The secondary zoospore of Aphanomyces astaci and A. laevis (Oomycetes, Saprolegniales)

The ultrastructure of the secondary zoospores of Aphanomyces astaci and A. laevis was compared. The general appearance of the organelles and their compartmental–ization is the same, but some subtle differences were found. A. laevis has a less distinct distribution of fuzzy vesicles around the border of the water expulsion apparatus than A. astaci. The content of the U–body of the A. astaci zoospore is paracrystalline but the A. laevis U–body has a tubular content. In the peripheral cytoplasm of both species are seen vesicles with a granular content. Flattened cisternae are found in a narrow zone just below the plasmalemma. These two structures are confined to the zoospore stage of the fungi. The lipid–microbody complex of A. laevis cysts is not present in the zoospore. The ultrastructural organization of uniflagellate and bi–flagellate zoospores is compared.