Fine structure of the apicomplexa oocyst of Nematopsis sp. of two marine bivalve mollusks

The fine structure of a n apicomplexan parasite oocyst found in gill tissues of 2 bivalve molluscs (Cerastoderme edule and Ruditapes decussatus) from Portugal is described.Monozoic oocysts, composed of a dense ellipsoidal wall with 0.35 ,um thickness containing a single vermiform uninucleated sporozoite, were 13.3 X 4.5 pm The apical zone of the wall contained a circular micropyle covered by a conical operculum.Externally the wall was surrounded by numerous thick and long microfibrils attached to the wall and connected to the host cell.The sporozoite occupied the internal oocyst lumen amidst fine granular material.Subsequently, the gradual disappearance of the attached microfibrils of the oocyst wall concomitantly with the degenerat~on and disintegration of the host cells was interpreted as a sign that thls apicomplexan species was pathogenic to these host specles.Lacking observation of different life cycle stages and other hosts.identif~cation of the species of t h ~s apicomplexan gregarine of genus Nematopsis was not possible.