Characterization and potential pathogenicity of Ceratomyxa koreana n. sp. (Myxozoa: Ceratomyxidae) in cultured olive flounder (Paralichthys olivaceus).

Myxosporean parasites have been understudied in Korea, with only a few species described in marine environments. This study aimed to identify and characterize a new Ceratomyxa species associated with emaciation in olive flounder (Paralichthys olivaceus) farmed in Jeju Island, Korea. Morphological analysis revealed that the isolate had spores with an average length of 10.8 ± 1.0 μm and thickness of 41.9 ± 7.4 μm, characterized by two subspherical polar capsules (4.0 μm). The specimens morphologically resembled C. protopsettae Fujita, 1923, but were differentiated from it by their smaller spore thickness and smaller subspherical polar capsules. This parasite showed a 100% infection rate in emaciated fish, and histopathological findings revealed severe gallbladder distension, cholestasis, and liver vacuolization. Tissue tropism analysis using real-time polymerase chain reaction (PCR) revealed that the parasite burden was highest in the gallbladder, with systemic dissemination to the liver, kidney, and muscle. Furthermore, a highly specific and sensitive real-time PCR assay without cross-reactivity with other myxosporean parasites was developed. These results suggest that this new Ceratomyxa koreana n. sp. may be a potential causative agent of emaciation in olive flounder, and the developed molecular biological tool provides an important foundation for early diagnosis and disease management in aquaculture.