Description: In this thesis the ingestion, remaining and egestion of microplastics by means of polyacrylic fibers and polystyrene beads by a brackish water organism were investigated. As test organism the offshore living, common ditch shrimp Palaemon varians was chosen. In the first experiment P. varians was fed with fluorescent polyacrylic fibers in combination with different flake food concentrations. After 3 h the stomachs of the animals were dissected, the numbers of fibers were counted and it was noted where the fibers remained in the organism. The counted numbers of fibers within different food concentrations available were statistically compared to each other. In a second experiment, a time-line was performed to investigate the stomach residence times of microplastic fibers, beads and food. For this, every 2 h the presence of beads, fibers and food in the stomach was estimated. In order to document the way of egestion the number of faeces was counted, and it was investigated whether they contained microplastic or not. Moreover, mucuslike structures suggested regurgitation whose occurrence was also noted. Due to time-lapse recordings of the digestive systems of the animals the regurgitation could be documented. The animals ingested microplastic fibers, whereby, after 3 h they were only found in the stomachs, but neither in the midgut gland nor in the gut. It could be estimated that with available flake food the number of fibers in the stomachs was higher compared to the stomachs of the animals that were fed with fibers only (without food). It could be concluded that flake food can act as a vector for P. varians to ingest microplastics. Furthermore, the food concentration was varied leading to a decreased fiber-mass-food-mass-ratio within higher food concentrations and therefore a decreased probability for microplastics to get adherent onto it. Therefore, animals ingested a lower number of microplastic fibers. The animals voided their stomachs from microplastic fibers on average after 7 h and from microplastic beads after 9 h. The process of egestion was followed by regurgitation and defaecation. The microplastic fibers and beads were mostly found at the matrix and rarely in the matrix of the faeces. The stomach residence of food with microplastics lasted for 11 h, while the residence time in the control group that was fed only with food was 8 h.