SOME ULTRASTRUCTURAL OBSERVATIONS OF A THRAUSTOCHYTRID (PROTOCTISTA, LABYRINTHULOMYCOTA) FROM THE CLAM RUDITAPES DECUSSATUS (MOLLUSCA, BIVALVIA)

Light and transmission electron microscopy techniques were used to stu dy different stages of a thraustochytrid, a protist found on the bival ve molluscan Ruditapes decussatus. Vegetative cells developed on-the g ill epithelium at the base of the cilia. The cells of the protist divi de by binary fission giving rise to zoosporangia, each containing 4 un inucleate biflagellated zoospores with a bilateral array of tubular ma stigonemes on one of the flagella. Each zoospore develops within a thi n nonlaminated wall that is surrounded by a thicker, multilayered zoos porangial wall. Zoospores were ellipsoidal to spherical, 6.1 +/- 0.6 F un in diameter. Thraustochytrids have been included in the Kingdom Pro toctista and placed in a separate phylum Labyrinthulomycota (= Labyrin thomorpha). Based on the ultrastructural morphology of different devel opmental stages and absence of sagenogens and ectoplasmic nets, the pr esent microorganism should be included in thraustochytrids. A comparis on of ultrastructural observation of the different species in this gro up provides insufficient data for identifying the species from R. decu ssatus.