EPITHELIOCYSTIS IN SILVER PERCH, BIDYANUS-BIDYANUS (MITCHELL)

Cultured juvenile silver perch, Bidyanus bidyanus (Mitchell), were aff ected by epitheliocystis, with a prevalence of up to 75%. The conditio n affected 2-96% of gill filaments. Granular basophilic cysts ranged f rom 10 to 87 mu m and had a round-to-oval shape. The infected cells we re positioned centrally within a lamella, often close to its tip, and their identity was difficult to determine, but the involvement of pill ar cells could not be precluded. The size of pathogen was within the r ange previously reported. The pathogen had two morphological forms, wh ich were positioned differently within the cyst. The form located more in the centre was rod-shaped with an electron-dense core and electron -lucent vesicles on the sides of the core. Its mean length was 498.14 nm (SD = 47.68 nm), and its mean width was 145.14 nm (SD = 9.53 nm). T he form present in a more peripheral position had an irregular shape, often did not show the electron-dense core and did not have the electr on-lucent vesicles. Its mean length was 704 nm (SD = 170.29 nm) and it s mean width was 152.4 nm (SD = 16.40 nm). Both forms were enclosed by a double trilaminar membrane. This is the first confirmed report of e pitheliocystis in Australian freshwater fish.