Environmentally relevant concentrations of tramadol and citalopram alter behaviour of an aquatic invertebrate

•Opioid drug Tramadol and antidepressant Citalopram alter crayfish behaviour at environmentally relevant concentrations.•Our results highlight the ecological importance of PhACs in aquatic ecosystems.•Simple method for assessing PhACs effects on aquatic organisms is presented.•Further research is necessary to assess the effects in detail. Environmental pollution by pharmaceutically active compounds, used in quantities similar to those of pesticides and other organic micropollutants, is increasingly recognized as a major threat to the aquatic environment. These compounds are only partly removed from wastewaters and, despite their low concentrations, directly and indirectly affect behaviour of freshwater organisms in natural habitats. The aim of this study was to behaviourally assess the effects of an opioid painkiller (tramadol) and antidepressant drug (citalopram) on behaviour patterns of a clonal model species, marbled crayfish. Animals exposed to environmentally relevant concentrations of both tested compounds (∼1 μg l−1) exhibited significantly lower velocity and shorter distance moved than controls. Crayfish exposed to tramadol spent more time in shelters. Results were obtained by a simple and rapid method recommended as suitable for assessment of behaviour in aquatic organisms exposed to single pollutants and combinations.