Macrostomum lignano is a flatworm known for his outstanding regeneration potential and stem cell system. A pluripotent stem cell type is responsible for development and renewal of all cell types of the animal. Our interest originates from applications of marine alkaloids and commercial dyes, which we want to use for investigations of pH regulation in the context of a possible future ocean acidification scenario. Primary experiments show the applicability of Macrostomum lignano as a model for application of fluorescence dyes. Not all marine animals are easy to use for such purposes because of efficient removal and detoxification mechanisms of potentially toxic secondary metabolites, and in this context especially dyes. Our preliminary experiments show promising data regarding acidic compartments of the animals and allowed for Ca++ signal measurements in intact and moving animals. Individuals can be cultivated at room temperature and adults are about 1,5mm of length. It is transparent and therefore an object of interest for optical investigations of many kind. As this organism is amenable to pharmacological and genetic (RNAi) interventions, it is an ideally suited marine organism that should allow us to decipher the reactions to a dramatically changing environment from the organismal up to the molecular level