The parasite has devastated the whitefish population and is now threatening the trout.
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.