pH Regulates the Formation and Hatching of Tomonts, Which Affects Cryptocaryoniasis Occurrence in Aquaculture.

are the main pathogens of white spot disease in marine teleost. However, the occurrence of cryptocaryoniasis is influenced by several abiotic factors including the pH. To explore the effect of pH on the life cycle of (encystment, cleavage, and hatchability), protomonts and tomonts of were incubated in seawater of 10 different pH levels (2-11). pH 8 was used as the control. The change in morphology and infectivity of theronts that hatched from tomonts against were then recorded. We found that pH 6-9 had no significant effect on the encystment, cleavage, and hatching of the parasites. However, pH beyond this limit decreased the cleavage and hatching of the tomonts. Furthermore, extreme pH decreased the number of theronts hatched by each tomont and the pathogenicity of the theronts, but increased the aspect ratio of the theronts. Infectivity experiments further revealed that extreme pH significantly decreased the infectivity of against . In conclusion, the can survive in pH of 5 to 10, but pH 6-9 is the optimal range for the reproduction and infectivity of . However, extreme pH negatively affects these aspects. is a ciliate parasite that causes "white spot disease" in marine teleosts. The disease outbreak is influenced by hosts and a range of abiotic factors, such as temperature, salinity, and pH. Studies have shown that change in pH of seawater affects the structure (diversity and abundance of marine organisms) of marine ecosystem. However, how pH affects the life cycle and survival of , and how future ocean acidification will affect the occurrence of cryptocaryoniasis, are not well understood. In this study, we explored the effect of pH on the formation and hatching of tomonts. The findings of this study provide the foundation of the environmental adaptation of , the occurrence of cryptocaryoniasis, and better management of marine fish culture.