Salinity regulates the formation and hatching of Cryptocaryon irritans tomonts, affecting infectivity to Larimichthys crocea

Abstract

Cryptocaryon irritans are the main pathogens of “white spot disease” on the body surface of marine teleosts, but the seawater environment affects pathogen development and virulence. This study investigated C. irritans tomont formation, cell division, hatching, and infectivity to Larimichthys crocea under eight salinity gradients (S0, S5, S10, S20, S30, S40, S45, and S50) to understand how C. irritans grows, develops and harms host fish in a variable natural environment. The results showed that the protomont hatching rate in S5, S10, S20 and S30 significantly increased over time, but the highest and lowest values appeared in S20 and S30, respectively; and the hatching rate in S20 is significantly higher than S30.The rate of tomont formation increased over treatment time, and the highest and lowest rates were observed in S20 and S0 groups, respectively. Seawater at extreme salinity (S0, S40, S45, and S50) significantly reduced the encystment duration to <5 min. In addition, the tomont division rate at S5, S10, S20, S30, S40 and S45 significantly increased over time, and the highest and the lowest values were recorded in S20 and S45, respectively; and the division rate at S20 is significantly higher than S45. The hatching rate in S5, S10, S20, S30, and S40 also significantly increased over time, but the highest and lowest values appeared in S20 and S5, respectively; and the hatching rate in S20 is significantly higher than S5. The tomonts treated with seawater of 5–45‰ salinity divided evidently, but most tomonts under 0 and 50‰ salinity dissolved. TEM observation showed the pellicular alveoli of tomonts in S40 and S45 were wider than S20, the chromatin was loose, and the cyst wall was separately compressed into thin-layered layers. Finally, the aspect ratios of the theronts hatched at S5, S10, S20, S30, and S40 significantly increased than those at S20, and the theronts became long and thin. The infectivity of the theronts hatched from tomonts under S5 and S40 salinity treatments were significantly lower than the control (S20) because of the changes of theront number and morphology. In conclusion, salinity changes regulate the formation and development of C. irritans tomonts. Low (S5) or high (S40) salinity inhibits the infectivity of incubated theronts.