Fluoxetine and microplastics are increasingly detected in freshwater ecosystems, yet their combined effects on aquatic invertebrates remain poorly understood. This study evaluates the individual and combined effects of the antidepressant fluoxetine (FLX) and microplastics (MP) on predatory avoidance behaviour, movement, reproduction, and egestion in the freshwater snail Physa acuta. Snails were exposed to four treatments: control, fluoxetine (350 ng/L), microplastics (10 µg/L), and a combined fluoxetine and microplastics treatment, with 17 replicates per treatment.
Predatory avoidance behaviour was measured as the proportion of snails using refuge on or above a tile and by escaping their container. Movement was quantified as the number of grid crossings on a 1 cm² grid. Reproduction was assessed by counting egg masses and eggs per egg mass, and egestion was measured by weighing the dry waste mass.
We hypothesize that fluoxetine will alter behavior and reproductive output due to its interaction with conserved serotonergic pathways, while microplastics will impair digestion and reduce energy allocation to growth and reproduction through physical and physiological stress. Additionally, we predict that combined exposure will produce stronger negative effects than either stressor alone due to potential contaminant interactions. This study contributes to understanding how multiple emerging contaminants may interact to influence freshwater invertebrate fitness and ecosystem processes.