Ultrastructure of vegetative and sporulation stages of Haplosporidium ascidiarum from the ascidian Ciona intestinalis L.

Specimens of the ascidian Ciona intestinalis L. collected in the Bay of Nap les showed the occurrence of an eukaryotic microparasite characterized by s porogonial plasmodia with internal sporulation stages. The microorganism wa s identified as Haplosporidium ascidiarum and was located in the host's sto mach lumen, Transmission electron microscopy examinations were performed on specimens of C. intestinalis collected during spring and summer, Mono- and binucleated cells characterized by enlarged mitochondria and by the occurr ence of reticles and vesicular organelles were frequently observed. Plasmod ia of H. ascidiarum appeared abundant and measured 6-10 x 8-14 pm. They cle aved internally producing 6-8 asynchronous sporoblasts, The anterior-poster ior cell arrangement was observed at the beginning of the sporulation phase within the plasmodia. The sporoblasts and immature spores showed a complex sequence of spore wall formation, characterized by the sequential assembla ge of fibrous components producing the spore wall. The spherular complex an d lid opercule formed prior to the spore wall completion. Organelles and el ectron dense bodies were also observed in the cytoplasm of early vegetative stages or in the maturing spores, frequently clustered in the anterior reg ion. The sporogonial plasmodia were the most frequent stages observed in th e host.