Ultrastructural Studies on the Tegument of the Metacercariae of Meiogymnophallus minutus and Meiogymnophallus fossarum (Digenea: Gymnophallidae) in Cerastoderma edule (Bivalvia) from Portugal

Unencysted metacercariae of Meiogymnophallus minutus (Cobbold, 1859) and the sibling species Meiogymnophallus fossarum (Bartoli, 1965) infect the edible cockle, Cerastoderma edule (L.), with free metacercariae occurring simultaneously in the same host specimens in Portugal. Observations on the tegument of both species show that changes in the morphology of the body surface occurs in the intermediate host. Scanning and transmission electron microscopy revealed an alveoli-like tegument with palmate, digitated spines protruding from each individual alveolus. Uniciliated papillate receptors, which may or not may be recessed, were abundant, surrounding the suckers as well as being distributed on the body surface. Transmission electron microscopy revealed a significant number of 3 types of densely packed secretory vesicles, thin cysternae, and mitochondria in the surface tegument of the body, oral cavity, and esophagus. No distinguishing differences were observed in the surface ultrastructure of M. minutus and M. fossarum metacercariae.