Mechanism of infection by Labyrinthuloides haliotidis (Protozoa: Labyrinthomorpha), a parasite of abalone ( Haliotis kamtschatkana) (Mollusca: Gastropoda)

Biflagellated zoospores of the thraustochytrid Labyrinthuloides haliotidis (Protozoa: Labyrinthomorpha), a pathogenic parasite of juvenile abalone ( Haliotis kamtschatkana), lost their flagella on contact with the abalone host. Within 4 hr, and prior to the complete development of the ectoplasmic net, sagenogenetosomes were produced and extracellular lytic activity disrupted the plasmalemma layer of host epithelial cells adjacent to the parasite. The damaged epithelial cells were lysed and the parasites entered into the resulting lesion. The ectoplasmic nets extending from the sagenogenetosomes were well developed within 24 hr and may have released lytic agents since net elements were observed deep within the cytoplasm of host cells. Nevertheless, the ectoplasmic net enabled the parasite to move into and within the head and foot tissues of the abalone. As demonstrated by their ultrastructural morphology, the motile zoospores were transformed to vegetative cells within 4 hr and binary fission was observed in the resulting vegetative cells within 24 hr.