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.

 

 

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.

 

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Are Mussels Living Rocks or Important Natural Resources?

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.

 

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Why Are Mussels So Imperiled?

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.

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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).

 

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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.

 

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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.

 

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Why Do We Care?

• 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.

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How Can You Help?

To protect mussels the following things can be done.

• Conserve energy in an effort to limit the construction of new hydroelectric power plants.
• Limit or cease pesticide use to conserve soil and prevent runoff into nearby lakes and streams. Integrated Pest Management (IPM) strategies can replace pesticide use--find out more about it.
• Help control soil erosion by planting trees and plants to avoid runoff of soil into freshwater areas.
• Remove aquatic weeds from boat trailers and motors before using again to prevent the spread of things such as zebra mussels. DO NOT throw weeds back into the water.Help support watershed management programs by calling your local resource management agency.
• Support and follow zebra mussel quarantine, inspection, and decontamination programs.

Mussels of the St. Croix River | Mussels of the Ohio River | The Zebra Mussel Threat

 

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Last updated: July 29, 2009