Polypodium (animal)

Polypodium is a monotypic genus of strange parasitic animals; the only genus in family Polypodiidae. Polypodium hydriforme is a parasite of the eggs of sturgeon and similar fishes (Acipenseridae and Polyodontidae). It is one of few metazoans living inside the cells of other animals.

The taxonomic position of Polypodium is unclear. It was formerly classified as Narcomedusae, but is now treated as its own class Polypodiozoa, most often placed in the cnidarians. Based on 18S rRNA DNA sequence analysis, a relationship with Myxozoa has been proposed (Zrzavý & Hypša 2003), but this is controversial. If correct, P. hydriforme could be united with the Myxozoa in a clade Endocnidozoa.