Abstract: Cryptocaryon irritans theront stage development characteristics were examined by observation of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Tomont in each period collected from the diseased Pseudosciaena crocea, and the strains were maintained in experimentally infecting Pseudosciaena crocea from which the sample used for ultrastructural observation was collected. The result of observing different stages of theront off the tomont by electron microscope and light microscope showed: theront off the tomont after 0-4 h, ciliary body of the worm body was more dense and the cell structure was complete, swimming quickly in culture fluid; theront off the tomont after 8-16 h, Cryptocaryon irritans larvae significantly reduced the long diameter and the number of its body cilia, with thin cytoplasm, swimming slowly; theront off the tomont after 20-24 h, the larvae body cilia was few. And the larvae were nonmoving, sinking to the bottom. Meanwhile, the cell shapes of teardrop changed into the irregular and begain apoptosis, the cell size in a serious recession significantly, with long diameter of only 10 μm around. In this study, the morphological and structural characterization changes of the larvae in different periods are associated with the process of the loss of infectivity, reflecting the main reason that the influences on infectivity of Cryptocaryon irritans is energy attenuation.

Key words: Cryptocaryon irritans, contagious, artificial infection, ultrastructure