumptive effects of infection on filtration activity. The presence of C. lingua and R. roscovita alone had no significant effect on clearance rate. The observed reduction in filtration activity resulted in lower infection rates of experimental mussels. Moreover, mussels reduced clearance rates by 42% in the mere presence of chemical cues from the first intermediate host Littorina littorea, while exudates of H. elongata had no additional negative effect. It is evident that a non-consumptive parasite effect can significantly reduce the filtration activity of experimental blue mussels. Considering the magnitude of this reduction and the widespread occurrence of parasitic trematodes, it is reasonable to expect that fear of parasitism has ecological consequences for the functioning role of mussels.

Parasites are known to have direct negative effects on host fitness; however, the indirect effects of parasitism on host fitness sans infection are less well understood. Hosts undergo behavioural and physiological changes when in proximity to parasites. Yet, there is little experimental evidence showing that these changes lead to long-term decreases in host fitness. We aimed to determine if parasite exposure affects host fitness independent of contact, because current approaches to parasite ecology may underestimate the effect of parasites on host populations. We assayed the longevity and reproductive output of Drosophila nigrospiracula exposed or not exposed to ectoparasitic Macrocheles subbadius. In order to preclude contact and infection, mites and flies were permanently separated with a mesh screen. Exposed flies had shorter lives and lower fecundity relative to unexposed flies. Recent work in parasite ecology has argued that parasite–host systems show similar processes as predator–prey systems. Our findings mirror the non-consumptive effects observed in predator–prey systems, in which prey species suffer reduced fitness even if they never come into direct contact with predators. Our results support the perspective that there are analogous effects in parasite–host systems, and suggest new directions for research in both parasite ecology and the ecology of fear.