Using genome sequencing, researchers have discovered that parasites called myxozoans are actually microscopic jellyfish. These common parasites, which have only a few cells, live inside both vertebrate and invertebrate animals. Myxozoans are a diverse group of over 2,100 different parasites whose name means “mucus animals.”
The DNA analysis indicates that myxozoans belong to the phylum of cnidarians, which includes over 10,000 species like jellyfish, sea anemones and corals. Myxozoans’ body structure is like a tiny, stripped-down jellyfish without a stomach or mouth. But they share one major similarity with jellyfish: its stinger. The same cellular structure that gives jellyfish their sting is found in the microscopic parasite.
"This is a remarkable case of extreme degeneration of an animal body plan," said Paulyn Cartwright, a researcher on the project and an evolutionary biologist at the University of Kansas. "First, we confirmed they're cnidarians. Now we need to investigate how they got to be that way."
In a statement, Cartwright said that the parasite also has a much smaller set of genes than jellyfish. "These were 20 to 40 times smaller than average jellyfish genomes," she said. "It's one of the smallest animal genomes ever reported. It only has about 20 million base pairs, whereas the average Cnidarian has over 300 million.”
The findings will likely have importance for biological classifications, as well as fighting the parasite itself. Myxozoa parasites don’t always kill their hosts, but can cause serious problems for important fish stocks, like salmon and trout. Myxozoans can cause “whirling disease” in salmon, a neurological problem that causes the fish to swim in circles.
Researchers say this finding may even expand how we define what an animal is. Previously, myxozoans were grouped with single-celled organisms, which aren’t considered animals. But if this microscopic organism evolved from more complex jellyfish down to something with just a few cells, they may call into question the boundary of that definition.
The findings were published this in the Proceedings of the National Academy of Sciences.