Caffeine pollution changes anti-predator behaviour in Gammarus pulex similarly to manipulative parasites

Caffeine is an emerging environmental contaminant in freshwater ecosystems that is primarily introduced through wastewater. Even at low concentrations, caffeine can affect the physiology and behaviour of aquatic organisms. Still, its ecological significance remains little understood. We examined the impact of environmentally relevant caffeine concentrations on the anti-predator behaviour of the freshwater amphipod Gammarus pulex. Gammarids play a central role in aquatic decomposition processes and are a primary food source for various aquatic predators. We examined the effects of caffeine in relation to a well-documented effect caused by the manipulative parasite Polymorphus minutus. This acanthocephalan parasite manipulates the anti-predator behaviour of its intermediate host, the gammarid, by altering its photophobia, thereby increasing its transmission to the final host, water birds. Using a two-choice experimental design, we assessed the phototactic behaviour of P. minutus-infected and uninfected G. pulex after 20 h of high- and low-concentration caffeine exposure. High concentrations of caffeine increased the activity of exposed individuals and reduced photophobia to a degree comparable with that of a P. minutus infection. These results suggest that human-derived chemicals can shift antipredator behaviour in a direction similar to an evolved parasite-mediated manipulation, which could increase predation risk and ultimately alter ecosystem dynamics.