In rivers across the country, freshwater mussels are dying. Not all rivers, and not all mussels, but the U.S. Fish and Wildlife Service is alarmed by rapid population declines in some locations, and its scientists are trying to figure out why it’s happening.

Mussels are unique creatures that feed by filtering water while nestled in the river bottom. They are closely connected to the whole ecosystem, including unique relationships with fish to assist in reproduction. There are 41 species of mussels in the St. Croix River, one of the highest numbers of anywhere in the world.

The St. Croix has not been affected by the die-offs (yet), but it’s still part of the research. Federal biologists are trying to see what might be causing the die-offs by also seeking to better understand healthy populations, like those found in this river.

The Clinch River in West Virginia and Tennessee is one of the worst cases. Also known for having high mussel diversity, biologists have observed repeated “mass-mortality events” since 2016. The river bed has been blanketed in the white shells of dead pheasantshell mussels.

“The smell will knock you off your feet,” USFWS biologist Jordan Richard told The Guardian. “You see what was a healthy looking river, but now there’s just dead bodies scattered everywhere.”

Mussels in other rivers across the United States have also been decimated. The closest problem to the St. Croix so far is the Embarrass River in northeastern Wisconsin. That’s where DNR biologists saw “unusually high mortality” of the snuffbox mussel, a federally-endangered species also found in the St. Croix.

“These events are very alarming,” said biologist Eric Leis of the USFWS’s La Cross laboratory. “In several of the sites, the populations have gone from being strong to rapidly declining to nearly undetectable (in some cases) within a few years.”

Connecting the clues

One clue about why the mussels are dying is quickly evident: it is only specific species of mussels that are dying, while other kinds continue to thrive. That means it probably isn’t pollution, which would likely affect all mussel species. It points to a pathogen, a bacteria or virus that was attacking select species.

But very little is known about mussels, including what microbes naturally occur in the aquatic creatures. It’s thought that some bacteria are beneficial, helping the mussels convert nutrients into calcium, which the mussels use to build their shells. Other bacteria can actually strengthen mussel immune systems.

Without knowing what a healthy mussel hosts, it’s hard to say what might be killing the Clinch’s pheasantshells, or other species in other rivers.

So while USFWS researchers worked to identify what bacteria and viruses could be connected to the die-offs, they also examined mussels from unaffected locations, including the St. Croix River. While pheasantshell mussels don’t live in the St. Croix, the scientists looked at a cousin, the mucket. Sampling for healthy mussels also took place elsewhere in Wisconsin, and throughout the Mississippi River’s tributaries.

Just detecting the microbes in mussels required innovative science. The researchers pioneered new methods for sampling a fluid called “hemolymph,” which helps a mussel’s immune system and other biological functions, to discover what microscopic organisms share the mussel’s shell.

With samples from St. Croix River mussels and from other rivers in the upper Mississippi River system to compare to the Clinch River, some hints are beginning to emerge. The scientists caution that the work is ongoing and the results uncertain.

One unique species of bacteria was found in two different mussel populations while a die-off was happening, but the scientists don’t think that bacteria is actually harmful to the creatures. Instead, its presence may point to some other factor in the environment. There is an intriguing and disturbing connection: in another study, it was found to be connected to oil-contaminated soil. Because of how mussels live in the bottom sediments of rivers, they are particularly sensitive to toxins that can accumulate there.

“It seems possible, at this point, that it may be an indicator of something in the environment (possibly some sort of hydrocarbon) during the mortality events,” Leis said.

Urgent investigation

But the complex connections between water, mussels, sediment, and everything else in a river requires further study before coming to any conclusions.

The cause of the die-offs could also be that the mussels are normally able to fight off a virus, but some other environmental change is stressing and weakening them. Leis points out that all the die-offs have happened in the fall, indicating there could be a connection to falling water temperatures. Such a connection is common in viruses that infect fish, he says.

In December, the research team published an article in a peer-reviewed journal about the work so far, documenting their techniques and naming the microbes they believe should be studied further.

The USFWS has developed a network with the University of Wisconsin and scientists and agencies around the country to collect samples from dead mussels and send them to the La Cross laboratory for analysis. They will soon be testing samples from die-offs in Oregon, Washington, Wisconsin, and Michigan.

The scientists are also focused on developing methods to identify healthy mussels that can later be used to repopulate areas. They need to be able to quickly and harmlessly determine if a specimen is carrying a harmful microbe, or if it can safely be transported to another location.

“When you consider the ecosystem services (filtering pathogens and impurities out of the water) provided by mussels there is much cause for concern,” Leis said. “Whatever is happening is alarming and we need to do what we can to help affected populations and protect those that are unaffected.”

Anyone who sees that appears to be sick, dying, or dead mussels is asked to inform the USFWS La Crosse Fish Health Center at 608-783-8444.

More information

• Holden, Emily. “‘The Smell Will Knock You off Your Feet’: Mass Mussel Die-Offs Baffle Scientists.” News, October 14, 2019. https://www.theguardian.com/environment/2019/oct/14/the-smell-will-knock-you-off-your-feet-mass-mussel-die-offs-baffle-scientists.
• Eric Leis, Sara Erickson, Diane Waller, Jordan Richard, Tony Goldberg “A Comparison of Bacteria Cultured from Unionid Mussel Hemolymph between Stable Populations in the Upper Mississippi River Basin and Populations Affected by a Mortality Event in the Clinch River,” Freshwater Mollusk Biology and Conservation, 22(2), 70-80, (19 December 2019)
• Leis, Eric, Diane L. Waller, Susan Knowles, Tony Goldberg, Joel G. Putnam, Jordan Richard, Sarah Erickson, Emilie Blevins, and Jesse Weinzinger. “Building a Response Network to Investigate Potential Pathogens Associated with Unionid Mortality Events.” Ellipsaria 20, no. 4 (2018): 44–45.