Diagnose_species: Alveolate endoparasite in dinoflagellates. Motile cells with two unequal flagella arising from the anterior part of the cell. Sporangium formed inside the host and covered by small spicules (processes). Each sporangium produces numerous swarmers. There are two routes of host infection, either nuclear in thecate host or cytoplasmatic in athecate species. Strain specificity in target hosts, showing local adaptation to Mediterranean strains are as follows: type host: Alexandrium (strain specificity, not A. taylori), Karenia, Gymnodinium catenatum and nolleri, Protoceratium reticulatum (strain specificity). Akashiwo sanguinea and Scrippsiella trochoidea not significantly infected.