Research team aims to discover why freshwater mussels are disappearing

by Sydney Cromwell

Through her snorkel mask, Erin Singer McCombs was sure she was seeing a small fish. Then, she looked again and realized it wasn’t a fish at all.

It was a mussel, using a “lure” to attract the fish it needed to complete its life cycle.

“I was actually tricked,” said McCombs, the southeast conservation director for the nonprofit American Rivers.

That experience is one of the reasons that McCombs says freshwater mussels are “just astounding.” The North Carolina resident was introduced to mussels and other river-bottom species as a student doing field work in Alabama.

“I just became absolutely captivated by the cryptic,” she said.

They’re more than just a fascination. McCombs said mussels are a critical and underappreciated part of the freshwater ecosystem. And scientists don’t know why they’ve been disappearing for decades.

McCombs is part of an ongoing study of more than 100 streams across the country — including four in Alabama — in an effort to find a way to salvage America’s mussels.

‘THE LUNGS OF A RIVER’

The freshwater mussel doesn’t have the curb appeal of furry and feathered endangered species or some of its tastier cousins, like oysters. It lives out its life in a place that most people rarely think about: the bottom of streams and rivers.

But within that shell, the mussel is a constant filtration device, cleaning the drinking water of both wildlife and people. 

“They’re kind of like the lungs of a river. They’re constantly breathing in and out, and they’re filtering,” McCombs said.

Wendell Haag, a research fisheries biologist with the U.S. Forest Service, said mussels pull sediment out of the water and their waste becomes a food source for other bottom-dwelling species, forming the foundation of a freshwater food web.

“They can dominate the biomass in the bottom of the stream and so when mussels are abundant, they represent in total a tremendous amount of filtering activity,” he said.

In return, mussels depend on the fish traveling upstream in order to reproduce. Mussel larvae are microscopic and spend the first part of their lives as parasites, living on the fish’s gills or skin until they become juveniles. Then they fall off the fish and land on the riverbed, where they’ll spend their adult lives.

Some mussels can use any fish to transport their larvae. Others rely on just a single species. And they have different ways of reaching that goal, including the larvae-filled bait, resembling worms, fish or crayfish, that fooled McCombs.

“They’re all about partnerships,” she said.

Some mussel species can live up to 100 years, although others typically live between 10 and 50 years. There are around 300 species of freshwater mussels in the U.S., according to the Center for Biological Diversity.

Alabama has around 180 species, including some species that aren’t found anywhere else. According to the Alabama Department of Conservation and Natural Resources, about half of Alabama’s mussel species are some level of threatened, endangered or extinct.

“Alabama has the highest diversity of freshwater mussels of any comparably sized area on earth,” Haag said.

THE VANISHING OF THE MUSSEL

McCombs says scientists first began to notice freshwater mussels disappearing around the 1970s. Today, the Xerces Society for Invertebrate Conservation estimates as many as three-quarters of American mussel species are in danger. Eight species were declared extinct in 2021.

Early on, many scientists believed that human development was solely to blame. After all, human behavior has polluted and dammed up waterways and warmed the planet, making rivers less healthy and preventing the fish travel that mussels rely on.

McCombs described freshwater mussels as a “canary in the coal mine” for the impacts of climate change and human development on native habitats.

In the Cahaba River, there are almost no juvenile mussels anymore, only adults. Jason Throneberry of the Nature Conservancy of Alabama has said this is due in part to dams further downstream on the Alabama River, blocking fish migration.

However, McCombs said more research has shown that the causes of mussel decline aren’t so straightforward. The ways that these populations are collapsing don’t always match up with dam construction or other human interference.

Part of the issue is that scientific knowledge about freshwater mussels has been limited.

“We don’t know very well what the effects of losing mussels in a stream are. We’re still trying to understand their role,” Haag said.

“There’s a lot that we don’t know. We don’t even know exactly what they eat,” McCombs said.

Now, scientists are considering whether a contagious virus or competition from invasive species, like the Asian clam, are at the root of some of these population losses.

Haag said “there’s a lot of conjecture now and speculation” about mussel decline, but few rigorous, large-scale research projects to see whether those ideas are correct.

McCombs and Haag are in the midst of a study that may finally give clear answers.

“What we’re really interested in is to understand why this decline is happening, so we can make sure that we’re advocating for practice and policy to protect freshwater mussel populations in the future,” McCombs said.

A LANDMARK STUDY

American Rivers and the U.S. Forest Service’s Southern Research Station are in the second year of a three-year study of mussel populations across 13 states.

McCombs said the original scope of the research was limited to about 40 rivers and streams, but she was surprised by the cooperation and enthusiasm from local partner organizations, and the project was expanded to more than 100 locations.

Those sites include Paint Rock River and Flint River in north Alabama and Big Canoe Creek and Terrapin Creek in central Alabama.

“We were all just floored by the enthusiasm, the support and the willingness to collaborate. This project has grown so far beyond what we originally envisioned,” McCombs said.

Haag said some of the rivers and streams have healthy mussel populations, while others have seen declines.

McCombs, Haag and their team are visiting different sites in each of the three years of the study. First, they visit a stream in June, where they place a cage full of baby mussels into the waterway. 

The team returns in September to collect the mussels and see if there have been any changes in their size, the water quality of the river, the presence of pathogens or invasive species, surrounding land use and other factors.

McCombs collected mussels from the four Alabama sites in September of this year. After the final fieldwork is completed in summer 2023, the research team will compare the results from different sites to find any trends that might provide clues about how mussels are declining.

“We’re hoping that we will be able to see data that informs really practical ways to protect mussels,” McCombs said.

She said that data could influence the advocacy American Rivers does at state and federal levels to protect water quality and biodiversity.

“Hopefully we will have an actionable story to tell,” she said. “… Rivers, the lifeblood of communities, need help.”

Haag said he hopes the results of the research will also be helpful for land management and conservation organizations to find better practices.

“The scale of this project is very important because you can’t study what’s going on in Alabama in a vacuum. It’s much more effective and informative if you can put it in a larger context,” he said.

THE NEXT GENERATION

Several groups around the country are trying to propagate native mussel species in a bid to save them. 

That includes the Alabama Aquatic Biodiversity Center in Perry County, which raises baby mussels and snails to release back into the wild. Since 2010, the AABC has released hundreds of thousands of mollusks into their native environments, including 14 different mussel species.

McCombs said she supports these propagation efforts, but she wants to make sure they’re getting at the “root cause” of population loss so those baby mussels that are released have the best chance of surviving and reproducing.

There’s no time to waste, however, McCombs said.

“Time is running out for us to be able figure out what’s harming these populations,” she said. “… We need kind of an all-of-the-above approach.”

McCombs said she is hopeful about the future of these species.

“Whether you ever see a mussel or not, they’re out there all day long filtering the water, enabling us to have clean water,” McCombs said. “… I hope that we will have a future with mussels.”

Main article image of purple wartyback mussel shells courtesy of Erin Singer McCombs.