A group of microscopic parasites that infect both vertebrate and invertebrate hosts have been found to actually be tiny jellyfish that have evolved a highly reduced form, reports Phys.org.

So-called Myxozoa were once believed to be unicellular organisms, but upon closer inspection they are multicellular and even possess nematocysts, the characteristic cells of all jellyfish that produce the sting. It's a dramatic discovery that could redefine exactly what it means to be an animal.

"This is a remarkable case of extreme degeneration of an animal body plan," said Paulyn Cartwright, principal investigator on the research project "First, we confirmed they're cnidarians. Now we need to investigate how they got to be that way."

Cartwright and colleagues at the University of Kansas sequenced the genome of Myxozoa, and found that they only have about 20 million base pairs. Compared to the average Cnidarian (the phylum that includes jellyfish, corals and sea anemones), which has more than 300 million base pairs, the Myxozoa genome is severely reduced. That's not surprising, since myxozoans have a stripped-down body plan that includes only a few cells, but it's remarkable to imagine how this simple creature evolved from a much more complex jellyfish.

Polypodium hydriformeThe study looked at the genomes of two distantly related myxozoan species, Kudoa iwatai and Myxobolus cerebralis, and another cnidarian parasite, Polypodium hydriforme. (Photo: Ekaterina V. Raikova /Wikimedia Commons)

"Because they're so weird, it's difficult to imagine they were jellyfish," said Cartwright, whose research work is mentioned this week in the Proceedings of the National Academy of Science. "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."

Myxozoans are infectious parasites that can wreak havoc on commercial fisheries, because they are known to infect fish and can devastate populations. In salmon, for instance, they cause whirling disease, which is a neurological infection that makes the fish swim in circles. The chart at bottom explains

The now-established evolutionary history of myxozoans places them firmly within Animalia, even though they behave and, in many ways, look more like single-celled Protozoa. It's a classification that certainly shakes up how we usually think of the taxonomy and evolutionary relationships of organisms.

The discovery reiterates that evolution does not always move from simple to more complex design over time. Rather, complex species sometimes evolve more simple designs when that's what natural selection dictates. Myxozoa is a particularly extreme example of this principle, though it's possible this discovery could prompt researchers to re-examine other organisms too.

"If it can happen once in evolution, it certainly can happen again," said Cartwright.