The fine structure of sporogony in Myxidium zealandicum (Protozoa: Myxosporidia)

Studies of the ultrastructure of the trophozoite (plasmodium) and the process of sporogony and polar capsule development of Myxidium zealandicum from gill and kidney of the North American eel Anguilla rostrata are described. Although the basic steps and characteristics of sporogony are similar to those previously reported for other myxosporidians, we report three new and significant observations in this study: (1) the identification of a uninucleate germ cell for the binucleate sporocyst; (2) the asynchronous maturation of polar capsules within a single sporocyst; and (3) the existence of a mitochondrial layer at the periphery of the Plasmodium that may have a transport function. We also observed cylindrical and spherical structures, either of which might be the primordium of the polar capsule, but we were unable to resolve this problem. These observations are discussed with reference to previous descriptions of myxosporidians.