Intraspecific Variation in the Characteristics of <i>Cryptocaryon irritans</i> Isolated in Japan

Cryptocaryon irritans is an obligate parasitic ciliate and the causative agent of cryptocaryoniasis. This parasite is widely distributed in tropical and subtropical marine waters, and numerous outbreaks in aquaculture have been reported worldwide. Previous studies have documented variations in biological traits, such as cell size, serotype and environmental conditions for infection, including salinity and temperature, suggesting considerable intraspecific diversity. Molecular and serological variations have been reported in Japan; however, the extent of biological diversity remains unclear. Understanding this variation is critical because it may influence experimental outcomes and control strategies. In this study, C. irritans was collected from three geographically proximate coastal sites in Japan, and clonal cultures were established. Biological characteristics-including cell size, serotype, pathogenicity and development and infectivity under different salinity and temperature conditions-were compared to assess intraspecific variation. Genetic diversity was also evaluated using phylogenetic analysis. Biological characteristics differed even among isolates from nearby sites, indicating the presence of biologically distinct populations within Japan. Moreover, phenotypic comparisons and phylogenetic analyses showed that distinct traits were observed not only between genetically distant isolates but also within the same group, underscoring high intraspecific diversity. These findings may contribute to guiding future research and developing effective control strategies for cryptocaryoniasis.