First report and characterization of Ceratomyxa platichthytis from cultured starry flounder, Platichthys stellatus.

This study reports Ceratomyxa platichthytis infection in cultured starry flounder, Platichthys stellatus, from Pohang, Korea, representing the first record of this species in an aquaculture system and the first molecular characterization since its original description. Fish showing abnormal gallbladder morphology and reduced condition were examined using parasitological, bacteriological, virological, molecular, and histopathological analyses. No major bacterial or viral pathogens, or other myxozoans including Enteromyxum leei and Parvicapsula anisocaudata, were detected. Mature spores were slightly crescent-shaped, with two subspherical polar capsules, and their morphometric characteristics were consistent with the original description of C. platichthytis. The SSU rDNA sequence obtained in this study was 1813 bp in length, and phylogenetic analysis showed that C. platichthytis formed a genetically distinct lineage within the genus Ceratomyxa. Histopathological examination revealed infection intensity-associated alterations in the gallbladder, including trophozoite attachment to the epithelial surface, mucosal fold distortion, reduced epithelial cell height, microvilli deformation, and inflammatory cell infiltration in the lamina propria. Moderately infected individuals also showed reduced cytoplasmic lipid content in hepatocytes, suggesting a possible association between infection and host nutritional condition. In addition, a species-specific PCR assay developed in this study successfully detected C. platichthytis without cross-amplification with other myxozoans commonly reported in Korea. These findings provide the first molecular and histopathological characterization of C. platichthytis from cultured starry flounder and offer a diagnostic basis for monitoring this parasite in aquaculture systems.