Microplastic evacuation in fish is particle size-dependent

Abstract

The pollution of aquatic systems with microplastics (MPs) affects marine and freshwater fish species worldwide. However, little is known about the size-dependent retention time of these inert particles. To approach this question, the retention time of MP particles was examined in two freshwater fish species with distinct differences in gastrointestinal morphology: rainbow trout (Oncorhynchus mykiss) with a true stomach, and stomachless common carp (Cyprinus carpio). A special diet was developed that contained environmentally relevant concentrations of MPs with sizes ranging from 20 to 1,000 µm. The two species were exposed to three different concentrations and numbers of retained particle were determined up to 72 hr after administration. The results revealed significant differences in retention time between large and small MP particles in rainbow trout, in which the T50 value (time for 50% of particles to be evacuated) ranged from 12.1 hr for 42.7 µm particles to 4.0 hr for 1,086 µm particles. In contrast, the differences observed between sizes in common carp were considerably smaller, with T50 ranging from 7.3 hr for 42.7 µm particles to 4.6 hr for 1,086 µm particles. It is therefore concluded that large particles in rainbow trout must be actively transported out of the stomach, as the evacuation times are significantly shorter than those for food. Small particles, however, are passively excreted with the chyme. In common carp, the evacuation rates of all particle sizes were in the range of that for food, suggesting a passive excretion. The results are particularly relevant in the contexts of particle transfer through the food chain, the release of toxic chemicals, and the translocation into tissues and organs. For this reason, it is essential that the particle size is given more consideration in future studies of microplastics in aquatic environments.