Scanning electron microscopic observation of the in vitro cultured protozoan,Perkinsus olseni, isolated from the Manila clam,Ruditapes philippinarum

Background Perkinsosis is a major disease affecting the commercially important marine molluskRuditapes philippinarum(Manila clam) in Asian waters. In this study, we investigated the morphological characteristics ofPerkinsus olseni, the causative agent of perkinsosis, cultured under laboratory conditions at different stages of its life cycle using a scanning electron microscope (SEM). Results The prezoosporangia formed after induction with Ray's fluid thioglycollate medium (RFTM) developed into zoosporangia. During this process, a discharge tube formed a porous sponge-like structure that detached before the zoospores were released; thus, this organelle operated as a bung. Liberated zoospores gradually transformed into immature trophozoites, during which detachment of the anterior flagella occurred, but the loss of the posterior flagella was not clearly observed in the present study. Mature trophozoites underwent schizogony by cleaving the cell forming some merozoites in schizonts, which were released by the rupturing of the cellular membrane of the schizont within a few days. Conclusions Our morphological and ultrastructural studies contribute new information on the life cycle and propagation ofP. olseni.