Differential effects of pharmaceuticals and insecticides on swimming behaviour and survival in Gammarus pulex

Many freshwater systems are continuously exposed to waste streams like municipal wastewater and agricultural runoff, leading to exposure to chemicals that can cause mortality and behavioural changes in aquatic organisms. While research has advanced our understanding of pesticide effects on behaviour of aquatic organisms, the impacts of pharmaceuticals are less understood. Psychopharmaceuticals are particularly interesting because they can act on nervous systems, potentially affecting the behaviour of aquatic organisms. Sublethal behavioural effects can be crucial in ecotoxicological research for environmental pharmaceuticals and are often detected below lethal concentrations. Gammarids, especially Gammarus pulex, are widely used in ecotoxicological studies due to their ecological role and sensitivity to pollutants. This study aims to evaluate the sensitivity of six swimming behaviour endpoints in G. pulex compared to the conventional endpoints immobility and mortality, using different chemicals with distinct modes of action: insecticides imidacloprid and chlorpyrifos and the pharmaceuticals carbamazepine and citalopram. After a 2-hour exposure, the mobile organisms were assessed for their swimming speed, acceleration, curvature, thigmotaxis and startle response (magnitude and duration). Our study reveals that G. pulex exhibits varied behavioural responses to different chemical pollutants. While behavioural endpoints can indicate harmful effects on aquatic organisms, they are not consistently more sensitive than traditional endpoints, such as immobility and mortality. The insecticides imidacloprid and chlorpyrifos show development of immobility and mortality without prior sublethal behavioural effects, suggesting a limited utility of behavioural endpoints as early warning indicators. In contrast, the pharmaceuticals carbamazepine and citalopram demonstrate adverse effects through behavioural changes before immobility and mortality occur. Further research is essential to understand the mechanisms underlying these varying sensitivities of behavioural endpoints to different compounds, emphasising the importance of considering both chemical type and endpoint relevance in toxicity testing protocols.