Gastrodermis ultrastructure of the different life stages of the polyopisthocotylean monogenean gill parasite Discocotyle sagittata

The polyopisthocotylean Discocotyle sagittata is a blood-feeding monogenean that infects the gill lamellae of rainbow trout, Oncorhynchus mykiss, and brown trout, Salmo trutta. The ultrastructure of their alimentary tract, at different stages of the life cycle, was previously unknown. Here, we show that the gastrodermis of the oncomiracidium, subadult, and adult D. sagittata follows the same structural organization as that of other blood-feeding polyopisthocotyleans, being composed of digestive cells alternating with a connecting syncytium. Digestive cells of the oncomiracidium are found in three developmental forms: undifferentiated, developing differentiated, and differentiated (presumably functioning) cells whereas those of adult and subadult are present in a single functioning state with variable size and content. The apical cytoplasm of adult digestive cells forms conical outgrowths, a feature which is absent in the oncomiracidium. The connecting syncytium of the oncomiracidium has no evidence of metabolic activity, while that of adult and subadult is metabolically active. The lamellae of the connecting syncytium of adults and subadults are more numerous and larger, and their terminal portions are expanded, compared with those of the oncomiracidium. Parallel, tubular, membranous structures are characteristic of the apical cytoplasm of the connecting syncytium of the oncomiracidium. Luminal lamella in the oncomiracidium, subadult, and adult form balloon-like structures enclosing some luminal contents, but those of the oncomiracidium are larger, bounded by nucleated cytoplasmic layer, and enclose more luminal contents. The possible functions of these structures and mechanism of digestion in both oncomiracidium and adult are discussed.