Microplastic pollution is an emerging threat to marine biota. Uptake of microplastics can impair nutrition and affect the performance of organisms. However, the vulnerability to microplastics seems to vary between species for yet widely unexplored reasons. In addition to synthetic particles, marine invertebrates, such as shrimps and mussels, are in their habitats exposed to large amounts of natural microparticles. These include grains of sand, fragments of diatom or bivalve shells, chitin remains and cellulose fibers. Shrimps collected in their natural environment contained up to 3.000 sand grains and numerous fragments of bivalve shells in their stomachs. Sand grains may have been ingested to support maceration of food. The stomach can be cleaned from ingested particles by regurgitation. The animals have mechanisms that allow them to tolerate the uptake of microparticles and prevent possible harmful physiological and cellular effects.

In an experimental approach, the brown shrimp (Crangon crangon) was exposed to different natural and synthetic microparticles. The antioxidant defense of the midgut gland was analyzed by means of antioxidant enzymes. The results, so far, show no differences in the superoxide dismutase activity. Additionally, fluorescent microbeads of 0.1, 2.1, and 9.9 μm diameter were administered and digestive organs were anatomically investigated. Only the smallest particles (0.1 μm) entered the midgut gland, which is the principal site of nutrient resorption in crustaceans. A fine-meshed chitinous filter system in the stomach of the shrimps prevents the passage of particles larger than about 1 μm. Further studies on the shrimps and mussels are in progress.

C. crangon appears well adapted to handle natural microscopic particles. This trait seems also to be advantageous in coping with microplastic pollution. The results form the basis for better defining the hazard potential of different species and thereby making a reliable risk assessment. A realistic assessment of the risk posed by microplastics makes a constructive contribution to current environmental and socio-political discussions and future prospects.