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.

Light and transmission electron microscopy were used to study different stages of Perkinsus atlanticus (Apicomplexa) during induced zoosporulation, with fluid thioglycollate medium and seawater. Cytokinesis and nucleokinesis of different developmental stages were studied every 12 h during the incubation period of 72 h, at which time the zoospores became free. Uninucleated and flagellated zoospores present the apical complex formed by conoid, polar ring, micronemes, rhoptries and subpellicular microtubules observed at different sections. Ultrastructural details were compared with the other two species of the genus Perkinsus.

Faecal coliform (FC) levels in surface water and clams (Ruditapes decussatus) and variations in environmental parameters were studied in two bivalve harvesting areas in the Alvor lagoon (southern Portugal). Land use and cover characteristics in adjacent subwatersheds were also analysed to assess their contributions as sources of faecal contamination. High FC levels in clams from the harvesting area in the most urbanized subwatershed (impervious surface coverage approximately 10.5%) were positively associated with rainfall and with the cooler periods of the year. FC levels in clams from the least urbanized subwatershed were generally very low and did not present any detectable seasonal trend. From these results, it was concluded that the periodic deterioration of the microbiological quality of clams derives from the cumulative impact of the reservoir of faecal contamination created in urbanized areas, entering into the lagoon during storm water runoff.

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