Closer to Understanding Enigmatic Mussel Declines

A cluster of about five mussels on the bottom of a streambed
“In terms of biomass, mussels were once the most dominant organisms in the bed of a stream,” says Haag. “If you combined the weight of all the creatures in the bottom of the stream, 90 percent of that would be mussels.” Photo by Monte McGregor, used with permission.

Just by existing and eating, mussels improve water quality. They are filter feeders, which means they eat small pieces of organic matter that float past them.

But mussels are dying, often in streams that otherwise seem healthy. Many streams that formerly supported diverse mussel communities now are essentially defaunated. These events are enigmatic because other animals in the streams seem unaffected.

“It’s not just endangered species that are disappearing,” says Wendell Haag, a fisheries research biologist with the USDA Forest Service. “Almost all mussel species have declined dramatically and continue to decline in many areas.”

Mussels in relatively cool, unproductive streams appear particularly vulnerable to enigmatic declines, according to a recent study that Haag led. The study was published in the journal Freshwater Science.

“We found strong evidence that chronic stressors negatively affect growth in defaunated streams,” says Haag. “However, we did not identify any contaminants that can explain reduced mussel growth and mussel declines in all the defaunated streams we studied.”

Haag and his colleagues raised over 4,000 baby mussels from glochidia (microscopic larvae) to juveniles. When the study began, their shells were about a quarter of an inch across. All of the mussels were Lampsilis cardium, a species native to most of east-central North America. Its common name is the plain pocketbook.

The researchers put the small mussels in 23 streams across Kentucky. All of the streams in the study once supported many mussel species. At the time of the study, 16 still did, and the remaining 7 streams were essentially defaunated – mussels had inexplicably disappeared over the last 30 years.

Over a dozen juvenile mussels are held in the palm of someone's hand. All the mussels have two-toned shells
As the two-toned shells show, a mussel’s environment can change rapidly, and in ways that affect their growth. Photo by Wendell Haag, USFS.

“Because we were focusing on enigmatic declines, we excluded streams where mussel declines appeared to result from obvious impacts such as impoundment, channelization, or severe water pollution caused by municipal or industrial discharge or extensive coal mining,” says Haag, who also recently published a paper on enigmatic mussel declines in the journal Freshwater Mollusk Biology and Conservation.

To keep the mussels in one place for monitoring, the researchers put batches of 16 mussels in small cages. They used two types of cages: a silo and a sediment cage. The sediment cage exposed the baby mussels to sediments of the streambed, while the silo kept them in the water column. This was intentional – pollution is a suspected cause of enigmatic mussel declines, and pollutants concentrate differently in the flowing waters of a stream versus sediment.

For four months, the team collected water samples and measured stream temperature. The Kentucky Division of Environmental Program Support measured over 150 different substances in the samples, including:

  • pH,
  • nutrients, or total organic carbon,
  • and concentrations of metals,
  • nitrites and nitrates,
  • pesticides.

In their analysis, the researchers also included land cover and local geology – especially whether karst topography was common.

Because of the large number of variables, the researchers used a principal components analysis to show which variables were significantly related to mussel survival and growth. The results showed that, overall, mussels grew more slowly as the influence of row-crop agriculture increased.

But there were important nuances to the study. Agricultural contaminants were not present in all defaunated streams. Two of the defaunated streams were in forested areas with little row-crop agriculture, and agricultural contaminants were rare. Surprisingly, there were no consistent patterns of sediment contamination among streams, and sediment contaminants were not related to mussel growth. All this led the researchers to consider whether factors other than row-crop agriculture might be related to mussel growth.

Only three characteristics were associated with all seven defaunated streams: low total organic carbon, low water temperature, and a high percentage of karst. These characteristics generally indicate streams that are relatively unproductive, but researchers do not yet know why mussels might be more vulnerable under such conditions.

historical photo shows rows of mussel shells with circular holes cut out of them
From the mid-1800s to the early 1900s, settlers made buttons from freshwater mussel shells. Overexploitation and the availability of zippers and plastic buttons eventually ended the practice. Image courtesy of Wikimedia Commons.

Another interesting observation was that mussels in Kinniconick and Tygarts Creeks had two-toned shells – dark brown near the center and golden yellow on the edges. In the study, the dark color was associated with slow growth and the lighter color with faster growth. “This pattern may indicate growth inhibition early in the study followed by higher growth later in the study,” says Haag. “The important thing about that is it suggests that stream conditions that influence mussel growth can change quickly.”

One factor that the researchers didn’t measure was the abundance of Asian Clams (Corbicula), which arrived in eastern North America in the 1960s and now occur in all of the study streams. “Because Corbicula is so widespread and has been present in the U.S. for so long, we’ve all had a tendency to ignore it,” says Haag. But competition for food with Corbicula might be an explanation for the prevalence of enigmatic mussel declines in unproductive streams, where food resources are less abundant. Haag and his collaborators are now focusing on assessing potential impacts of Corbicula on native mussels.

About 70 percent of the 300 mussel species native to the U.S. are in danger of extinction.

“It’s a major conservation problem, and we don’t know what’s causing it,” says Haag. “It’s essential that we discover the causes of these declines.”

The Daniel Boone National Forest in Kentucky provided some funding for the study.

Daniel and Steve Mills and Boy Scout Troop 37 in Lawrenceburg, Kentucky constructed about half of the silos as part of Daniel’s Eagle Scout project.

Read the full text of the study in Freshwater Science.

Read the full text of the study in Freshwater Mollusk Biology and Conservation.

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

Access the latest publications by SRS scientists.

Subscribe to our newsletter!

Receive weekly updates