Sparicotyle chrysophrii infections were highlighted during a parasitological monitoring carried out on cultured Gilthead sea bream Sparus aurata Linnaeus 1758 from aquaculture sites in Corsica. Factors and biological mechanisms regulating parasites populations, particularly controlling the infestation and survival of monogeneans have been studied. Variations in infection support the hypotheses that the main factor determining the infection of S. chrysophrii was temperature and that the parasite life cycle could be influenced by fish size and fish age. One hundred forty-nine of 280 S. aurata studied were infected. Seasonal differences in prevalence of infection were observed in all farms studied. A similar pattern was observed for parasite abundance with maximal infection levels occurring when water temperature were lower. A statistically significant dependence between infection and host size was also observed. The study of the surface topography of S. chrysophrii with scanning electron microscopy highlighted morphological adaptations not previously described. Parasites have developed diverse mechanisms of survival within the host that facilitate the establishment of infection. We mainly studied sensory structures and the anchoring systems of the parasite, i.e., filaments of eggs and haptor, enabling it to ensure the dispersal and survival of the species.

The parasites of 536 European sea bass, Dicentrarchus labrax, were studied between January 2012 and December 2013 in six Corsican fish farms. The indicator value (IndVal) method, which combines measures of fidelity and specificity, has been used in this study. Because of its resilience to changes in abundance, IndVal is a particularly effective tool for ecological bioindicator. The IndVal method showed how season can influence the occurrence of parasite species in cultured sea bass and also identified parasites as bioindicators relative to fish farm location. The combination of specificity and fidelity highlighted several parasite species as significant indicators. A randomization test identified five parasite species as having a significant indicator value for season (the monogenean Diplectanum aequans; the copepods Lernanthropus kroyeri and Caligus minimus; the isopod Ceratothoa oestroides, and the myxosporidian Ceratomyxa labracis). If gills parasites are compared, they can be seen to be indicator species for two different seasons. The only Monogenea species D. aequans had fidelity and specificity more pronounced in winter, whereas both copepod species and the Isopoda revealed highest rates of infestation corresponding with an increase of water temperature. Four species have a significant indicator value for site location (D. aequans, L. kroyeri, C. minimus, and C. oestroides). The fact that the farm 6 was isolated on the east coast of Corsica may not have allowed the parasite to infect other farms. The presence of copepods on a single farm can also be explained according to salinity variations. Data for species composition and infection levels should help to improve the monitoring and management of parasitism in cultured sea bass populations.

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