Light and electron microscopy of the pharynx and gastrodermis of the monogenean gill parasite Macrogyrodactylus clarii from the catfish Clarias gariepinus (Burchell, 1822)

The functional morphology of the digestive system in monogeneans is important in understanding feeding behaviour, dietary intake, and metabolic activity of the caecal epithelium. The present study used light and transmission electron microscopy to reveal detailed structure of the pharynx and gastrodermis of the viviparous gill monogenean Macrogyrodactylus clarii to compare with the congeneric skin monogenean Macogyrodactylus congolensis and other gyrodactylids. The basic components of the pharynx and gastrodermis of M. clarii are similar to M. congolensis. The pharynx comprises two regions: an anterior highly muscular region and a posterior glandular syncytium with 6 protrusible papillae. The syncytial epithelium lining the mouth and pharyngeal cavity is a modified layer with its own cell bodies, and not an extension of the general body tegument. Eversion of the pharynx has not been observed, but we postulate on the mechanism by which the pharynx and associated muscular structures function during feeding.TEM observations confirmed the similarity between the digestive system of M. clarii and M. congolensis with notable exceptions: the absence of melanin pigments and microorganism-like structures, presence of unique gastrodermis outgrowths, fibrotic vacuoles and small electron-dense secretory bodies and finally formation of deep intestinal crypts with numerous parallel intestinal lamellae in M. clarii. We discuss the possible roles of the luminal lamellar loops, gastrodermis outgrowths, deep intestinal crypts, lipid-like droplets, fibrotic vacuoles and different types of vesicles and vacuoles present in the gastrodermis cytoplasm.