Ingestion of polyethylene microplastics impacts cichlid behaviour despite having low retention time

Microplastics, particles between 0.001 and 5 mm in diameter, are ubiquitous in the environment and their consumption by aquatic organisms is known to lead to a variety of adverse effects. However, studies on the effects of microplastics on prey fish have not shown consistent trends, with results varying across species and plastic type used. Here, we manipulated the levels of microplastic (MP) exposure among juvenile convict cichlids (Archocentrus nigrofasciatus) by feeding them brine shrimp (Artemia spp.) exposed to 0, 10, or 100 MP ml-1 of virgin polyethylene microspheres (10-20 μm) for a 10-day period. We then tested groups of 3 cichlids in a 2-day maze trial, in which we measured the latency to explore and time to complete a novel maze. We found no impacts of microplastic exposure on foraging rate, growth, or competitive aggression. However, our results demonstrate that microplastics exposure shaped exploratory behaviour and maze performance. Despite these effects, we found very little microplastics remaining in the fish's bodies after the experiment. A companion experiment demonstrates that most plastic particles were egested within 24 h. Our current results show that pristine microplastics at non-lethal levels have consequences on cichlid behaviour and decision-making but not growth.