Fine structural observations of the natural spore excystment of Minchinia Sp. (Haplosporida).

The ultrastructure of the sequential phases of the spore excystment and the earliest plasmodial development of the haplosporidian Minchinia sp., pathogenic to the Portuguese clam Ruditapes decussatus (Mollusca, Bivalvia), were observed. Before spore excystment the endosporoplasm became lighter and its typical structures were rendered more visible. Like the endosporoplasm, the liberated amoebula and the uninucleate plasmodia evidenced a similar morphology: spherulosome, haplosporosomes, lipid droplets and mitochondria. During this phase a complex network of microfilaments and microtubules was visible at the periphery of the plasmalemma. Just before the nuclear division of the uninucleate amoebula, the spherulosome membranes seemed to fuse with the amoebula plasmalemma. The spherulosome and haplosporosomes disappear after the binucleate plasmodial stages.