An ultrastructural study of taxonomically significant characters of the Thraustochytriales and the Labyrinthulales

Labyrinthulales and Thraustochytriales are marine, biflagellate protists of uncertain taxonomic affinities. Initial classifications of the Thraustochytriales in the Mycota and Labyrinthulales in the Mycetozoa, Rhizopodea and algae were based on their different superficial morphologies. These differences are not paralleled at the ultrastructural level. Both orders are characterized by a unique organelle termed the sagenogenetosome, an ectoplasmic net and a wall comprised of dictyosome-derived scales. These characters are considered of taxonomic importance and support close phylogeny of the orders to each other and their removal from the Mycota, Mycetozoa and Rhizopodea.