Predation of Cyclopoid Copepods on the Theronts of Ichthyophthirius multifiliis: Shedding Light on Biocontrol of White Spot Disease

White spot disease, caused by the parasitic ciliate Ichthyophthirius multifiliis, is a significant threat to the freshwater fish farming industry worldwide, resulting in massive mortality and economic losses. Eliminating the free-swimming theronts from the culture environment is considered crucial for the control of I. multifiliis infection. It is well-documented that planktonic ciliates are valuable food resources for macro-zooplankton in aquatic ecosystems. In this study, we developed a fluorescence labeling method for alive theronts and found that cyclopoid copepods Thermocyclops taihokuensis, Mesocyclops spp., Macrocyclops sp., and Paracyclopina sp. present predation on the theronts in co-culture experiments. Laboratory challenge tests further confirmed that the presence of zooplankton in the culture water body significantly reduced the infection of I. multifiliis in goldfish (p < 0.01). Results from this study revealed that cyclopoid copepods have the potential to be used as biological control agents against white spot disease in aquaculture.