Myxozoans are microscopic parasites that wreak havoc on commercial fish stocks by causing neurological problems: Salmon and trout develop what’s called whirling disease and start swimming in circles. Researchers studying the genomes of these common sea pests reveal that myxozoans are actually parasitic “micro jellyfish,” complete with stinging tentacles. They’re highly reduced, stripped-down cnidarians, the phylum that includes jellyfish, corals, and sea anemones, according to findings published in Proceedings of the National Academy of Sciences this week.
There are over 2,180 species of myxozoans, all obligate endoparasites and typically just a few cells measuring 10 to 20 microns in width. Back in the 1880s, they were thought to be single-celled organisms (like protists), and not animals. It wasn’t until their DNA was sequenced did researchers realize that they were in fact animals, just drastically simplified ones. "Animals are usually defined as macroscopic multicellular organisms, and this is not that," Paulyn Cartwright from the University of Kansas says in a statement. "Myxozoa absolutely redefines what we think of as animal."
Intriguingly, their cells contain a complex little structure that helps facilitate attachment to the host. It’s called a polar capsule, and it bears a striking resemblance to nematocysts, the stinging cells of jellyfish tentacles. To better understand the dramatic evolutionary transition from a free-living cnidarian to a microscopic endoparasite, Cartwright and colleagues analyzed sequences from two distantly related myxozoan species, Kudoa iwatai and Myxobolus cerebralis. Then they compared these to the genome of a less reduced cnidarian parasite called Polypodium hydriforme.
The team confirmed that myxozoans are, in fact, cnidarians and a sister group to Polypodium. "This is a remarkable case of extreme degeneration of an animal body plan," Cartwright adds. And not only have these micro jellyfish evolved a stripped-down body plan of just a few cells, but myxozoans also have one of the smallest animal genomes ever reported: about 20 million base pairs only. The average cnidarian has over 300 million.
Despite the extreme paring down, myxozoans have retained a key jellyfish characteristic – nematocysts and the genes needed to make them. Still, they lack many of the traits we used to think are vital for animal development. They’re a spectacular example of how obligate parasitism can lead to a massive reduction of body plan, genome size, and gene content.