Abstract
Abstract. Microgemma hepaticus gen.nov. sp.nov. is described from the liver of juvenile grey mullet, Chelon labrosus (Risso). Development occurs within xenomas (diameter 500μm) which have microvillar surfaces, encircling bands of mitochondria and a reticulate hypertrophic nucleus. Vegetative developmental stages, meronts, are plasmodial and divided by plasmotomy. These stages are enclosed by host membranes. Sporogonic stages are free in the cytoplasm and divide by multiple exogenous budding. Uninucleate spores (2·4μm × 4·2μm) possess 7–9 coils of the polar filament and a lamellar polaroplast. Xenomas are associated with liver connective tissue, and cause necrosis of adjacent liver cells in certain circumstances. Host response to infection involves leucocyte infiltration and granuloma formation, with spores being destroyed by repeated macrophage phagocytosis and necrosis and resulting in gradual resolution of the lesion. Although juveniles apparently tolerate large parasite burdens there is some evidence of a contribution by the parasite to stress‐related mortality. The transmission of the disease and its potentially high pathogenicity to larval fishes is discussed.
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