On August 12, Montana officials realized that the mountain whitefish of Yellowstone River were dying en masse. They sent corpses off for testing and got grave news in return: The fish had proliferative kidney disease—the work of a highly contagious parasite that kills between 20 and 100 percent of infected hosts. Tens of thousands of whitefish were already dead, and trout were starting to fall.

Humans can spread the parasite from one water source to another. So, on the morning of August 19, Montana Fish, Wildlife and Parks closed a 183-mile stretch of the Yellowstone River, banning all fishing, swimming, floating, and boating. “We recognize that this decision will have a significant impact on many people,” said FWP Director Jeff Hagener in a press release. However, we must act to protect this public resource for present and future generations.”

The press statement and all the subsequent news reports referred to the organism behind the fishes’ woes as a “microscopic parasite.” A few select outlets actually named the thing—Tetracapsuloides bryosalmonae. But none of them realized how extraordinary it really is.

It is part of a group called the myxozoans. They spend most of their lives as microscopic spores that are made of just a few cells. Despite their appearances, these creatures are animals. And although they are obscure, you have definitely heard of their closest relatives—jellyfish, corals, and sea anemones. Yellowstone River is now closed because more than half a billion years ago, a jellyfish-like animal started transforming into a parasite.

There are over 2,000 species of myxozoans (pronounced “MIX-oh-zoh-uns”), and all of them are microscopic parasites. Myxobolus cerebralis is a typical member, and the most well-studied one. It infects rainbow trout and other freshwater fish, causing an illness called whirling disease. The parasite attacks the spinal cartilage of young fish, leading to skeletal deformities and nerve damage. The youngsters often chase their tails or swim in corkscrews—hence the disease’s name. Unable to feed or escape from predators, up to 90 percent of them die.

The dead fish release large numbers of tiny seed-shaped spores, each comprising just six cells. These can stay dormant in the environment for decades. They reactivate once they are swallowed by the right host—the sludge worm, an aquatic relative of the more familiar earthworm. Once inside the worm’s gut, the spores deploy two small but sophisticated structures called polar capsules, which are like coiled harpoons. They launch outwards, attaching the spores to the worm’s gut and allowing them to burrow inwards.

One of the star-shaped spores.

As they multiply, they produce a different kind of spore, which looks like a jack or the head of a grappling hook. The worms poop these into the water, where they latch onto the skin of passing trout. Using those same polar capsules, the jack-shaped spores inject the fish with infectious amoeba-like cells that crawl through their bodies. When the cells reach spinal cartilage, they start reproducing, causing the symptoms of whirling disease. Eventually, they produce more of the seed-shaped spores, which leave the fish to start the whole complicated life cycle anew.

The two spore types—the seeds and jacks—are so different that for the longest time, scientists believed that they were entirely separate organisms belonging to distant branches of the myxozoan family tree. It was only in 1984 that Ken Wolf and Maria Markiw proved that they were just two halves of the same creature’s life cycle.

Classification problems have plagued the myxozoans since they were discovered in 1838. They’ve been mostly billed as protists—a grab bag group of small organisms that include amoebas and the parasites behind malaria. But some scientists have always seen them differently. They noted that protists all consist of a single cell; myxozoans comprise many. They also pointed to the polar capsule and its extendable harpoon; it looks uncannily like the stinging cells of cnidarians—the animal group that includes jellyfish and corals. Maybe they were cnidarians?

It was an unpopular view. Most biologists just couldn’t imagine how the tiny, simple, parasitic myxozoans could possibly have evolved from large, complex, free-living jellyfish-like ancestors. They don’t even have muscles, tissues, or organs. Beyond the polar capsules, they have almost nothing in common with cnidarians.

Nothing, that is, except their genes. When researchers like Mark Siddall started sequencing myxozoan genes in the 1990s, it became clear that these oddballs are probably animals, and likely cnidarians. Paulyn Cartwright confirmed that last year by comparing the full genomes of five myxozoan species. She showed that their closest relatives are indeed cnidarians—and specifically the jellyfish, box jellies, and their kin.

The myxozoans probably split away from these other cnidarians over 500 million years ago. During the intervening aeons, they have lost almost all the features of their former selves. As science writer Jennifer Frazer once wrote: “Once upon a time, a jellyfish became a parasite, and its descendants became unrecognizable.”

The details of that process are still unclear, but one living animal might shed some light on it. It’s called Polypodium hydriforme, and it’s the closest living relative of all myxozoans. It infects the eggs of sturgeons and paddlefish; as Cartwright says, it’s a caviar parasite. Within each egg, Polypodium develops into a bizarre colony of inside-out larvae—each has a ‘gut’ that sits within its ‘skin’, and several of them are connected in a single long sac. Once the eggs are released into the water, the larvae invert their bodies and break apart from each other.

In this free-swimming form, they look very jellyfish-like, with identifiable tentacles, mouths, and guts. Perhaps the ancestors of myxozoans went through a similar phase in their evolutionary history, when they were already devoted parasites, but still kept some obvious traces of their cnidarian heritage.

As they evolved further down the parasitic path, they lost these ancestral physical features. They did away with many genes too. “They have the smallest known animal genomes,” says Cartwright, “and they lack some of the genes that we consider hallmarks of animal development.” For example, the all-important Hox genes, which direct the construction of animal bodies and which I wrote about last week, are simply missing in myxozoans.

However, these parasites have kept the genes that cnidarians use to build their stinging cells; they now use those to build the polar capsules instead. “They’ve adapted the stings for attaching to their hosts,” says Cartwrght. “But everything else that makes you a proper multicellular animal was lost.”

Who would have thought that jellyfish, of all creatures, would have evolved into a dedicated parasite? And yet, these animals are hardly alone. An estimated 40 percent of animal species are parasites, and a recent study estimated that parasitism has evolved among animals on at least 223 separate occasions. Wasps and flies have done it. Worms and flatworms have done it. Fish and barnacles have done it. Even jellyfish—or, at least, jellyfish-ish creatures—have done it.

It’s easy to look down upon these creatures, with their minimalist bodies and pared-down genomes. But they are highly successful. There are lots of them, and they clearly excel at exploiting more complex hosts. Myxozoans mostly attack fish, but some species have been known to infect turtles, amphibians, and even shrews. None of them infect humans, but they cause us problems nonetheless by threatening aquaculture and recreational fishing.

And there’s so much we still don’t know about them. For example, Tetracapsuloides—the species that just closed Yellowstone River—is an oddball, even for a myxozoan. Unlike most of its kin, its intermediate hosts aren’t worms but bryozoans—a group of aquatic, filter-feeding animals. And it has an even more complicated life cycle than its peers. Along with the usual spores, it can also form a sac-like stage that contains hundreds of parasitic cells.

One of its relatives, called Buddenbrockia, is stranger still. It goes through a stage where it looks very much like a worm—a long, sinuous, muscular animal. But unlike actual worms, this one has no mouth, gut, or brain; no left or right; no top or bottom; no head or tail. It does, however, have polar capsules—it’s a jellyfish that turned into a worm.

These things are much more than simple microscopic parasites. And yet, in the Yellowstone crisis, Cartwright notes that “no one has mentioned that the parasites are actually relatives of jellyfish!” She adds, “We have to change our way of thinking. We have to think of them within the evolutionary framework of where they came from. I suspect we’ll understand a lot more about their basic biology then.”