Exploring the impact of high salinity and parasite infection on antioxidant and immune systems in Coris julis in the Pityusic Islands (Spain).

Climate change associated with human activities alters marine ecosystems and causes imbalances and abrupt changes in sea conditions. Scarce freshwater resources for human consumption often prompt the construction of desalination plants, which discharge significant amounts of brine into the sea, potentially elevating salinity levels. Furthermore, global trade together with higher temperature and pollution can facilitate the spread of parasites. The aim of this study was to assess the potential effects of salinity, an abiotic stressor, and Scaphanocephalus sp. parasitic infection responsible for black spot disease, a biotic stressor, on Coris julis, a common fish in the Balearic Islands (Spain). Fish were sampled from an area affected by a desalination plant, one with a high rate of parasite infection and a control area, and biomarkers were analysed in the liver, gills and epithelial mucosa. Both salinity and the parasite induced increases in catalase (CAT) and glutathione s-transferase activities in the liver, while superoxide dismutase (SOD) did not show significant changes. The effects of salinity were evident to a greater extent in the gills with an increase in the activity of all enzymes, as well as in the production of reactive species. The effects of the parasite were mainly observed in the mucus with significant increases in CAT and SOD activities. Regarding immune response markers in the mucus, both stressors induced an increase in lysozyme and alkaline phosphatase activities, and in the case of the parasite, also an increase in immunoglobulins. Malondialdehyde, as an indicator of oxidative damage, remained unchanged. In conclusion, both abiotic and abiotic stress induce a stress situation in C. julis that responds by activating its antioxidant and immune defence mechanisms but does cause oxidative damage. The differential tissue response to different stressors highlights the value of analysing multiple tissues to detect early indicators of diverse impacts on marine fauna.