Ultrastructure study of the intramolluscan stages of Meiogymnophallus minutus (Digenea: Gymnophallidae) in Scrobicularia plana (Bivalvia) from Portugal

The ultrastructure of the tegument of the sporocyst and contained cercariae of Meiogymnophallus minutus found in Scrobicularia plana collected in Ria de Aveiro estuary (Portugal) were studied by scanning (SEM) and transmission electron microscopy (TEM). TEM revealed microvilli extending from the surface of the sporocyst tegument that contributed to the lacetrimmed surface morphology observed by SEM. The cercarial surface was papilliform and covered densely with single-pointed tegumental spines. On TEM, besides the spines and several mitochondria, translucid and electron-dense spherical secretory vesicles were evident in the matrix of the tegument but were not seen on the sporocyst tegument. Uniciliate sensory receptors found in the cercaria of M. minutus were examined under SEM and TEM. Structural differences between the oral and the ventral sucker were observed. On the lip of the ventral sucker, large and strong spines were arranged in concentric rows, and two groups of six sensory papillae were placed concentrically and equidistantly. The oral sucker, surrounded by a crown of dense and long microvilli, exhibited a ring of uniciliate sensory papillae and some tegumental spines along the lip. The surface morphology of the M. minutus cercaria is compared with that of other Gymnophallidae cercariae.