Caffeine made freshwater crustaceans less wary, like parasite-infected prey

In laboratory tests, higher caffeine exposure made Gammarus pulex more active and less likely to avoid light, resembling a parasite’s effect.

Editorial illustration — not from the study.

Researchers tested how environmentally relevant caffeine concentrations affected Gammarus pulex, a freshwater amphipod involved in decomposition and commonly eaten by aquatic predators. They compared infected and uninfected animals after 20 hours of exposure to high or low caffeine concentrations.

Higher caffeine concentrations increased the amphipods’ activity and reduced their avoidance of light. The change was comparable to the reduced photophobia caused by the parasite Polymorphus minutus, which alters its host’s behaviour in ways that can help the parasite reach water birds, its final host.

How caffeine changed behaviour

In a two-choice test, higher caffeine exposure increased the activity of Gammarus pulex and reduced its photophobia—the tendency to avoid light. The shift in behaviour was comparable to that seen in amphipods infected with Polymorphus minutus. The abstract does not report numerical effect sizes or sample sizes.

Why the behaviour matters

Caffeine enters freshwater mainly through wastewater, so human-derived chemicals can reach organisms that form part of aquatic food webs. The findings suggest that caffeine can shift anti-predator behaviour in a direction similar to an evolved parasite-mediated manipulation, potentially increasing predation risk and affecting ecosystem dynamics.

Evidence and caveats

The study is a controlled laboratory experiment using a two-choice behaviour test after 20 hours of caffeine exposure. It examined activity and light avoidance rather than directly measuring predation or changes in an ecosystem. The abstract does not provide the exposure concentrations, sample sizes or effect sizes, and it does not show whether the observed behavioural changes occur in the wild or alter predator populations.

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