Polypodium

Polypodium is a genus of 75–100 species of true ferns, widely distributed throughout the world, with the highest species diversity in the tropics. The name is derived from Ancient Greek poly (πολύ) "many" + podion (πόδιον) "little foot", on account of the foot-like appearance of the rhizome and its branches. They are commonly called polypody or rockcap fern, but for many species unique vernacular names exist.

They are terrestrial or epiphytic ferns, with a creeping, densely hairy or scaly rhizome bearing fronds at intervals along its length. The species differ in size and general appearance and in the character of the fronds, which are evergreen, persisting for 1-2 years, pinnate or pinnatifid (rarely simple entire), and from 10-80 cm or more long. The sori or groups of spore-cases (sporangia) are borne on the back of the frond; they are globose and naked, not covered with a membrane (indusium).

Polypodies have some use in herbalism, but are today most important in horticulture where several species, hybrids, and their cultivars like Polypodium 'Green Wave' are commonly used as ornamental plants for shady locations. Polypodium have a bitter-sweet taste and are among the rather few ferns that are used in cooking; in this case as a spice e.g. for nougat.

Polypodium hydriforme

Polypodium hydriforme is a species of parasite attacking the eggs of sturgeon and similar fishes (Acipenseridae and Polyodontidae). It is one of few metazoans living inside the cells of other animals.

Polypodium hydriforme is the only species in the genus Polypodium (monotypic genus). It is also the only species and genus within the whole family Polypodiidae.

Taxonomy

Unusual characteristics have led to much controversy regarding its phylogenetic position within metazoans.

Polypodium has traditionally been considered a cnidarian because it possesses nematocysts, the stinging structures characteristic of this phylum.

However, molecular phylogenetic studies using 18S rDNA sequence data have challenged this interpretation, and have indicated that Polypodium is a close relative to myxozoans and that together they share a closer affinity to bilaterians than cnidarians (Zrzavý & Hypša 2003). Due to the variable rates of 18S rDNA sequences, these results have been suggested to be an artifact of long branch attraction.