In order to investigate the common fish diseases in indoor culture of large yellow croaker (L.crocea), this study reports the occurrence of scuticociliatosis, a parasitic disease, on the surface of L.crocea in a laboratory circulating water recirculation system. The histopathological analysis of skin, liver and muscle of L.crocea was carried out to explore the pathogenic mechanism of scuticociliatosis. At the same time, in order to further identify the body surface parasitic Scuticociliatida ciliates of L.crocea, the pathogen was isolated, and the morphological characteristics of Scuticociliatida ciliates were analyzed by in vivo formaldehyde fixed microscopic observation and scanning electron microscope observation. The results revealed the presence of whitening symptoms on the head, skin, and fin base of L.crocea. Gill cover congestion and severe cases of white massive skin ulceration were observed, along with the infestation of a significant number of Scuticociliatida ciliates. Notably, no apparent pathological alterations were detected in the body tissue. Histopathological analysis showed that the skin and subcutaneous tissue were the main sites of infection with Scuticociliatida ciliates. Tissue necrosis, cytolysis and increase of inflammatory cells occurred in most areas of muscle tissue. Myofibril was obviously broken and muscle fiber space was enlarged, while the boundary between liver tissue cells was not obvious, the central vein was oppressed, mononuclear cells increased and parenchyma cells enlarged. It was observed that the body of the pathogen was mainly slightly pointed at the top, blunt and round at the back end of the body, and contained an obvious telescopic vesicle as well as a long flagellum. The pathogen was identified as Pseudocohnilembus persalinus by morphology. Taken together, the histopathological results showed that the Scuticociliatida ciliates could infect the body surface tissue of L.crocea, resulting in loss of protection and resistance. At the same time, this study can provide a certain reference basis for indoor healthy culture of L.crocea.
