Emerging monogenean infections in farmed meagre Argyrosomus regius (Asso, 1801)

Introduction Monogenean outbreaks threaten the health of farmed fish stocks and severely affect aquaculture production. The meagre Argyrosomus regius is a promising species for diversification in Mediterranean fish farming: in the wild, this species is known to host a variety of ectoparasites, however reports of infection and pathology in farmed fish are rather infrequent. The present investigation reports the occurrence of an outbreak of monogenean infection in A. regius broodstock, with identification of the parasite species involved and associated histopathological findings. Methodology During a mortality outbreak registered in a meagre broodstock batch, parasites were isolated from gills and skin and preserved in 70% ethanol and 10% buffered formalin. Gill samples were also fixed in 10% buffered formalin for histology. The parasites were subjected to morphological analysis in light and scanning electron microscopy, and to molecular analysis amplifying and sequencing the complete ITS and partial 28S rDNA. Results and Conclusions Two monogenean species were identified, based on their morphology: the calceostomatid Ktariella polyorchis, reported for the first time in farmed A. regius, and the diplectanid Diplectanum sciaenae, already reported from A. regius broodstock in Spain. K. polyorchis were mainly found on the skin, mainly along the dorsal part of the body and tail, and in the oral cavity, while D. sciaenae were recovered from the gills. At histological level, the presence of D. scienae was associated to severe gill damage with diffuse epithelial hyperplasia and sloughing off in the lamellae, focal necrosis, inflammatory infiltration, hyperemia, telangiectasias and hemorrhages. Histological lesions were similar to those observed in previous reports and confirmed D. scienae as health-threatening parasite of meagre. Our findings suggest a role of both K. polyorchis and D. sciaenae as pathogenic parasites of A. regius farmed in the Mediterranean area, although at the moment they have been reported only from broodstock. In addition, we provide the first sequence data for these two monogenean species, as a further diagnostic tool for their correct taxonomical identification in possible future outbreaks.