Neuroactive pollution alters shoaling in fish, but group phenotype matters

Jake Mitchell Martin , Jack Lewis Manera, Kehinde Olajid, Jack A. Brand, Mirjam Amcoff, Alexander Kotrschal, Alexander Szorkovszky, Erin S. McCallum, Daniel Cerveny, Tomas Brodin, Michael Grant Bertram, Niclas Kolm

Collective behavior governs how animal groups evade predators, forage, navigate, and share information. Yet it is rarely the focus of research on human-induced environmental change, particularly in the case of chemical pollution. Here, we show that exposure to the common neuroactive pollutant temazepam disrupts shoaling dynamics in fish (Poecilia reticulata), with effects shaped by group phenotype. Using 111 groups from artificial-selection lines divergent in collective coordination, we quantified group dynamics and the local interaction rules from which they emerge. In wild-type groups, temazepam weakened alignment and attraction between neighbours, reducing group polarisation, speed, and cohesion, particularly during early group establishment when social information is rapidly integrated. Conversely, groups selected for high coordination maintained or increased these traits. We provide the first evidence that neuroactive pollution alters local interaction rules in ways that scale up to collective motion, and that group phenotype can buffer or amplify pollutant effects.