In the last decades the production of plastic increased continuously. Simultaneously, environmental pollution by plastic became a rising issue. Marine litter can have adverse effects on animals. Some species may get trapped in lost fishing nets or they may starve to death upon ingestion of plastic which may clog their digestive tracts. Degradation of plastic items generates a continuously increasing number of smaller-sized particles. Microplastic, finally ranging in the µm-size classes can have adverse effects on marine invertebrates upon ingestion. Most of these effects can be attributed to the cellular level. How can particles in the microscale harm organisms? In this study the ingestion of microplastic by marine invertebrates and, moreover, the possible transfer into cells of the digestive tract will be examined. As model species we chose the Atlantic ditch shrimp (Palaemon varians). This species inhabits coastal regions, estuaries, and brackish water systems which are most affected by anthropogenic pollution. Effects will be determined in the cells of the midgut gland of P. varians. Measuring the formation of reactive oxygen species (ROS) is a suitable method to detect cellular stress. Quantification of ROS-formation will be done by confocal laser scanning microscopy and the aid of the fluorogenic substrates Dihydroethidium (DHE) and 2’, 7’ - Dichlorodihydrofluorescin diacetate (DCFDA). The results will help to identify cellular reactions after exposure to microparticles and indicate the toxicological impact on cells and whole organisms.