Mucosal immunology in fish

Mucosal immunology is the study of the immune responses that occur at mucosal surfaces, such as the gut, skin, gills, and nose. These tissues in fish are closely associated with the external environment. The mucosal immune system must provide protective immunity against pathogens but remaining tolerant to nonharmful environmental substances. In the last two decades, mucosal immunology has attracted increasing attention in medicine and veterinary sciences, not only for its critical role in explaining some essential aspects of how immune homeostasis is maintained, but also for its application in developing vaccines against the many infectious diseases that enter the bodies of fish through mucosal membranes. Regarding mucosal immunity, fish have one advantage: the skin mucus, which forms a continuous layer surrounding the fish body and can be collected using noninvasive methods. At present, the analysis of skin mucus can provide information about the fish (for example, about well-being or immune status) and also about the aquatic environment (e.g. presence of pollutants or stressors) in which the fish are living. The current knowledge about mucosal immunity has inspired numerous efforts to identify new biomarkers in fish mucus and, therefore, have contributed to reducing the number of specimens sacrificed for research purposes. In the present chapter, we will briefly review features of fish mucosal immunology and their roles in wound repair, healing, infection, vaccination, exposure to contaminants, and stress, while providing examples of mucosal immunity improvement by dietary administration of immunostimulants, mainly in Mediterranean species such as gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax). Furthermore, a comparison of the available methodologies to collect and preserve fish mucus and some examples of analysis with mucus samples as resources and tools for fish welfare evaluation will be reviewed and discussed.