Ultrastructural features of Cryptocaryon irritans, a ciliate parasite of marine fish

The parasitic, reproductive, and free living phases of Cryptocaryon irritans Brown 1951, a ciliate parasite of marine fish, were studied by means of transmission and scanning electron microscopy. The ciliature of this protozoan is arranged in 78-80 monokinetid meridians which run lengthwise converging at the oral cavity and at the posterior pole of the cell. In the trophont, a crown of pointed ciliar triplets fused at the tip delimits a small cytostome whose radially ridged walls lead to a shallow cytopharynx. The trophont feeds on whole host cells and tissue debris. An electron-dense, foam-like, PAS-positive substance fills the pellicular alveoli of the growing trophont. The mechanism of its formation is yet to be determined and several possible functions for it are hypothesized. The macronucleus in the young trophont consists of four linked bead-like segments twisted in a crescent-shaped alignment; up to five micronuclei are adjacently located. At this stage, the macronucleus is homeomeric. Along with trophont growth, the macronucleus increases in volume and its coarse network of chromatin expands. As the trophont leaves the host, development proceeds onto the protomont and tomont stages, during which a substantial reorganization occurs in the cell. The dense chromatin clumps apparently coalesce while the electron-lucent matter expands and the four macronuclear segments fuse into one thick, elongated strand which coils throughout the protoplasm. The micronuclei are no longer detectable in the protomont. The tomont then begins to undergo palintomic division, yielding scores of tomites. In the tomite, the macronuclear chromatin bundles are thin and abundant within the electron-lucent matrix. The micronuclei reappear. Following excystment, the emerging infective theront actively seeks out its host. At this stage its oral apparatus appears as a narrow slit surrounded by cilia shorter than the somatic ones, and is presumably not yet functional. The macronucleus is homeomeric again, has assumed its characteristic quadripartite shape with adjacent micronuclei.