America's Mussels: Silent Sentinels
Text and photos from the USFWS Midwest Region website

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, along with Kentucky and Tennessee) more than over 25% of the 140 known species are currently listed as Federally endangered, threatened, candidate or State species of special concern. 13 species are currently considered to be extinct.  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 expel 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.

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

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.