Nanoplastic Exposure Impairs Host Fitness and Parasite Transmission With Transgenerational Maternal Reproductive Costs in the Daphnia ‐Fungal Parasite System

ABSTRACT Nanoplastics are emerging contaminants in aquatic ecosystems, but their direct and maternally transmitted effects on host–parasite interactions remain poorly understood. We investigated the effects of polystyrene nanoplastic exposure (50 nm, 5 mg L −1 ) on Daphnia magna infected with the fungal parasite Australozyma monospora across two host generations. Direct nanoplastic exposure occurred in the parental generation (F0), whereas F1 offspring were raised in nanoplastic‐free medium to isolate maternal nanoplastic effects. Direct nanoplastic exposure severely impaired F0 host fitness, reduced reproductive probability and total offspring production, while halving median survival time relative to controls. Nanoplastic exposure also impaired parasite maturation likelihood by decreasing host survival prior to transmission. Infection prevalence and spore production did not differ between nanoplastic‐exposed and control hosts among individuals that survived to the transmission stage, suggesting that nanoplastic exposure compromises parasite fitness primarily through accelerated host mortality rather than through within‐host suppression. In the F1 generation, offspring of nanoplastic‐exposed mothers had lower reproductive probability and delayed age at first clutch, despite experiencing no direct nanoplastic exposure. Host survival and parasite fitness outcomes were not altered in F1. Together, these results demonstrate that nanoplastic pollution disrupts host–parasite dynamics both directly and across generations, imposing reproductive costs that persist in unexposed offspring through maternal effects. Such transgenerational effects compromise host population recovery and amplify parasite transmission in nanoplastic‐contaminated freshwater environments.