An Animal Living Backwards
Deep within the river systems of the Volgograd Oblast, a bizarre life cycle unfolds inside the eggs of sturgeon. The culprit is Polypodium hydriforme, one of the few multicellular animals known to live inside the cells of another animal. For years, which can be a significant portion of the host's life, this parasite develops within the sturgeon's growing oocytes (egg cells). Its initial infectious stage is a tiny, binucleate cell that measures just 15-30 micrometers in diameter.
The most peculiar aspect of its parasitic life is its inverted body plan. Most animals in the phylum Cnidaria, which includes jellyfish and corals, have two main cell layers: an outer ectoderm for protection and an inner endoderm for digestion. Polypodium develops with these layers reversed. Its digestive endoderm is on the outside, directly absorbing the yolk of the host egg, while its ectoderm, complete with developing tentacles, faces inward. The entire growing parasite—which develops from a larva into a long, folded structure called a stolon—is further enclosed in a protective cell that aids in digesting the host's cellular material. This "inside-out" arrangement is an adaptation to its intracellular existence.