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.
A Brief and Mysterious Freedom
Just before the sturgeon host is ready to spawn, a rapid transformation occurs. The parasite everts, turning itself right-side-out. During this process, it engulfs the remaining egg yolk, which fills its newly formed digestive cavity and provides nutrients for its next life stage. When the sturgeon releases its eggs into the freshwater, the now-emerged Polypodium stolon is freed.
Once in the water, the stolon fragments into multiple, individual medusoid-like organisms. Each one is about 1-2 millimeters tall and resembles a tiny jellyfish or hydra. These free-swimming creatures possess stinging cells called nematocysts, a hallmark of cnidarians that were absent in the parasitic stage. They move about, and some observations suggest they can catch and eat tiny worms and rotifers. They develop sexual organs, but their complete reproductive cycle remains partly mysterious. The "male" gonads produce infectious binucleate cells, but the "female" gonads appear to be abortive. How the next generation of sturgeon becomes infected is not fully understood, completing the life of this inside-out animal.