|Two specimens of free-living Polypodium|
|Polypodium hydriforme |
Polypodium hydriforme is a species of a 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. It is also the only known intracellular cnidarian parasite.
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 shown that Polypodium is a close relative to myxozoans and 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.
Evans et al. (2008) have 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 taxa. This support the placement of Polypodium within Cnidaria. This accords with the fact that Polypodium possesses nematocysts and a cnidarian-like body plan. The ML combined analysis places Polypodium as its own class Polypodiozoa. By contrast, the combined parsimony analysis and the ML analyses of 28S alone place Polypodium within the hydrozoan clade Leptothecata.
Description and life cycle
Habitat of Polypodium hydriforme is freshwater.
Although the fresh water habitat of Polypodium is unusual for cnidarians, it is not unheard of, especially within hydrozoans. For instance, the model organism Hydra and the jellyfish are both exclusively fresh-water hydrozoans. Hydra and Craspedacusta are distantly related and they are not closely related to Polypodium. Thus, it appears that in the evolution of cnidarians, invasion to fresh-water habitats has happened at least three separate times.
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