Polypodium hydriforme is a species of parasitic jelly monster in the Cnidaria phylum. It's the only species in the Polypodium genus, which is the only genus in the Polypodiidae family. There's nothing quite like it!
It isn't in fact the only parasite in Cnidaria, but it is one of the only multicellular parasites which lives INSIDE cells.
Which cells? What creature is raised to lofty heights by means of such a unique and unexpected form of suffering?
Alas! It's caviar!
Polypodium hydriforme parasitises the expensive eggs of Sturgeons and Paddlefish in the northern hemisphere. They do this while the eggs are still within the adult fish, though it's not clear how exactly they get in there.
So let's start with a larval Polypodium which has somehow managed to get itself into a fish egg that's still in the fish.
The larva is inside out. We know that members of Cnidaria are made up of just two layers of cells, an endoderm on the inside which does the digesting and an ectoderm on the outside which has the stinging cells. Polypodium is the other way round. Their inner, digesting cell layer is on the outside, facing all that yummy egg yolk.
Being Cnidaria, that one larva soon develops into a stolon, which is a whole line of individuals. These are like polyps but are still inside out, feeding on egg yolk on the outside and with tentacles and stinging cells on the inside.
Stolon, recently emerged from an egg
They escape when the eggs are laid, revealing themselves to be rather strange looking, freshwater polyps. They are a bit like Hydra, but with almost no stalk.
"Polypod" means many feet, which I guess refers to the four special tentacles they use for walking around and attaching to things.
It takes a little while for them to develop a mouth so in the meantime they subsist on that last gutful of yolk. Later on they can use their tentacles and nematocytes to catch tiny worms, flatworms and rotifers.
As you may or may not suspect, these adults can reproduce by fission, where they simply divide into two individuals. They also develop sex organs for yet more reproduction.
Again, it's not known how exactly their offspring infect fish eggs before they're even laid. I hope we find out soon, I can't help but think it involves something really weird!