Parasites are weird and often unsettling: Some are good at mind-control, some alter their hosts for their own benefit, some can even have positive side effects. But a new parasite discovery recently upset evolutionary biologists’ understanding of life itself. Or at least, how they classify life.

 

 

A group of parasites that scientists formerly thought were protists—a huge category of microorganisms—are actually members of Cnidaria, the phylum that includes jellyfish and coral. Somewhere along the evolutionary line, the recently re-classified parasites, myxozoans, left behind all forms of mouths, guts or ability to survive outside of a host, according to a press release from the University of Kansas. 

During this evolutionary clense, they also pared down most of their genes. The average myxozoan genome is about 20 to 40 times smaller than that of an average jellyfish, Paulyn Cartwright explains in the release. Cartwright is an evolutionary biologist and an author of a new paper that reclassifies the creatures, recently published in Proceedings of the National Academy of Sciences.

"Because they’re so weird, it’s difficult to imagine they were jellyfish," she says in the release. But they did retain one key feature: Myxozoans still have a complex structure that looks like the stinging cells of jellyfish, called a nematocyst, that Cartwright calls "little firing weapons." 

 

The creatures are so small (only 10 to 20 microns) that scientists originally thought they were single-celled organisms, but at their largest life stage they do consist of a handful of cells.

But before you panic about potential dangers, their hosts are fish. Also, not all parasites kill their hosts. Most myxozoans live in harmony with fish—only needing them to pass the infection on to the next generation.

Yet a few species do cause disease that can cause trouble for aquaculturists hoping to raise fish for human food. "Whirling disease" is one myxozoan infection that can cause neurological problems and skeletal deformation in young fish that forces them to swim in a corkscrew-like pattern. Other myxozoans can create cysts in muscle tissue that makes it unattractive or soft and watery.

 

The researchers aren’t sure what caused the parasites to change so drastically from their jellyfish-like ancestors, but they are interested in finding out more. "Myxozoa absolutely redefines what we think of as animal," says Cartwright in the press release. She also suspects that it might not be the only microscopic animal out there. "If it can happen once in evolution, it certainly can happen again."