Asian Clams and Native Mussel Growth

Abundance of invasive bivalve associated with slower mussel growth

holding about a dozen Asian clams in two hands
In their native range, Asian clams are also called good luck clams. But in the Rockcastle River system, they were associated with slower mussel growth. Photo by Robyn Draheim, Center for Lakes and Reservoirs, Portland State University.

Native freshwater mussels grew more slowly when invasive Asian clams (Corbicula fluminea) were abundant. The study was led by Wendell Haag, a USDA Forest Service research fisheries biologist. It was published in the journal Freshwater Biology.

Mussels live out of sight – buried in the river bottom, eating algae and other small particles of organic material. Mussels are filter feeders and key members of aquatic ecosystems.

If mussels lack charisma, their names do not. The common names of four species Haag studied are the Cumberland bean, painted creekshell, plain pocketbook, and fluted kidneyshell.

Unfortunately, mussels are disappearing worldwide, and about 70 percent of the 300 mussel species native to the U.S. are in danger of extinction. Addressing mussel declines is difficult because their causes are mostly unknown.

Dams have caused some mussel extinctions, and coal mining, water pollution, and other factors also are proposed explanations. In a 2019 study, Haag and his colleagues found that mussel declines were associated with reduced growth of juvenile mussels, but water quality did not consistently explain reduced growth.

Haag and his colleagues followed up on that study by measuring mussel growth at 17 sites in the Rockcastle River system in southeastern Kentucky. Historically, the river system supported over 30 mussel species, but mussels have declined dramatically since the 1960s. “Some tributaries have lost essentially their entire mussel fauna,” says Haag. Mussel declines in the Rockcastle River have previously been attributed to coal mining.

In addition to water quality, the researchers also measured the abundance of Asian clams at each site. Asian clams are an invasive species that arrived in the U.S. in the 1920s and first appeared in the Rockcastle River about 1968.

However, the effects of Asian clams on native mussels are poorly known. “Because Asian clams have been here so long, most mussel researchers grew up with them, and we’ve had a tendency to ignore them,” says Haag.

Haag and his colleagues placed 30 juvenile mussels of each of the four species at each study site. They placed them on the river bottom for 84 days in concrete silos that allow mussels to feed normally but allow researchers to retrieve the mussels later.

Mussel survival was high at all sites. However, there were huge growth differences at different sites. “Growth varied among sites by an order of magnitude,” says Haag.

a stream with trees on both sides
“In the 1980s, Horse Lick Creek  was full of mussels,” says Haag. “Now they’re virtually gone – you can find only a few old shells.” Water quality is excellent and other aquatic animals still thrive here. Horse Lick Creek is one of the study sites. Photo by Chad Von Gruenigen, Kentucky Division of Water.

The researchers found no evidence of severe water quality degradation from coal mining at any site, and mussel growth was not strongly related to any facet of water quality. Instead, most of the variation in growth among sites was explained by only two factors: water temperature and Asian clam abundance.

Mussel growth was lowest at cool sites with the greatest numbers of Asian clams. “Because mussels are cold-blooded, we expected to see slower growth in cooler streams,” Haag says, “but we were surprised by the strong relationship between growth and Asian clam abundance.”

Based on these results, Haag and his colleagues developed a model that predicted that mussels would gain about half as much mass over 84 days for every ten-fold increase in Asian clam abundance, regardless of temperature.

Asian clams, which are also filter feeders, may reduce the amount of food available to mussels. “We don’t yet know if Asian clams directly affect native mussels or what the mechanism is,” says Haag. “But our model can be used by researchers in other areas to test our predictions and help answer those questions.”

Haag and his colleagues are currently conducting a series of laboratory experiments to better understand how Asian clams affect native mussels.

The study is important because it suggests that some previously proposed factors may not explain mussel declines. “To be clear, coal mining can have devastating effects on streams, but the relatively low amount of mining in the Rockcastle River system does not seem adequate to explain the dramatic mussel declines we’ve seen there in the last 40 years,” Haag says.

Instead, previously overlooked factors like Asian clams and disease may explain mussel declines across large areas, as Haag proposed in a 2019 review. “It’s vital that we understand causes of mussel declines so that scarce conservation resources can be directed in the most effective way,” says Haag.

Read the journal article in Freshwater Biology.

For more information, email Wendell Haag at wendell.haag@usda.gov.

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