From the Article:

Researchers have discovered a new parasite in freshwater mussels after investigating hundreds of mussels that died along the Embarrass River in Shawano County six years ago.

The findings were recently published in the international, peer-reviewed journal Parasitologia. Researchers with the U.S. Fish and Wildlife Service, the U.S. Geological Survey and Wisconsin Department of Natural Resources were among agencies that collaborated to study the mass die-off in 2018. Biologists observed sick mussels lying on their sides on the surface of the river with open shells when they’re normally buried in the river bottom.

Researchers examined 29 mussels from six different species within the river. They then cut them in half and examined the organs of some samples to see if the mussels were healthy or diseased.