The Filter Feeders of the Three Rivers: Why Freshwater Mussels Matter

What are Mussels?

They don't swim, they don't make noise, and yet they clean our rivers every day. Mussels are a type of bivalve mollusk, meaning they have two shells hinged together. These organisms are essential but underappreciated river dwellers. Acting as nature's water filters, mussels pull pollutants, algae, and sediment out of the water column, helping keep our rivers clean and balanced. They are extremely important for the overall health of the ecosystem, making them a top conservation priority, particularly in Pennsylvania. At one time, Pennsylvania's drainage basins supported at least 67 species of native freshwater mussels. But today, they're at high risk: across North America, 7% of mussels are already extinct, and 70% of the remaining species are threatened, endangered, or rare. Freshwater mussels form the backbone of a healthy aquatic ecosystem. The biodiversity of our rivers often depends on the success of these quiet, hard-working filter feeders.

Display of native freshwater mussels of Pennsylvania, labeled for identification. 3RWK

Nature's Water Filters

Mussels are considered ecosystem engineers due to their ability to change the composition of the entire water column. These organisms use a siphon effect with their gills to take in oxygen and filter out food from the water column. They partially bury themselves in the sediment with only their siphons exposed, allowing them to properly pump water in and out of the body. Freshwater mussels are valuable filter feeders, each cleaning as much as 18 gallons of water every day. Mussels don't just remove unwanted particles; they also cycle nutrients into the sediment, boosting productivity for other organisms and preventing excessive algae growth. These aquatic organisms are also considered bioindicators, meaning that their health reflects the overall health of the ecosystem. When there is a decline or disappearance in mussels, it's often a sign of pollution, habitat destruction, or invasive species. Additionally, mussels benefit the rest of the food web by providing habitat for benthic organisms and smaller fish. They also provide a food source for other animals such as river otters and muskrats, further benefiting the ecosystem.

Pittsburgh's Mussels: A History of Decline

Mussel populations have faced many challenges, particularly in cities that have undergone major urban development, such as Pittsburgh, PA. Historically, runoff from Pittsburgh’s booming steel industry and coal mining operations seeped into the rivers, degrading water quality and contributing to dramatic mussel population declines. Throughout the 19th and 20th centuries, raw sewage, heavy metals, and acidic mine drainage turned parts of the Allegheny, Monongahela, and Ohio Rivers into aquatic dead zones. Pollution from industry, urban and agricultural runoff, dredging and channelization, and habitat destruction are all direct causes of population decline in the three rivers region. The construction of dams and river navigation systems also altered flow patterns and disrupted mussel reproduction, as many species rely on host fish to carry their larvae upstream. Without healthy fish passage, mussel life cycles were broken. Pennsylvania's pearl button industry in the late 1800s and early 1900s further reduced native mussel populations. Mussels were heavily exploited for their shells during the "Pearl Rush," and many ecosystems collapsed due to overharvesting.

Factory workers sit atop a massive pile of waste shells, a byproduct of button manufacturing. Washington County Historical Society / Twin Cities Pioneer Press

A mussel shell with circular cutouts where buttons were punched. National Pearl Button Museum / Moda Fabrics Supplies

Due to recent conservation efforts, there is hope for Pittsburgh's mussel populations. As the city transitioned away from heavy industry, the rivers have slowly begun to recover. Clean water laws, sewage treatment upgrades, and coal mine remediation efforts have improved water quality. However, challenges remain. Invasive species, fracking runoff, and climate-related stressors continue to threaten progress. Along with habitat degradation and pollution, a newer and less visible threat has emerged: radioactive waste from fracking. This industrial byproduct poses a significant risk to the health of aquatic ecosystems and highlights the need for ongoing attention and protection of Pittsburgh's rivers.

Pollution from ruptured oil barge near Duquesne Works on Monogahala River in 1973. John L. Alexandrowicz / U.S. National Archives and Records Administration

Radioactive Waste and the Hidden Threat to Mussels

Another major concern for Pittsburgh's mussels is the wastewater produced by fracking. Not only does fracking consume large volumes of water, but the process also generates waste contaminated with harmful chemicals. A recent study revealed elevated levels of radioactivity in freshwater mussels collected from the Allegheny River. This contamination was traced back to a landfill that accepted oil and gas waste during the height of the fracking boom.

The waste included naturally occurring radioactive materials like radium-226, which seeped into nearby waterways. Although the facility no longer accepts radioactive waste, the study found that contamination still persists in sediments downstream. Mussels collected from these areas showed elevated levels of radioactivity in both their tissues and shells.

This poses a serious risk not only to mussels but to the broader food web. Radioactive materials can accumulate and intensify as they move up the food chain — from mussels to muskrats or otters, and eventually to apex predators like bald eagles. This finding highlights the importance of monitoring radioactivity in aquatic ecosystems and preventing contamination at its source.

Invasive Species: Zebra Mussels

While Pennsylvania's native freshwater mussels work quietly to clean and stabilize our rivers, a different kind of mussel has made its way into local waterways — and it's causing major problems. Zebra mussels, an invasive species from eastern Europe, first appeared in the Great Lakes in 1988, likely transported in the ballast water of ships. Since then, they have spread rapidly and have been found in all three of Pittsburgh's rivers. Unlike native mussels, zebra mussels reproduce quickly. A single female can produce up to a million eggs per year, and their microscopic larvae, called veligers, drift easily through the water, making it difficult to contain their spread. Once established, they attach themselves in massive numbers to hard surfaces like boats, water intake pipes, docks, and even native mussels. These dense colonies smother native species, depriving them of space, oxygen, and food.

Zebra mussels are also aggressive filter feeders, each capable of straining about one liter of water per day from the microscopic plants and animals known as plankton. This reduces the amount of food available for other aquatic organisms. Even more concerning, zebra mussels selectively filter green and brown algae while rejecting blue-green algae, which then grow unchecked. This imbalance can lead to toxic algal blooms that harm aquatic life and contaminate drinking water sources. Moreover, zebra mussels frequently form colonies on the shells of native mussels, hindering their ability to feed and breathe.

Controlling their spread is crucial. Boaters, paddlers, and anglers can help by cleaning, draining, and drying all gear before moving between waterways. Even a small amount of water or residue can introduce zebra mussels to new areas, putting more ecosystems at risk.

Zebra mussels attached to native mussels can severely hinder their movement and feeding. External view with zebra mussels clustered on the shell surface. Evan Clark / 3RWK

Internal view showing zebra mussels inside a native shell. Lucinda Mortimer / 3RWK

The Return of the Pink Heelsplitter

​The pink heelsplitter is a native mussel species in Pennsylvania that has made a remarkable comeback in recent decades thanks to river cleanups and stronger pollution regulations. In 2019, researchers from the Western Pennsylvania Conservancy found two pink heelsplitters in the Kiskiminetas River, commonly called the Kiski, while searching for a type of salamander. The species had not been seen there in over a century.

The Kiskiminetas River (also known as the Kiski), a tributary of the Allegheny, was once severely polluted by untreated mine drainage during the height of the coal industry and supported little to no visible aquatic life before 1980. Improvements brought by the Clean Water Act, the efforts of local watershed groups, and the decline of coal and steel production have all helped restore the river. Since freshwater mussels are strong indicators of water quality, the return of the pink heelsplitter is a powerful sign that the health of the river is steadily improving.

Pink heelsplitter mussel shell found in the Allegheny River. Jess Friss / 3RWK

Why are Mussels Essential for Ecosystem Health?

Mussels are extremely important for monitoring aquatic health, purifying the water system, providing a food source to other organisms, and contributing to biodiversity. Healthy mussel populations generally indicate a healthy aquatic system, which often means productive fishing and good water quality that humans benefit from. Otters, raccoons, muskrats, herons, and some fish all benefit from healthy mussel populations, further expanding biodiversity and ecological success. Mussels also provide an important substrate for algae and insect larvae to attach to. When mussels are present in large numbers, they may become underwater gardens that attract fish and other organisms to feed. Additionally, as mussels firmly anchor themselves to the stream bed, they can stabilize the river bottom, minimizing the negative effects of floods or erosion. Mussels are extremely important organisms in Pittsburgh's rivers, and they are a top conservation priority, considering many species of mussels are endangered or extinct.

Conservation and Action

Freshwater mussels face serious threats from pollution, habitat loss, and invasive species, but efforts across Pennsylvania are helping protect and restore their populations. Three Rivers Waterkeeper tracks and maps major polluters in the Pittsburgh region to identify those exceeding permitted discharge levels. Waterways are monitored for pollutants as well as sewage overflows following heavy rain events. Changes to the landscape, such as industrial development and wetland loss, are also tracked to help protect critical mussel habitat. Public comments are submitted during permitting processes to ensure aquatic ecosystems are properly considered.

Direct conservation work also supports mussel recovery. In the Kiski and Clarion Rivers, tributaries of the Allegheny River, mussels have been hand placed to encourage the formation of new beds. Positioning them on clean, stable riverbed substrate increases their chances of survival and reproduction. Ongoing surveys help track mussel growth and assess the overall health of the surrounding aquatic environment.

While monitoring and restoration efforts remain essential to protecting Pennsylvania's freshwater mussels, individual choices at home also play a powerful role.

What You Can Do

There are several simple but effective actions individuals can take to support freshwater mussel conservation. First, reducing lawn fertilizer use and properly disposing of household chemicals helps limit the amount of pollution that enters local waterways through runoff. Planting native vegetation in your yard or garden can also help absorb rainwater and reduce the flow of pollutants into rivers.

When boating or fishing, it's important to always clean, drain, and dry your equipment before moving to a new body of water to prevent the spread of invasive species like zebra mussels. Finally, supporting local conservation organizations and clean water policies — whether through donations, volunteer work, or voting — helps ensure long-term protection for mussels and the health of our rivers.