A phylum of single-celled fungi that live as parasites inside minute aquatic organisms such as diatoms and water moulds. Because of the microscopic and obscure nature of cryptomycotes, their diversity and widespread distribution have emerged only by comparing DNA samples from aquatic microbes with those of known fungal groups and finding the presence of a characteristic fungal ‘signature’. Like the related microsporidia, members of the Cryptomycota lack cell walls during their intracellular feeding stage and have flagellated spores, but they can produce resistant spores with chitin-containing walls.