Can Mussels Save Philadelphia’s Waterways?

The Philadelphia Water Department’s Lance Butler and William Whalon in the Manayunk Canal / Photography by Gene Smirnov

“Feeding time,” says Shannon Boyle, pushing back her chair and pulling her attention away from the microscope she’s been bent over for the better part of an hour. She’s counting freshwater mussel larvae, called glochidia. It’s exacting work; each one is about a third the size of a grain of sand.

Inside the tank are about 30 freshwater mussels — alewife floaters, tidewater muckets, eastern elliptios, eastern pondmussels, and yellow lampmussels. They are filter feeders — they eat the particulates by filtering them through their systems — so they clean water simply by eating.

In the wild, freshwater mussels eat the contaminants that pollute fresh water: nitrogen, phosphorus, pathogenic bacteria such as E. coli, microplastics — even metals, pharmaceuticals, and personal care products, according to the U.S. Fish and Wildlife Service.

For now, the mussels live in a tank in a glass-enclosed lab in the basement of the Fairmount Water Works, a sprawling campus that sits on the east bank of the Schuylkill River between Boathouse Row and the Philadelphia Museum of Art. Now a free museum and environmental education center, the building is open to the public, including the lab.

Both inside the lab and throughout the city, Boyle and a small team of other aquatic biologists are studying the freshwater mollusks native to our urban rivers, streams, and canals. The goal: to bring native freshwater mussels back into Philadelphia’s waterways, including the Schuylkill and the Delaware, as a kind of technology to help clean the water — not just for drinking but to make it swimmable and fishable again.

The lab, where Boyle works as an environmental scientist, is part of the Philadelphia Water Department. The research makes PWD the first and currently only water company in the nation using freshwater mussels to clean the waterways within its own watershed. They are also propagating, or breeding, mussels. “We have the first, the only that we know of, municipally owned and operated freshwater mussel hatchery in the United States,” says Lance Butler, who runs the lab and launched the freshwater mussel project in 2017. The hatchery was developed in partnership with the Academy of Natural Sciences of Drexel University, the Partnership for the Delaware Estuary, and the Water Works.

Other cities, including Washington, D.C., are working on mussel projects. But those projects are not being overseen by their water departments. PWD is the first to do it all under one metaphorical roof. Theirs is part of a larger, three-pronged, tricolor (gray, green, and blue) infrastructure project to clean Philadelphia waterways.

Gray infrastructure is the treatment plants, pipes, and conveyance systems; green is land-based systems that slow stormwater runoff, like vegetative swales and permeable paving. And blue infrastructure is Butler’s freshwater mussels.

The mussel project began on the heels of a “very ambitious” green infrastructure program, part of PWD’s stormwater compliance efforts, says Kelly Anderson, director of the Office of Watersheds at PWD, which oversees the project. “We recognized the value that these creatures could provide to our local waterways,” she says. “After focusing on the land, it felt natural to move to aquatic ecosystem services, working with an adaptive species in a constantly evolving program.”

The investment in the science and technology of blue infrastructure, especially, is cutting-edge. The World Economic Forum just this summer called on cities throughout the world to do exactly what PWD has been doing for almost a decade: Rather than choosing between concrete and nature, the group is appealing to cities the world over to leverage green and blue infrastructure and integrate them with gray infrastructure.

Enter the mussel.

There are more than 300 species of freshwater mussels in the world. They’re often unnoticed and underappreciated, but aquatic biologists call them “the livers of our rivers”: One adult mussel can filter between six and 10 gallons of water a day. “They’re really small little workhorses out there, filtering,” says Butler.

But many are endangered. Freshwater mussels were decimated in the early 1900s during an era of unregulated harvesting by the button industry, by unchecked pollution, and by the proliferation of dams, which are physical barriers for the fish mussels rely on for their life cycle. Mussels are, in fact, one of the most endangered animals on Earth and the most imperiled invertebrates in the world. Seventy percent of freshwater mussel species native to North America are threatened, and more than two dozen are already extinct. But they can also be a natural indicator of a river’s health, says Jess Jones, a restoration biologist with the U.S. Fish and Wildlife Service.

When you have a bed of a million mussels filtering 10 million gallons of water a day, stripping all the particles out of the water and increasing water clarity so you can see through it, “the plants on the bottom can all of a sudden get sunlight for photosynthesis. It’s immediate, it’s dramatic,” says Danielle Kreeger, an aquatic ecologist at Drexel University who’s been studying the biology, ecosystem services, and enhancement of freshwater and saltwater habitats in and around Philadelphia for more than three decades. “They filter all these particles indiscriminately. They’re really quite elegant, sophisticated animals, internally … and they can live 50 to 80 years, some species even 100 years,” she says, cleaning our waterways the entire time.

The more mussels the better. “We know if we increase the number of mussel populations, the filtration ability is more and therefore the water clarity will follow,” says Monte McGregor, a research scientist and expert in the field who works with endangered mussels in Kentucky and is familiar with the PWD project.

In healthy environments, mussels actively pull in water and small particulates through siphons inside their shells. Suspended material — zooplankton, phytoplankton, bacteria, microbes, and other particulate matter — is pulled in through the siphons to the gills for oxygenation. What the mussels need for energy they process and digest; the rest gets excreted and settles to the river bottom. “It’s really kind of cool,” says Kreeger. “A mussel bed is like a biogeochemical hot spot in a river bottom. Fish abundance is 10 times higher on a mussel bed versus not because all the little fish are in there, foraging in the enriched sediments and then eating all those macroinvertebrates. And there’s more oxygenation. It’s like a little coral reef on the river bottom.”

Mussels tagged for tracking

While researchers have known since the mid-1800s that freshwater mussels are filter feeders, the science of using freshwater mussels for their ecosystem services is new in the past three decades. Kreeger has been on the forefront.

It wasn’t until about 15 years ago that most facilities had the technology or infrastructure to breed mussels, says McGregor. There are still fewer than two dozen mussel hatcheries in the U.S., and most of them are focused on propagating endangered mussel species. Without hatcheries breeding new mussels, it’s been slow going for aquatic biologists like Butler to show, on a grand scale, how mussel ecosystem services could be the main driver of the blue infrastructure movement.

What scientists have figured out, though, and what Kreeger says she’s been shouting from the rooftops for decades, is that if there’s a strong community of a foundation species — that is, the mussels that are not endangered — then all the species become more resilient. “When you look at who’s filtering water out there in the river, it’s not the rare species,” she says. “I mean, they all filter water, but if you have a rare, rare, rare species, you don’t have enough animals to filter and do much work.” Kreeger’s focus is the eastern elliptio and the alewife floater, both native to Philadelphia’s urban waters, both foundational mussels living healthy lives in the Delaware watershed.

“Paradoxically, the biggest mussel beds [of the entire watershed] are actually on the doorsteps of Philadelphia,” says Kreeger. “Everyone thought, ‘Well, that’s the urban corridor, it’s the last place you’d find them.’”

Kreeger was the one, in fact, who found them. “I was doing wetland surveys, got hot one day, dove off the side of the boat to cool off, and found mussel beds that we didn’t think were there,” she says. But there they were, living in the tidal part of the Delaware River between Trenton and Philadelphia. She could see them at the river bottom because the water above them was so much clearer. She found seven native species, including two species that had been believed to be locally extinct and several others listed by the state as rare, threatened, or endangered.

PWD has invested not only in Kreeger’s scientific research, but also in the work of city planners, engineers, and modelers to look at the entirety of the watershed. This more holistic approach stems, in part, from the 1996 amendments to the Safe Drinking Water Act, which require a stricter look at what’s flowing into rivers from the top for communities using those waterways for drinking water.

There’s about 10,000 square miles of watershed above the city of Philadelphia, says PWD’s Anderson, with 40 percent of Philadelphia’s drinking water coming from the Schuylkill and 60 percent from the Delaware. “[We’re] at the bottom of both the rivers,” she says, and it’s incumbent upon the water department to look “all the way to the top” of both. “It truly takes a watershed approach to manage a watershed,” she says.

And it’s not just about the mussels. “The fish are probably the most critical part of a mussel’s life cycle,” says Butler.

The fish are kind of like an unaware host and the mussels are guests who show up, pretty much uninvited — with a twist. The guests pretend to bring dinner and then say gotcha! It’s a fairly astounding relationship that involves a sophisticated level of trickery.

Nonswimmers, mussels have a unique reproductive strategy. Males release their sperm (or gametes) into the water. If a female mussel of the right species is nearby, she will siphon in those sperm while filter feeding. When fertilization works, glochidia (or larvae) will begin to develop inside the female. Because only about one in 10,000 mussels survives to adulthood, to maximize survival, female mussels use a host fish to help disperse the glochidia far and wide.

They lure the fish much like an angler with bait. Mussels make themselves look like their host fish’s favorite meal. “Honestly, to my knowledge, some of the most complex mimicry in the natural world,” says Fish and Wildlife’s Jones.

“Through evolution, the mussel has perfected its ability to mimic a worm, a crayfish, a minnow,” says Butler. When a host fish tries to bite what looks like a crayfish, for example, the fish chomps down instead on the mussel’s display — that thing that looks like a crayfish, fake eyes and all. While holding the fish in place, the mussel ejects its microscopic glochidia into the fish’s mouth, and those glochidia, hundreds of them, clamp on to its gills. They stay there for a few weeks while they ­metamorphose, then break out of the fish’s system and drop down to the river bottom. The ones that land in a hospitable habitat grow into adults, living and cleaning that part of the water for as long as they are alive.

Back in the lab, it’s propagation day — when the scientists replicate what happens in nature. Extra hands are on deck, 11 in all. Some are interns from Drexel’s externship program who work in Butler’s mussel lab; others are borrowed for the day from one of the other PWD watershed departments.

Propagation day takes place once every three to four weeks, and it means counting the microscopic glochidia, which is how Shannon Boyle spent her morning. Using a rejiggered nasal speculum, Boyle opens up a female mussel, peering in to see if there are any glochidia. If there are, she siphons them out of the mussel and collects them in beakers filled with water, then tests them for viability before inoculation.

Folding tables with cut-out holes for buckets are set up outside the glass lab. Visitors wander by, seemingly unaware that what’s happening in front of them could one day help clean Philadelphia’s water. They read the captions on images that adorn the stone walls and watch a cartoon video about mussels and their host fish.

Each bucket gets about two liters of water and 10,000 to 14,000 of the glochidia Boyle has counted. A volunteer scoops six brook trout, the state fish, into each bucket.

Then inoculation begins.

Butler watches the clock as the interns agitate the water, moving the fish around to get them to breathe harder so they’ll ingest the glochidia. After about five minutes, Butler scoops a fish out of a bucket and holds it under a magnifying lamp. “I’m going to open the gill and see if there are any little dots on it,” he says by way of explanation. Those dots are glochidia. “That looks really good,” he says. “The second gill and third gill look really good.” The first gill doesn’t have a lot of glochidia, so the fish goes back into the bucket and the agitation begins anew.

Inoculation in the mussel hatchery at Water Works

“It’s an exact and inexact science,” Butler says with a chuckle.

Inoculation lasts for about 15 minutes.

“We don’t have the cookbook with a recipe on how to do it,” says Butler, who has tried and failed many times. “You do not want to over-inoculate. A couple of months ago, I over-inoculated and I lost the whole batch” of glochidia, he says.

“Let’s go two more minutes and call it,” Butler announces to his volunteers.

As many as 500 glochidia can attach to each fish — it really just depends on how big the fish is. Last year, Butler says, they had 60 hybrid striped bass that gave them 12,000 juvenile mussels, an average of 200 per fish.

Once propagation is over, the fish go into a holding tank in the lab. There the glochidia can drop from their gills onto filters and then grow safely. When they’re two months old and measure about two millimeters, they’re moved to ponds around the area, where they live in submerged protective cages and are monitored quarterly by Butler and his team. It takes about a year and a half for the propagated mussels to grow enough to be tagged and released from the cages into a river.

Butler has added 3,000 freshwater mussels to the Schuylkill River, and a total of 10,000 across all Philly waterways. He’s now taking his work upstream to the Manayunk Canal, where water is running again after a $20 million investment by PWD. He’s got 15 cages of mussels — about 1,200 — growing there. In about a year, the healthy ones will be radio-tagged and planted in the canal’s sediment, where they, too, will be monitored by the team.

“With respect to science, this is all pretty new,” says Butler of the blue infrastructure he’s building. “The process is far from simple — it requires strict adherence to step-by-step protocols. … It’s a meticulous process, but also incredibly exciting science we’re undertaking here.”

And it’s not just the ecosystem that could benefit; these mussels could help lower water costs.

“These tidal wetlands, the mussel beds, the submerged aquatic vegetation, it’s not just that they’re good things for the ecology, but they can potentially cut costs by providing these ecosystem services in perpetuity, if you get these habitats established,” says Kreeger.

Butler’s 10,000 propagated mussels in Philadelphia’s waterways are still juveniles, but each is already filtering one to two gallons of water a day, on average. Multiply that by the millions of mussels he wants to put throughout the watershed, and the effect is massive. “Think about how many gallons of water a day they could process,” he says. “At that point, they’re just basically a treatment plant.”

But they’re not there yet. To get those millions of mussels into the water, Butler needs to grow his lab and get his blue infrastructure project recognized as an environmentally compliant technology that’s proven to clean water. “[We] have to be able to show the science, demonstrate the improvements … show the value to the decision-makers, the politicians, the regulators,” says Anderson. And that means continuing to move the project farther and farther upstream, high up into the watershed.

At the core of it all is the economics of the blue infrastructure; it’s also the sales pitch.

Fish and Wildlife’s Jones leans in on the money that comes from recreation. While mussels provide a host of ecosystems in the water, he says, creating healthier fish means improved fishing, and cleaning the water can lead to other forms of recreation too. That’s at least two different economic drivers that are proven beneficiaries of the muscle of mussels.

“Do not swim” and “Do not fish” signs are starting to take a toll. When people see them enough they want to know what can be done about it, says Kentucky’s McGregor. “So this was a novel idea from the [Philadelphia] water department: Let’s improve our filtration naturally, and maybe that will allow people to use the water better.”

PWD has spent the past few decades pushing out the message to connect the dots for people so they can better understand what it costs to make the water cleaner and more usable. “We have seen folks shift over the last 25 years recognizing the value of the rivers and streams,” says Anderson.

The next message Kreeger wants to push out is how much money the blue infrastructure project can save ratepayers. “We know that [mussels] have to filter volumes of water just to exist there. So it’s not in debate that they will remove large quantities of polluting particles,” she says. “[But] we need a lot more data, basically, to make the case that the ratepayers will save X amount of money if we increase the population of mussels by X amount. I have those data, I have those projections, but they’re estimates.” The proof will come, she says, once there are really large populations of mussels in the water, even in novel places like stormwater ponds.

Her early projections compare the cost of mussels and stream riparian buffers, streamside trees and shrubs used to prevent pollution from entering waterways that are approved by crediting agencies like the EPA for their pollution-fighting — specifically their removal of nitrogen. They cost $200 to $250 per pound of nitrogen removed. Kreeger’s estimates show that mussels cost $15 to $30 per pound of nitrogen removed. “So my estimates are about 10 times better.”

How we get to that future, though, is still up in the air. PWD is looking to create a state-of-the-art, first-of-its-kind freshwater mussel education facility with a lab and hatchery at Bartram’s Garden. Tucked away in Southwest Philly, the 50-acre public garden is a National Historic Landmark on the banks of the Schuylkill River and the oldest botanical garden in North America. “It may be the first of its kind of project in maybe the world,” says Anderson. “Our goal is to have this be a place where students and neighbors can engage with hands-on work, discover science.”

In partnership with Bartram’s Garden and the Partnership for the Delaware Estuary, the plan is to take the proof of concept from Butler’s lab and make it bigger, better, and faster, with a focus not just on mussels but on the entire ecosystem, including fish and insects. The goal would be to produce half a million mussels per year, with a secondary goal of producing enough mussels to provide ecosystem services for other parts of the state, like the Susquehanna watershed.

But the entire project is in jeopardy.

They’ve raised about $8 million, and $4 million more is needed to build this hatchery. With federal funding cuts, though, even the state money they’ve secured could disappear. “The whole project is at risk,” says Anderson. “It’s in limbo now.”

Back in the lab, Butler remains optimistic. They are in the final stages of inoculating brook trout with the glochidia from the eastern elliptio mussel and are working on propagating the eastern pondmussel with largemouth bass. Still in the nascent stages of the research, Butler continues to write the cookbook. “This year was about protocols and procedures,” he says, including finding additional host fish for mussels. The more host fish, the more propagation.

The results suggest they are on the right track: Brook trout are excellent alternative host fish for eastern elliptios, yielding anywhere from 40 to 80 juvenile freshwater mussels per fish, and hybrid striped bass look to be a promising alternative host fish for alewife floaters, yielding more mussels than their known hosts in nature.

“The hard work at our little hatchery is paying off,” Butler says. “Through our laboratory and field studies, we now have a much clearer understanding of the ecosystem services these unique organisms provide — they truly are our modern-day canaries in the coal mine.”

And maybe someday soon we’ll all reap the benefits.

Published as “Mussel Up” in the September 2025 issue of Philadelphia magazine.