, a Novel Scuticociliate Pathogen Isolated and Identified from Cultured Large Yellow Croaker ().

Scuticociliates are recognized as the causative agents of mass mortalities in certain cultured marine fishes, resulting in enormous economic losses. This study aimed to investigate a fatal infection caused by scuticociliates in farmed large yellow croaker () in Fujian province, China. Microscopic examinations of focal organs, including the brain, eyes, gills, and skin, revealed the presence of parasites. Active masses of scuticociliates were observed in these organs, and the ciliates were subsequently isolated and maintained in vitro. An immersion challenge experiment revealed that experienced cumulative mortalities reaching 73% within 7 d upon exposure to 1.0 × 10 ciliates mL. Based on the microscopic and PCR testing of infected fishes, the brain was comprehensively inferred as the main infection organ for the isolated strain. Microscopic and submicroscopic observations of the isolated scuticociliate, coupled with cortical ciliature patterns revealed through α-tubulin indirect immunofluorescence techniques, identified these scuticociliates as . The sequencing of two genetic markers (small subunit ribosomal RNA, SSU rRNA and cytochrome c oxidase subunit I, COI) further confirmed that the isolated strains exhibited the highest sequence similarity to most sequences in GenBank. However, significant differences in SSU sequences compared to the strain Ma/2, and the lack of published COI and ITS (internal transcribed spacer) sequences for Ma/2, indicate the need for further molecular data to resolve whether there are potential cryptic species within the complex.