Polypodium was discovered in 1871 by M.M. Owsiannikov within the eggs of
sterlet (
Acipenser ruthenus) from the
Volga river in
Russia. In 1885, Ussov named the species
Polypodium hydriforme and gave a morphological description of the parasite.
Polypodium was long considered a unique intracellular parasite among cnidarians. Its hosts include 14 species of
Acipenser, two species of
Huso, the
American paddlefish, 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
sexual organs: "female" ones showing
ovaria and gonoducts, and "male" ones with simpler organization. "Female" gonads are supposedly abortive; the "male" ones ultimately produce binucleate cells and become
gametophores which infect host fish.
Polypodium hydriforme displays many peculiar characteristics, some of them shared with myxozoa. == Habitat ==