The hidden strengths of freshwater mussels

Spending most of their lives buried in streambeds, freshwater
mussels can be easy to miss. You could wade across a dense bed of
them without noticing. An observant snorkeler might see scattered
shells and pairs of holes in the river bottom where the creatures’
siphons pierce the sediment.

Yet even though they’re hunkered out of sight, freshwater
mussels shape ecosystems.

They funnel food downward, fueling life in the riverbed and
clarifying water for other species. They help to mitigate nutrient
pollution, a widespread problem that leads to dead zones in some
waters.

And today they are in trouble, with one of the highest
extinction and imperilment rates on the planet. In North America
alone, 30 freshwater mussel species have gone extinct over the last
century, and 65 percent of those surviving are considered
endangered, threatened or vulnerable — primarily due to the
large-scale damming of rivers.

A blitz of dam construction from the 1920s to the 1980s
destroyed thousands of miles of habitat and fragmented far more.
Adapted to shallow, free-flowing waters, mussels can’t survive in
the deep, cold and oxygen-poor conditions that major dams create
for scores of miles downstream, says Wendell Haag, a fisheries
research biologist at the Kentucky Department of Fish and Wildlife
Resources’ Center for Mollusk Conservation.

Some mussels attract the fish whose gills will temporarily host
larval mussels by presenting a fleshy flap that looks like a tasty
meal, for example a small fish or crayfish. When the fish bites,
the mussel releases a cloud of thousands of mussel larvae, called
glochidia, some of which will take up residence in the fish’s
gills. After growing into juveniles, the young mussels drop off and
settle into the sediment.

CREDIT: CHRIS BARNHART / MISSOURI STATE
UNIVERSITY

Now a group of biologists has a two-pronged plan to pull mussels
back from the brink. They are working on building the mussels’ PR
by spreading word about the water-cleansing services they provide.
And they aim to put the creatures to work reclaiming waters, by
rearing them in large numbers then releasing them to the wild.

In so doing, the scientists are turning traditional conservation
on its head: Instead of protecting habitat to rescue a threatened
creature, the goal is to use mussels to rescue their habitats
themselves. “Mussels are biofilters,” says Caryn Vaughn, an
ecologist at the University of Oklahoma who coauthored an article about the ecological roles of
the creatures
in the 2018 Annual Review of Ecology,
Evolution, and Systematics
. “And if we can convince people
that’s important, then I think that’s a tool to save them.”

A complex life, out of sight

In healthy streams, mussels live in large beds that may hold
thousands of individuals of several species, each adult as big as a
baseball or larger. They’re long-lived — some species have
lifespans of more than 100 years. They lead flamboyant reproductive
lives that likely got their start more than 100 million years ago,
when an ancestor of today’s mussels evolved a strategy of having
its larvae hitchhike on fish.

Each female produces millions of these larvae, called glochidia,
and many mussel species make elaborate lures that resemble the prey
of their specific fish hosts. Once on board a fish, glochidia form
cysts on the creature’s gills or fins until they transform into
juveniles, drop off and settle to the stream bottom. Perhaps two in
a million will live to reproductive age.

Photograph of membranous sacs of mussel larvae that look like small minnows with black eyes and red mouths.

The Ouachita kidneyshell mussel packages its larvae, called
glochidia, in a membrane that resembles a small fish. When a real
fish snaps it up, the membrane bursts, releasing the glochidia,
which will live in the fish’s gills for a period.

CREDIT: CHRIS BARNHART / MISSOURI STATE
UNIVERSITY

This complex interaction with fish hosts helped mussels thrive
and spread. “Fish can swim upstream, while mussels cannot,” notes
Chris Barnhart, a biologist at Missouri State University who works
on propagating mussels for research and restoration. By riding on
fish, mussels can colonize upstream habitats — an innovation so
powerful that all living members of the freshwater mussel family,
the Unionidae, are descended from the long-gone ancestor that
adopted the tactic.

But the proliferation of dams has blocked fish movements,
cutting off many mussel populations from their hosts and ending
their ability to reproduce. Other populations dried out when water
diversions lowered flows in their home streams, or they fell victim
to water pollution from sewage plant discharges, industrial spills
and nutrient-heavy runoff from farms and cities. Invasive species
threaten still others (nonnative zebra and quagga mussels
outcompete native mussels, for example, and zebra mussels stick to
native mussels in great numbers). Even as conservationists focus on
rescuing species listed as threatened or endangered, the overall
decline in mussel numbers continues — contributing, in turn, to the
degradation of freshwater habitats throughout the US and the
world.

A graphic with three charts: The top one shows the proportion of freshwater bivalves in different regions around the world that are included in various threat categories such as extinct, critically endangered, endangered, vulnerable and threatened. In the middle is a pie chart of the proportion of freshwater mussel species in the US that are in these categories. At the bottom, a bar graph shows declines in mussel populations and diversity over a roughly 10-year period, at 10 sites on the Kiamichi River in Oklahoma.

Human activity hits mussels hard: Of the 270 North American
freshwater mussel species (top graph), 30 have gone extinct in the
last 100 years, 95 have been listed as endangered or threatened,
and a large proportion of the remaining species are considered
vulnerable (middle pie chart). A case in point is the Kiamichi
River (bottom graph), where several years of drought combined with
damming the river raised the water temperature, killing many
mussels. (A bar for decline in mussel density is not shown for Site
10 because the change was too slight for the scale of the
chart.)

Trying to make the most of water

When Vaughn began studying one US river — the Kiamichi — in the
early 1990s, she witnessed firsthand a precipitous decline. The
river, which originates in uplands in southeast Oklahoma, was home
to an abundance of freshwater mussels belonging to 31 different
species. Her long-term study documented a drastic drop-off: 60
percent of the mussel population has vanished over the last 20
years.

Significant loss began during a severe drought that started in
1998 and didn’t subside until 2005. “Drought is common in this
region, it’s cyclical, and has been going on for as long as people
have been keeping records,” Vaughn says. But the mussels had to
contend with something new: water management at a dam, built in
1982, that holds back flow of a major Kiamichi tributary. The drier
conditions became, the more water was held back for human use,
elevating the temperature of the remaining water and killing many
mussels.

Photograph shows a researcher holding water-sampling equipment as she stands in a shallow river alongside a canoe filled with various pieces of lab equipment.

Mussel fieldwork at the Kiamichi river in Oklahoma. Mussels play
an important role in nutrient cycling, removing organic matter from
the water, excreting dissolved nutrients back into it, and
depositing these nutrients in the sediment. In the Kiamichi,
mussels can process the entire volume of overlying water during the
summer.

CREDIT: CARYN VAUGHN

But Vaughn also witnessed something hopeful. As she and
colleagues reported in the journal Ambio, while the
Kiamichi lost significant numbers of species during the drought and
overall abundance dropped too, populations in the nearby Little River held
steady
. The key was a difference in management. At the dam on
the Little River, the largest releases of water were in late summer
and fall, the driest time of year, which protected mussels from
high temperatures in shallow water during the drought. In other
words, enlightened water management can help sustain mussels even
as climate change increases human demand for freshwater.
Conservationists are now suing to demand flows high enough to
protect endangered mussels in the Kiamichi.

Mussel power

Endangered mussels can provide legal leverage, but they lack the
gut appeal of a wolf or a falcon. “A freshwater mussel is the
opposite of a charismatic species,” says Vaughn. “People don’t see
it and don’t know what it’s doing.”

But in fact, an adult mussel is a powerful, durable and
efficient water filter inside a hard shell. It can filter up to 10
gallons of water daily, removing algae and organic matter and
transforming water from cloudy to clear so that bottom-dwelling
plants get more light.

It builds its own tissues from the material it filters, locking
up nitrogen, phosphorus and carbon for decades. And it deposits its
waste on the streambed, providing nutrients for bottom-dwelling
algae, insects and other invertebrates, which, in turn, feed
fish.

A riffleshell mussel patiently awaits a visit from a fish, which
becomes temporarily trapped while the mussel releases its larvae.
The tiny mussel offspring will live and develop in the gills of the
fish until the small, juvenile mussels are ready to take up life in
the stream bed.

CREDIT: CHRIS BARNHART / MISSOURI STATE
UNIVERSITY

A study of the Upper Mississippi River published in the
journal PeerJ  found that the relatively healthy mussel
population there filters more than 14 billion gallons of water
daily, removing tons of biomass and depositing tons of carbon and
nitrogen at the sediment surface. Bacteria that transform nitrogen
compounds into harmless nitrogengas thrive beneath mussel beds.

Other studies, published in Environmental Science &
Technology,
showed that the California floater, a threatened
mussel native to California and the Pacific Northwest, dramatically
lowers the amount of fecal bacteria in river water and lakes.

If freshwater mussels can be restored to their former abundance,
write ecologist Danielle Kreeger and her colleagues in the
Journal of Shellfish Research, there’s reason to think
that the creatures can mitigate nutrient pollution and reduce the
costs of drinking-water filtration
. “If your system had a
mussel population historically, and no longer does, I’m not
convinced it’s healthy until you have your natural mussel community
back,” Kreeger says.

Nutrient pollution is a widespread threat to aquatic ecosystems.
Sewage discharges and synthetic fertilizers used in intensive
agriculture release heavy loads of nitrogen and phosphorus to
rivers, triggering harmful blooms of algae and cyanobacteria. As
dead cells sink to the bottom, bacteria digest them, depleting
oxygen in the water. During intense blooms, fish and other aquatic
creatures may suffocate.

Photograph shows a close-up of a darter fish fin, which is covered in tiny translucent larvae of oyster mussels, an endangered US species.

The tiny larvae of the endangered oyster mussel (Epioblasma
capsaeformis
) spend part of their lives nestled in the gills
or fins of a host fish. Glochidia are visible here as translucent
blobs on the fin of a darter fish.

CREDIT: CHRIS BARNHART / MISSOURI STATE
UNIVERSITY

The Chesapeake, North America’s largest estuary, is a prime
case, says Kreeger, who works at the nonprofit Partnership for the
Delaware Estuary. Early settlers there recorded clear waters thick
with fish. The bottom sediments held a bounty of green plants,
mussels and oysters. But starting in the 1600s and accelerating in
the twentieth century, forest clearing and farming increased runoff of
nutrient-laden sediments into the waters
. Shellfish populations
diminished, and the Chesapeake became cloudy with sediment and
algal blooms, and native, bottom-dwelling plants and animals faded
away.

To limit nutrient runoff, farmers must use best management
practices, or BMPs — strategies such as minimizing fertilizer use
and planting wetland vegetation along drainage ditches. Restoration
of native bivalves — specifically, oysters — was recently approved
as a BMP. Efforts so far have focused on the eastern oyster, a
saltwater species that clears the water and is also a valuable
delicacy that Chesapeake watermen harvest. Freshwater mussels may
not be as tasty, but they could help to improve water quality just
like oysters and in a wider range of habitats, Kreeger says.

Lessons from the button trade

But before one deploys mussels, one has to learn to grow them.
Over the past 20 years, several labs in the US have worked on
honing mussel propagation techniques in the lab to raise animals
for restoration efforts. In doing so, they’ve turned to research
from almost a century earlier, when freshwater mussel shells were
used to manufacture buttons, forming the basis of a major US
industry.

The center of the button business was in the Midwest, where a
single mussel bed near New Boston, Illinois, produced more than 9,000 metric tons of shells from 1894
to 1897
— but was exhausted by 1899. It was just one of many
cases in which natural mussel beds were wiped out by
overharvesting. By the 1910s, researchers in Iowa and Missouri were
working to increase the growth and reproduction of mussels to keep
the button industry going.

Photograph shows a mussel shell with two button-sized holes drilled out of it; next to it are two mussel shell buttons. In the late 1800s, “pearl” buttons made from mussel shells were a hot commodity; by 1899 there were some 60 factories in the midwestern US producing millions of buttons each year.

In the late 1800s, “pearl” buttons made from mussel shells were
a hot commodity; by 1899 there were some 60 factories in the
midwestern US producing millions of buttons each year. The industry
severely depleted local mussel populations, but records from that
time have informed today’s mussel researchers who are propagating
mussels in the lab for release into the wild.

CREDIT: STEVE SIMMONS

They left information on how and when to find females carrying
glochidia, and which fish are hosts for local mussels. “We learned
a lot from reading those old papers,” says Barnhart. His own lab
found that the next step, attaching larval mussels to host fish,
was relatively simple: Get glochidia from the female, keep the
water in the tank stirred up, and add the right fish.

Barnhart’s team next focused on getting large numbers of
glochidia through their parasitic phase on host fish to produce as
many juvenile mussels as possible. But most microscopic juveniles
don’t survive, they found. The lab had to figure out how to get
them to grow to an inch or so, at which point “they’re bulletproof
and have a high probability of survival,” Barnhart says.

Still, rebuilding lost mussel populations in the wild is a
complex task even after successful lab-rearing. The Upper Clinch
River in Virginia, where pollution wiped out native populations, is
one of a handful of locations where restoration efforts have been
demonstrably successful.

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In 2005, after a cleanup, researchers tried releasing tiny
juvenile mussels, and host fish carrying glochidia, both with no
luck. Only when they released larger juveniles, cultured in a
lab for a year or more
, did the mussels dig in and, in time,
show behavioral signs of natural reproduction, says Jess Jones, a
restoration biologist with the US Fish and Wildlife Service who
worked on the effort. Female mussels holding larvae in their gills
were spotted rising to the sediment surface to display their lures
for host fish.

A new perspective on restoration

As efforts to restore endangered mussel species continue, and as
scientists learn more about water management practices that can
help them, Kreeger and colleagues are moving ahead with a plan to
apply the techniques on a grander scale, using common, not
endangered, mussel species. The goal is to deploy the mussels to
improve and protect water quality and thus help restore whole
ecosystems.

The project follows years of work propagating and culturing
local mussels and will focus on reviving their populations in the
Delaware and Chesapeake watersheds. Kreeger and colleagues conclude
that five species — the eastern elliptio, alewife floater,
tidewater mucket, eastern pondmussel and yellow lampmussel — would
be prime restoration candidates. All have high
filtration capacities, were historically widespread and abundant
and remain relatively common in the region.

Raising mussels in the lab is tricky; among other steps,
scientists must induce the parent mussels to release their tiny
larvae, called glochidia. For some species, such as this member of
the genus Epioblasma, this task is best accomplished by
presenting the mussel with a fish head on the end of a small
pipette, mimicking a promising host in which the glochidia can
develop. Once released, the glochidia are drawn into the pipette
and can be reared in tanks.

CREDIT: CHRIS BARNHART / MISSOURI STATE
UNIVERSITY

To that end, the Partnership for the Delaware Estuary recently
signed an agreement with the state of Pennsylvania to build a
mussel production hatchery in Philadelphia. “When the hatchery is
built and we switch on the lights and pumps, our goal is to produce
half a million mussels per year that would persist in our streams
and rivers, and provide a return on that investment in the form of
clean water,” Kreeger says.

The water must be somewhat clean to start with: Larval and
juvenile mussels can be poisoned by relatively low levels of
ammonia, a form of nitrogen common in waters polluted with sewage
or agricultural runoff. Decades of dam-building and pollution
caused the drastic decline of mussel populations in the Chesapeake
watershed. Work by Kreeger and others shows that some habitats can now support mussels again.

“There hasn’t been a facility before that’s been able to focus
on producing large numbers of common species,” Kreeger says.
“Therefore, we haven’t really had the opportunity to test a lot of
these concepts in a very significant way — where you put the
numbers up in a river, then see if you get the needle to move on
water quality.

“We’re looking forward to being able to finally test that.”