Forget everything you thought you knew about myxozoans. (We know it’s hard, but you can do it.) Scientists sequenced the microscopic organisms’ genome and discovered that they’re actually a type of teeny tiny jellyfish. The researchers published their results in the Proceedings of the National Academy of Sciences

Compared to other animals, the body of the jellyfish is as simple as it gets. It’s pretty much a bag with a single hole that serves as both mouth and anus. The bag is ringed with tentacles, and each tentacle is tipped with nematocysts, or stinging cells.

But compared to the myxozoans, most jellyfish are practically sophisticated. Myxozoa is the name of a group of more than 2000 different species of minuscule parasitic organisms that infect fish and other sea creatures. The myxozoans are incredibly primitive, made of just a few cells each. They don’t even have mouths. They do, however, have nematocysts—a defining characteristic of a jellyfish.

The little critters are “really weird,” ecologist Paulyn Cartwright said last week in a press release. When Cartwright and her colleagues at the University of Kansas sequenced the myxozoan genome, they found two surprises. The first was that the parasites are actually “micro jellyfish.”

"Because they're so weird,” Cartwright continued, “it's difficult to imagine they were jellyfish." And were may be the operative word: Cartwright and her colleagues believe that the myxozoans may have once been full-bodied cnidarians (the phylum that includes jellyfish), as opposed to the "highly reduced" ones they are now. But their body plans degenerated until there was nothing left but a few cells. Their bodies are so simple that for many years, scientists believed they were single-celled organisms.

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

The second surprise lay in the genome itself. The typical cnidarian genome has 300 million base pairs. The myxozoan genome has only 20 million. "These were 20 to 40 times smaller than average jellyfish genomes," Cartwright said. "It's one of the smallest animal genomes ever reported.”

The reclassification of the myxozoans into the cnidarian group challenges basic scientific ideas about the animal kingdom.

Certain genes, like the Hox gene, are considered crucial to animal existence. But the myxozoans don’t have Hox genes. They don’t have much of anything. They’re definitely cnidarians; their genomes, however dinky, confirm it. 

“But animals are usually defined as macroscopic multicellular organisms,” Cartwright said, “and this is not that. Myxozoa absolutely redefines what we think of as animal."