A team of US researchers has solved a six-year-old mystery surrounding a mass die-off of mussels in Wisconsin's Embarrass River, discovering a previously unknown parasitic fungus that infects female mussels.

The sick mussels were observed lying on their sides on the surface of the river with open shells, whereas typically, they are buried in the riverbed.

The research team, comprising scientists from the US Fish and Wildlife Service (USFWS) and the Wisconsin Department of Natural Resources, examined 29 mussels from six species within the river, reported Wisconsin Public Radio (WPR). They then cut them in half and examined the organs of some samples to see if the mussels were healthy or diseased.

The samples showed a new parasitic fungus among females of three species: mucket, fat mucket and plain pocketbook mussels. The team named the parasitic fungus Hirsutonosema embarrassi.

Although the parasite is not believed to have caused the mass die-off, its presence raises concerns about the potential impact on mussel reproduction and survival, especially given the declining populations of many freshwater mussel species.

Eric Leis, a fish biologist at La Crosse Fish Health Center in Onalaska, Wisconsin, told WPR that while it's unlikely the parasite caused the mass die-off, the parasitic fungus could impact reproduction and mussel survival.

He said this is significant because two-thirds of North America's mussel species are threatened or vulnerable. Like other shellfish, mussels are crucial in maintaining clean water and healthy ecosystems as filter feeders.

Leis noted that parasitic infections are typically tied to hosts with immune systems that are compromised or unable to fight off disease.

Jordan Richard, a biologist with the USFWS endangered species program, told the news outlet that multiple factors contribute to vulnerability. He paralleled this to the COVID-19 pandemic, which has claimed over 7 million lives globally since 2020. Despite the high death toll, he highlighted that the majority survive, with preexisting conditions playing a significant role in susceptibility to the virus.

The discovery of the new parasite is expected to allow researchers to screen for it in future restoration efforts, helping ensure that only healthy mussels are reintroduced into declining populations.