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America's Mussels, or clams, are a group of animals so inconspicuous they are often mistaken for rocks. Laying on the bottom of lakes, rivers, and creeks, they rarely move and eat by filtering water for microscopic food particles. They can even reproduce without moving. 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.
Mussels are not in trouble simply because they're delicate creatures that are on their way out anyway. Although mostly 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 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 animals have trouble surviving.
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 numbers 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.
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.
Many dams have been built on most 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. As a sedentary animal, this is great strategy for populating lakes and rivers. However, because dams are a barrier to fish, they also prevent mussels in the upstream from moving downstream, and vice versa.
The ebony shell (Fusconaia ebena) and elephant ear (Elliptio crassidens) mussels dramatically examplify the impact of dams on mussel distribution. Skipjack herring is the primary fish host for these mussels. The skipjack lived 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 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).
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.
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.
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.
To protect mussels the following things can be done.
Last updated: April 1, 2014