Some ultrastructural observations of a thraustochytrid (Protoctista, Labyrinthulomycota) from the clam Ruditapes decussatus

Light and transm~ssion electron lnicroscopy techniques were used to study different stages of a thraustochytrid, a protist found on the bivalve molluscan Ruditapes decussatus. Vegetative cells developed on the gill epithelium at the base of the cilia. The cells of the protist divide by binary fission giving rise to zoosporangia, each containing 4 uninucleate biflagellated zoospores with a bilateral array of tubular mastigonemes on one of the flagella. Each zoospore develops within a thin nonlamlnated wall that is surrounded by a thicker, multilayered zoosporangial wall. Zoospores were ellipsoidal to spherical, 6.1 * 0.6 pm In diameter. Thraustochytrids have been lncluded in the Kngdom Protoctista and placed in a separate phylum Labyrinthulomycota (= Labyrinthomorpha). Based on the ultrastructural morphology of different developmental stages and absence of sagenogens and ectoplasmic nets, the present microorganism should be included in thraustochytrids. A comparison of ultrastructural observation of the different species in this group provides insufficient data for identifying the specles from R. decussatus.