Black Spot Disease Related to a Trematode Ectoparasite Causes Oxidative Stress in Xyrichtys Novacula

Xyricthys novacula is very popular fish species in the Balearic Islands and a main target for recreational fishing. Over the last years, spots were observed on many individuals, which seems to be progressively more common. The aim of the present study was to identify the parasite, determine its abundance in a marine protected area (MPA) and in a non-protected one with more anthropic influence, and to evaluate the antioxidant and immune responses to this parasite presence by studying biomarkers in both liver and epithelial mucus. Analysis of genetic sequences established with 98.6-98.4% certainty that the parasite is a digenean fluke Scaphanocephalus sp. An average abundance of 12.3 +/- 11.3 and 1.3 +/- 1.3 parasites per individual was found for the non-protected area and the MPA, respectively. The activities of lysozyme and superoxide dismutase and total immunoglobulin concentration were significantly higher in mucus of X. novacula with more parasites. Similarly, in liver, a higher parasite load is related to higher activities of catalase, glutathione peroxidase and glutathione S-transferase, whereas malondialdehyde remained similar. In conclusion, Scaphanocephalus sp. affects X. novacula, inducing an immune and antioxidant response in epithelial mucus and in liver. The potential influence of the environment on parasite transmission, prevalence and abundance require further research to determine whether it makes fish more susceptible to infections.