A study published Monday in the Proceedings of the National Academy of Sciences takes a crack at the mystery. According to their genomic analysis, this group of parasites represents a degeneration in body and genome size. In other words, these guys aren't some primitive remnant of what jellyfish used to be -- they actually made themselves smaller and more simplistic over time.
Why would an animal become less like an animal? For a parasite, it's actually a really good strategy: The more you can rely on your host for energy and reproduction, the more your species can flourish. It's not a glamorous life, but I doubt a microscopic jellyfish loses much sleep over it.
"Because they're so weird, it's difficult to imagine they were jellyfish," lead study author Paulyn Cartwright, associate professor of ecology and evolutionary biology at Kansas University, said in a statement. "They don't have a mouth or a gut. They have just a few cells. But then they have this complex structure that looks just like stinging cell of cnidarian. Jellyfish tentacles are loaded with them -- little firing weapons."
It's hard to even call myxozoans animals -- though we must, if they're truly members of cnidarian.
"Hox genes are one example, which are important to development of all animals, and these lack them," Cartwright said in a statement. "But Myxozoa is definitely an animal because its evolutionary origin is shared with jellyfish, and we use species' ancestry to define them. But animals are usually defined as macroscopic multicellular organisms, and this is not that. Myxozoa absolutely redefines what we think of as animal."
If you think "devolving" from a jellyfish-like critter to a microscopic parasite is extreme, just think about what happened with viruses: Some scientists now believe that viruses came from cellular life that became parasitic, then got more and more simplistic as it relied on its hosts over time. You can hear a great episode of RadioLab about that embedded above -- starting at around the 26 minute mark.
It could be that these brilliant evolutionary degenerations are more common in nature's history than we'd expect.