Fine structural features of the free‐living stages of Amyloodinium ocellatum (Dinoflagellata, Thoracosphaeraceae): A marine fish ectoparasite

Amyloodinium ocellatum is a protozoan parasite that causes amyloodiniosis in marine and brackish water fish, threatening global aquaculture. The present study investigates the morphology and ultrastructure of the free‐living stages of A. ocellatum (tomont and dinospore) using light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Dinospores measured 13.03–19.66 μm in length, 12.32–18.71 μm in width, and were laterally flattened. Dinospores had a transverse flagellum for propulsion and a longitudinal flagellum for direction control. The cyst wall had three distinct layers and included cellulose. The outer wall was coated with numerous bacteria. The orange‐red speckled eyespot was observed all tomont developmental stages and in the dinospore of A. ocellatum. Tomonts proliferation required successive nuclear division, the formation of new cyst walls, and cytoplasmic segregation. The cytoplasm comprises mainly the matrix, organelles, and inclusions. The matrix was grainy and evenly distributed. In addition to organelles, including mitochondria with tubular cristae, Golgi apparatus, and endoplasmic reticulum, the cytoplasm had starch grains and lipid droplets as inclusions. The A. ocellatum cells lacked chloroplasts. This study provides the first ultrastructural view of the cytoplasmic structure of the free‐living stages of A. ocellatum.