Ultrastructure of Myxidium trachinorum sp nov from the gallbladder of the lesser weever fish Echiichthys vipera

Myxidium trachinorum sp, nov, is described from the gallbladder of the less er weever fish. Echiichthys vipera. Pseudoplasmodia attach themselves to th e gallbladder epithelium by filose processes, which are inserted between ho st cells. Pseudoplasmodia undergo endogenous cell formation at the secondar y and tertiary levels. In the proliferative cycle, primary and endogenous c ells are packed with digestive vacuoles formed by phagocytosis. In the spor ogonic cycle the pseudoplasmodium becomes a pericyte enclosing two secondar y cells (lacking digestive vacuoles) in a vacuole. These give rise to five cells each - two valvogenic, two capsulogenic and a binucleate sporoplasm, which mature into spores. Comparison of the disporic M. trachinorum with po lysporic species of Myxidium revealed significant differences in plasmodial ultrastructure, especially their attachments to host cells, surface charac teristics and mode of nutrition, and in formation of generative cells. Thes e suggest that the genus Myxidium may require revision.