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Polypodium hydriforme is a species of Cnidaria that parasite in the eggs of sturgeon and similar fishes (Acipenseridae and Polyodontidae). It is one of few animals that lives inside the cells of other animals. P. hydriforme is the Monotypic taxon in the class Polypodiozoa.
Polypodium has traditionally been considered a because it possesses , the stinging structures characteristic of cnidarians. Molecular phylogenetic studies using 18S rDNA sequence data temporarily challenged this interpretation, by finding that Polypodium is a close relative to and suggesting that together they share a closer affinity to than cnidarians.Zrzavý J. & Hypša V. (2003). "Myxozoa, Polypodium, and the origin of the Bilateria: The phylogenetic position of "Endocnidozoa" in light of the rediscovery of Buddenbrockia". Cladistics 19(2): 164–169. Due to the variable rates of 18S rDNA sequences, these results were suggested to be an artifact of long branch attraction, and myxozoans have also been classified within cnidarians.
Evans et al. (2008) performed phylogenetic analyses of metazoans with 18S and partial 28S rDNA sequences in a large dataset that includes Polypodium and a comprehensive sampling of cnidarian Taxon. This supports the placement of Polypodium within Cnidaria. This accords with its traditional classification, in particular with the fact that Polypodium possesses and a cnidarian-like body plan. are currently classified as cnidarians as well.
Polypodium hydriforme is an endocellular parasite with an unusual life cycle, a peculiar morphology, and high rates of DNA evolution. Polypodium spends most of its life inside the of acipenseriform fishes. In infected oocytes, Polypodium develops from a binucleate cell into an inside-out planula larva and then into an elongate inside-out stolon; the epidermal cell layer is located internal to the body and the gastrodermis is located externally. The embryo, larva and stolon are surrounded by a protective polyploid cell, which also functions in digestion. Just prior to host spawning, Polypodium everts to the normal position of cell layers, revealing tentacles scattered along the stolon. During eversion, the yolk of the host oocyte fills the gastral cavities of the parasite, supplying the future free-living stage with nutrients. The parasitic phase of its life cycle usually takes several years. Finally, upon emerging from the host egg in fresh water, the free-living stolon fragments into individual medusoid-like organisms that go on to multiply by means of longitudinal fission. In summer they form endodermal Sex organ: "female" ones showing Ovary and gonoducts, and "male" ones with simpler organization. "Female" gonads are supposedly abortive; the "male" ones ultimately produce binucleate cells and become Gametophore which infect host fish. Polypodium hydriforme displays many peculiar characteristics, some of them shared with myxozoa.
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