Ultrastructural characterization of Amyloodinium ocellatum (Dinoflagellata): Insights into trophont morphology and parasitic mode.

Amyloodinium ocellatum is a parasitic dinoflagellate that infects marine fish in tropical and subtropical regions, causing significant economic losses in aquaculture. This study used light microscopy (LM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron tomography (ET) to comprehensively characterize the morphology, ultrastructure, and specialized attachment apparatus of A. ocellatum trophonts. The trophonts primarily parasitized the gill filaments of infected fish, reaching 89.8 ± 10.3 μm after 48 h of infection. Under LM, trophonts appeared as dark, spherical or pear-shaped structures that remained stationary throughout the parasitic process. SEM revealed surface folds and persistent transverse grooves, while TEM identified starch granules, lipid droplets, accumulation bodies, and microtubules. Trophonts rapidly formed rhizoids post-infection, which branched into root-hair-like projections penetrating the host epithelium. These projections functioned both as anchors and nutrient absorbers. During parasitism, trophonts induced degeneration and necrosis of host cells, leading to sloughing of epithelial cells and wound formation. ET further demonstrated that trophonts possessed numerous root-hair-like projections that deeply infiltrated host cells. This study provides the first systematic description of the morphology and ultrastructure of A. ocellatum trophonts and sheds new light on the fine structural features of the parasite's attachment apparatus.