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

Light and transmission electron microscopy 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 nonlaminated wall that is surrounded by a thicker, multilayered zoosporangial wall. Zoospores were ellipsoidal to spherical, 6.1 +/- 0.6 Fun in diameter. Thraustochytrids have been included in the Kingdom 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 species from R. decussatus.