| Abstract: | ABSTRACT. The development of Ichthyophthirius multifiliis trophonts in gill epithelium of channel catfish was studied on the first five days post-exposure (PE) by light and electron microscopy. Trophonts increased in average diameter from 48 μm at one day PE to 248 μm at five days PE. Although theronts invaded any part of the gill epithelium, at three days PE most parasites were found adjacent to major blood vessels, particularly the afferent vessel. From one to three days PE, 70–90% of trophonts were immediately adjacent to gill epithelium, but at four and five days PE only 50% of trophonts were closely apposed by host epithelial cells. Notable ultrastructural changes in the trophonts over the five-day period occurred in the mucocysts and lipid inclusions, both of which increased markedly in number. The few crystalline mucocysts present at one day PE were near the cell periphery but rarely attached to the plasma membrane. At three days PE, crystalline mucocysts were significantly more abundant, and at five days PE, they occurred in large numbers throughout the cytoplasm as well as at the periphery. At three days PE, secretory mucocysts were first observed. Contractile vacuoles were more prominent at five days PE than earlier in development. Development of mucocysts and lipid reserves is probably essential to survival and reproduction of the ciliate after it leaves the host. |
