Feeding behavioral response of freshwater fish to plastic particles with naturally formed biofilm

The pervasive plastic pollution in marine and freshwater ecosystems has led to increasingly unpredictable interactions between aquatic organisms and microplastics (particles < 5 mm), including varying instances of plastic ingestion. The incorporation of microplastics into aquatic food chains poses significant risks to health and survival of the hydrobionts, as well as potential threats to biodiversity. The ingestion of microplastics by fish can occur inadvertently or deliberately, whether plastic particles are recognized or not as food objects. In natural aquatic environments, microplastics are often colonized by biofilms consisting of complex microbial communities, including protozoa, algae, and fungi. We hypothesized that the biofilm present on microplastics influences the possibility of their consumption by fish, enhancing the attractiveness of these particles as food objects and providing a gustatory trap. Thus, we assessed the feeding behavior of juvenile African catfish Clarias gariepinus and climbing perch Anabas testudineus in the presence of two types of plastic particles: expanded polystyrene (EPS) and extruded polystyrene (XPS). Both fish species are omnivorous and feed at the water surface. We used positively buoyant microplastics sized from 3.0 to 4.5 mm, which were exposed for 30 days under natural conditions to allow biofilm formation. Our findings indicated that climbing perch actively reacted to biofilm-covered plastic objects by biting and chewing them, but never ingested them. In contrast, catfish frequently ingested both types of plastic pellets, although they rejected the XPS pellets after prolonged periods of up to 12 hours. Based on these observations, we identified four patterns of feeding behavior in fish related to microplastic particles: refusal, testing and rejection, ingestion and rejection, and consumption. The first three behaviors are likely effective mechanisms for avoiding plastic consumption. Our results shed light on the species-specific behavioral traits that enable some fish to ignore visible plastic particles and successfully avoid consumption.