Endangered
Species Facts
America's
Mussels: Silent Sentinels
Are
mussels living rocks or important natural resources? |
Why are mussels so imperiled?| Why do
we care? | Is there hope? | How
can you help?
America's
Mussels, most of us call them clams, are a group of critters that are
so inconspicuous that they are often mistaken for rocks. They lie on
the bottom of our lakes, rivers, and creeks. They barely move and they
eat by filtering water for microscopic food particles. Even their reproductive
life seems boring. The male disperses sperm and the water current carries
it to the female where fertilization occurs. But throughout much of
North America, and particularly in the Midwest, these rock-like creatures
are sending an urgent message.
 |
This
female Higgin's eye pearlymussel packages her larvae into an enticing
lure that will attract a specific fish. When the target fish approaches,
she will expell her larvae at the fish. The larvae attach to the
fish's gills or fins, and hitch a ride for a few weeks while they
continue their transformation into a juvenile mussel. |
North America
has the highest diversity of freshwater mussels in the world. And within
North America, historically the Midwest had some of the highest numbers
of mussels species. Currently, however, in the Midwest (Minnesota, Wisconsin,
Iowa, Missouri, Illinois, Indiana and Ohio) more than half of the 78
known species are classified as Federally endangered, threatened or
State species of special concern. No other group of animals in the Midwest
is so gravely imperiled. To put this in perspective, The Nature Conservancy
reports that about 70 percent of mussels in North America are extinct
or imperiled, compared to 16.5 percent of mammalian species and 14.6
percent of bird species.
Are mussels
so imperiled simply because they're delicate creatures that are on their
way out anyway? NO. Although mussels look sedentary, they can move.
Many species have adapted to the constantly changing situations in streams
and rivers. They can also close their shells to avoid short term exposure
to toxins or other unfavorable environmental conditions. Thus mussels
are tough creatures that can withstand harsh conditions if those conditions
are temporary. The fact that so many species of mussels are imperiled
in the Midwest shows that there have been significant, long-term changes
to our lakes and waterways. And those changes have been so dramatic
that these aquatic species are having trouble surviving. Back
to the top
In the
past, mussels were used by native Americans in the construction of tools
and utensils. Mussels were also an important food source. After European
settlement, mussels were harvested for their shells to make pearl buttons.
By the early 1900's, this multi-million dollar business was in full
swing along the Mississippi River. Because there were no controls on
this industry, mussels were over-harvested and the number of many species
dwindled. This, along with the use of plastic replacements, put an end
to the pearl button industry.
In the
1960's, the Japanese cultured pearl industry began to expand. If tiny
pieces of sand or mussel shell are inserted into oysters they surround
the disturbance with a silicate, creating a pearl. Thousands of tons
of live mussels are harvested from North American rivers each year and
shipped to Japan where pieces of the shells are used as "seed pearls.".
The demand and high prices from this industry has resulted in increasing
the occurrences of illegal mussel harvesting.
Mussels
play an important role in the aquatic ecosystem. Many different kinds
of wildlife eat mussels, including raccoon, otters, herons, and egrets.
Mussels filter water for food and thus are a purification system. They
are usually present in groups called beds. Beds of mussels may range
in size from smaller than a square foot to many acres; these mussels
beds can be a hard "cobble" on the lake, river, or stream
bottom which supports other species of fish, aquatic insects and worms.
Although
their lives appear boring, their reproductive strategies are quite fascinating.
After the male has dispersed sperm that is carried by currents to the
female where fertilization occurs, the fertilized eggs are transformed
into a larval state inside the female. She then packages the larvae
into an enticing lure that will attract a specific fish. When the target
fish approaches, she will expell her larvae at the fish. The larvae
attach to the fish's gills or fins, and hitch a ride for a few weeks
while they continue their transformation into a juvenile mussel. When
the transformation is complete, they drop off of the unharmed fish,
and begin their life as a young adult mussel. Not only is this method
of reproduction interesting to biologists and students of nature, the
fact that mussels require specific species of fish to reproduce means
that mussels are also good indicators of the health of their host fish
populations. Back to the top.
Photo by G. Thomas Watters, Photographer |
| The
ring pink mussel(Obovaria retusa) has been extirpated
from the Midwest. |
Visualize
the Mississippi River (or the Ohio River or any one of the many rivers
and streams traversing our countryside) as it must have appeared to
the first European explorers. The riverbanks were forested, there were
many sandbars, logjams were found periodically, and the water was completely
free of man-made pollutants. Fish were probably plentiful, and if you
sampled the sand and rocks on the bottom, you would have found an abundance
of insects, worms, and other aquatic invertebrates. Then visualize the
Mississippi or another waterway as it looks today. With these visualizations
in mind, it's not too difficult to believe that many of the animals
living in those waterways would have a hard time surviving today. But
let's look at the changes that have been made to our rivers and streams
and how those changes have affected mussels. Return to
top
Dams
Numerous dams have been built on many of our rivers. Whether for hydropower,
recreation, or navigation, dams are a physical barrier, which is a major
threat to mussels. Part of the life cycle of young mussels includes
attaching to the gills of a specific type of fish. These young mussels,
which are actually larvae, are carried by the fish to different reaches
of the river. Eventually the larval mussel drops off the gills, lands
on the bottom, and if it's a suitable area, the mussel continues to
grow. This is how mussels populate lakes and rivers. Because dams are
a barrier to fish, dams also prevent mussels in the upstream portion
from moving to the downstream portion, and vice versa.
A dramatic
example of the impact of dams on distribution of mussels is provided
by the ebony shell (Fusconaia ebena) and elephant ear (Elliptio
crassidens) mussels. The primary fish host for these mussels is
the skipjack herring. The skipjack was present in the Mississippi River
watershed from the Twin Cities south to the Gulf of Mexico. It was actually
a migratory species in the upper Mississippi River watershed where it
was known to spawn in Lake Pepin and probably used other areas in the
watershed as well. But after a hydropower dam was erected on the Mississippi
River at Keokuk, Iowa in 1913, the skipjack herring could no longer
migrate north of the dam. Thus, this species of fish was extirpated
from the upper Mississippi River north of the dam at Keokuk. Along with
the extirpation of the skipjack, all (or most) reproduction of the ebony
shell and elephant ear mussels, upstream of the dam, ceased. Individual
live specimens of these species are still found today in the upper Mississippi
watershed. That is because these mussels are so long-lived and were
so abundant before the dam. However, reproduction of these species no
longer occurs north of the dam at Keokuk.
In addition
to acting as barriers to fish movement, dams impact mussels by changing
the flowing water environment of rivers. The reservoir area upstream
of a dam is a lakelike environment in a segment of river. In most cases,
riverine kinds of mussels cannot survive in lakes. So all the mussel
beds in the upstream reservoir area of the dam are eliminated. Depending
on the type of dam, releases from the dam may not mimic natural conditions,
instead unnaturally high releases of water alternate with unnaturally
low releases of water. The low water conditions are particularly hostile
to mussels (and most other kinds of wildlife in the river). Back to the top.
Sedimentation
and Pollution
Agricultural
production causes eroding soil to run off into rivers and lakes. Bridges
are also sites of high erosion and sedimentation. Large amounts of sediment
entering streams and rivers can bury gravel and rocky bottoms and smother
mussels. Many kinds of mussels cannot live on muddy or unconsolidated
sandy bottoms, they need the river bottoms to be rock, gravel, or firm
sands. The sediment in runoff often carries pesticides with it, which
further pollutes the water. Other contaminants such as PCBs, mercury and
lead are deposited in waterways from industrial plant discharges. A wide
variety of other toxic substances are released from industrial sites.
With navigation
on many rivers and industrial complexes located on riverbanks, there
is always a threat of an oil or toxic chemical spill. Also, many public
and private septic systems empty into our waterways. In the 1970s, NO
live mussels were found in the lower fifteen miles of the Minnesota
and the Mississippi Rivers, from the Twin Cities to Lake Pepin. It was
referred to as a mussel desert. Poor water quality was the reason that
these areas could not support mussels. Since that time, sampling indicates
that water quality is improving. In fact, a recent mussel survey north
of Lake Pepin documented the presence of good numbers of individuals
and numbers of species of mussels. Return to top
Exotic
Species
Exotic species are an ever-increasing threat to our native plants
and animals. For native mussels, infestation of zebra mussels has had
near catastrophic effects. Zebra mussels were inadvertently released into
Lake Erie from a ship carrying ballast water from the Caspian Sea. They
increase in numbers faster than non-native mussels and attach to almost
any hard surface, including native mussels. They reproduce so fast and
in such abundance that the native mussels' movement, feeding, and reproductive
behaviors are stifled. One mussel was found to have over 10,000 zebra
mussels on it. Back to the top.
- Monitors
of aquatic health: the presence of diverse and reproducing populations
of mussels indicate a healthy aquatic system which means good fishing,
good water quality for waterfowl and other wildlife species, as well
as insurance that our water is safe. Conversely, when mussel populations
are at risk, it indicates problems for other fish and wildlife species,
and people too.
- Ecological
value: mussels are natural filters, feeding on algae, plankton, and
silts, they help purify the aquatic system. Mussels are also an important
food source for many species of wildlife including otters, raccoon,
muskrat, herons, egrets, and some fish.
- Economic
value: freshwater mussels have been and continue to be a major economic
resource; first in the button industry and now in the cultured pearl
industry. Mussels from North American form the nucleus of the cultured
pearl industry in Asia.
- Education
and aesthetic value: the study of mussels, their natural history,
and habitat requirements provides interesting and important lessons
on the interconnectedness of the aquatic system and how species adapt
to their ecosystem.
- Cultural
value: Mussels played an important role in the cultural history of
prehistoric and recent native peoples of the Ohio and Mississippi
River basins. They were used as food and the shells were used for
ornamentation, tools, and as a commodity for trade. Indian shell middens
(the piles of shells that native Americans have left behind) extend
for miles along sites of old villages and encampments along the Mississippi
and Ohio Rivers.
- Biodiversity:
Mussels have, and hopefully will continue, to play an important role
in our aquatic ecosystems. Considering that less than 20 mussel species
are found in most other countries of the world, our North American
rivers and streams are truly "rich" with close to 300 species!!
Mussels do not get cancer. Researchers want to know why and mussels
may have additional values in the future that we cannot now predict.
The loss of any of these species will definitely have consequences
on how the aquatic ecosystem functions.
Is There Hope?
Passage
and implementation of the Endangered Species Act and the Clean Water
Act has contributed to the recovery of some species. The Endangered
Species Act provides protection of existing mussel communities containing
threatened and endangered species and has promoted efforts to recover
endangered mussels. Enforcement of the Clean Water Act has resulted
in improved water quality which has allowed recolonization of mussels
and fish in some areas.
Return to top
In order
to protect mussels, thereby protecting ourselves, the following things
can be done.
Clams
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