ULTRASTRUCTURAL DATA ON THE LIFE-CYCLE OF THE PARASITE, PERKINSUS-ATLANTICUS (APICOMPLEXA), ON THE CLAM, RUDITAPES-PHILIPPINARUM, IN THE MEDITERRANEAN

An Apicomplexan Perkinsus species has been found parasitizing the clam Ruditapes philippinarum (= Tapes semidecussatus) collected on the Med iterranean coast in the region of the Ebro Delta (Tarragona, Spain). L ight and transmission electron microscopy were used to study different stages of this parasite during zoosporulation induced by incubation i n thioglycollate medium and seawater. During incubation the trophozoit es began zoosporulation, which originated pre-zoosporangia and zoospor angia at different developmental stages. Successive cytokinesis and nu cleokinesis gave rise to pre-zoospores, which became elongate and diff erentiated in biflagellated zoospores. The latter presented large mito chondria and an apical complex formed by a conoid, polar ring, microne mes, rhophtries and subpellicular microtubules. The zoosporangium wall showed some typical lamosomes and a discharge tube developed in early phases of incubation. Ultrastructural data were compared with the onl y four species of the genus Perkinsus previously described. The morpho logical data, the host and the geographic proximity suggest that the s pecies located on the Mediterranean coast was Perkinsus atlanticus.