Perrot-Minnot_2025_Ecotoxicol.Environ.Saf_305_119220

Chemical contaminants, elevated temperature, and parasitism are expected to impose multiple stresses on aquatic organisms. Their sublethal effects on fitness may be mediated through multiple traits, and combine in additive or interactive ways. In freshwater ecosystems, at least half of paired-stressor effects are either additive or antagonistic, but few studies have considered parasitism. Using a fully crossed factorial design, we investigated survival, behaviour, and biomarker responses of the amphipod Gammarus fossarum to four-day exposure methomyl insecticide, in interaction with temperature and parasitic infection. Infection with the acanthocephalan Pomphorhynchus tereticollis was taken into account at two levels, (i) the population level, comparing uninfected gammarids from "non-naive" populations hosting the parasite to ones from "naive" populations; (ii) at the individual level, comparing infected and uninfected gammarids from non-naive populations. Overall, methomyl and temperature alone had limited effect, while parasitism had pervasive effects on most traits, including redox status and defensive systems. At the population level, the combined effects of infection and of either methomyl or elevated temperature were additive (survival, feeding rate, locomotion, acetylcholinesterase, antioxidant), or antagonistic (survival, feeding rate, locomotion), when not dominant. At the individual level, most combined effects of infection and either temperature or methomyl were dominant. We thereby provide evidence for mainly dominant and additive effects of parasitism combined with methomyl or elevated temperature. Their physiological and environmental drivers remain to be investigated. Given the pervasive effects of parasitism, alone or combined with other stressors, parasitism should be taken into account to gain realism in ecotoxicological studies.