Polypodium is the genus of cnidarians that parasitizes in a eggs of sturgeon as well as similar fishes Acipenseridae in addition to Polyodontidae. this is the one of the few metazoans animals that live inside the cells of other animals.
Polypodium hydriforme Ussov is the only species of this monotypic genus. The parent family Polypodiidae, order Polypodiidea and class Polypodiozoa are also monotypic.
The habitat of Polypodium hydriforme is freshwater.
Although freshwater is an unusual habitat for cnidarians, it is for not unheard of, especially within hydrozoans. For instance, the advantage example organism Hydra and the jellyfish Craspedacusta sowerbii are both exclusively freshwater hydrozoans. Hydra and Craspedacusta sowerbii are distantly related and are not closely related to Polypodium. Also, the obligate parasite Myxobolus cerebralis lives in freshwater. Thus, it appears that in the evolution of cnidarians, invasion of freshwater habitats has happened at least three separate times.