Background The large yellow croaker (Larimichthys crocea) is a commercially valuable fish in China, widely farmed in marine aquaculture. However, rapid growth in intensive farming has led to poorer water quality and higher stocking densities, causing frequent disease outbreaks and economic losses. Notably, ciliate protozoan infections, especially scuticociliatosis, pose a significant threat.

Objective This study aimed to isolate and identify the ciliate causing scuticociliatosis in large yellow croaker and to understand its pathogenic and biological traits, providing a basis for disease prevention and control.

Methods In April 2025, fish co-infected with trypanosomes and scuticociliates were collected from a net-cage farm in Ningde, China. The parasites were isolated, cultured, and identified via morphological and molecular analysis. A stable culture system was developed to study the effects of temperature, pH, and salinity on ciliate growth. Drug sensitivity tests were also performed to evaluate common aquaculture anti-parasitics.

Results Morphological and phylogenetic analyses confirmed the ciliate as Miamiensis avidus. Temperature tests showed high sensitivity: all cells died at 0°C within 12 hours, and population decreased at 30°C. Growth at 10°C was lower than at 20°C, with no significant difference at 25°C, indicating susceptibility to temperatures above 25 °C. Salinity tests revealed all cells died at salinity 0 within 12 hours, while growth at salinity 10 was lower than at salinity 30, with no significant difference at salinity 20, indicating broad salinity tolerance. The parasite could grow at pH 5.5 and pH 9.0, though less than at pH 7.4, with no significant difference at pH 6.5 and pH 8.5, showing strong pH adaptability. Drug sensitivity tests showed that carvacrol effectively eliminated M. avidus at a low dose (100 mg·kg⁻¹), indicating its potential as a low-toxicity treatment.

Conclusion The study confirms M. avidus as the cause of scuticociliatosis in sub-adult large yellow croaker, explores environmental factors affecting parasite growth, and identifies an effective low-dose treatment. These insights improve our understanding of M. avidus biology and support sustainable scuticociliatosis management in large yellow croaker aquaculture.