Fresh Water Mussels in Tennessee

TWRA's Role in Management

Proper management, protection, and monitoring of the surviving native mussel resources are therefore essential to preserve the biologically diverse group of mussels and their use by man.   As with any natural resource, a limited supply is available.   Protecting and enhancing the mussel's resources is the ultimate goal and primary directive of the Tennessee Wildlife Resources Agency.

Description and Status of Freshwater Mussels in Tennessee

Freshwater mussels, collectively called shellfish, clams, bivalves, and unionids, belong to an important group of animals known as mollusks.

Mussels occur in a wide variety of aquatic habitats, from small ponds and streams to our largest lakes and rivers. They provide many natural benefits.   Because they are filter feeders, these bivalves rely on water currents to supply nutrients for growth and reproduction.   Functioning as natural biological filters, they actually clean our lakes, rivers, and streams.

Mussels serve as indicators of water quality.   All are affected by pollution, although some are more tolerant than others, so they can be used to monitor levels of waterborne pollutants.   They efficiently remove silt and suspended organic particles and serve as a basis for studying environmental change over time.

Mussels themselves serve as food for other animals such as fish, muskrats, raccoons, otters, and birds.  Their complex life history makes them valuable for research, and they may have many other uses as yet undiscovered.

"Of nearly 300 recorded species of freshwater mussels in the United States, approximately 130 are or were known to occur within the political boundaries of Tennessee.   The mussel fauna of North America exhibits the greatest variety of species in the world and is concentrated mainly in the Southeast. Except for Alabama, the lakes, streams, and rivers of Tennessee once harbored the most diverse and abundant assemblage of these mollusks known in historic times. But with the settlement of land by European explorers and pioneers came lumbering of the forests, clearing and intensive farming activities, strip mining, industry, and construction of power dams. All of these factors, along with other related practices such as channelization of numerous rivers and the commercial exploitation of mussel shell, brought about major reductions in species distribution and abundance, local extirpation and, in at least a dozen cases, extinction." --- Parmalee and Bogan, "The Freshwater Mussels of Tennessee," 1998

Unfortunately, this decline is continuing at an alarming rate. Many mussel species are now considered endangered or threatened; some have populations limited to only one or two sites.   Forty-two species known from Tennessee are currently on the federal endangered list.   Several species are already extinct. Judging from the growing number listed as endangered, others may soon follow.

In the early 1900s, mussel shells were the primary source material for buttons.   Various shapes (mostly round) were punched out, polished and used in the textile industry.   As a result thousands of pounds of mussels were taken each year, often with little regard for efficiency.   Frequently a single button would be cut from a shell.   With the development of plastics in the 1940s, however, less emphasis was placed on mussels as a button source and native populations began to recover.

Recently, Tennessee's mussels have gained popularity in the cultured pearl industry.   A true pearl results from a natural foreign object becoming lodged within the shell of a mollusc which covers it in multiple layers of nacre.   Natural pearls occasionally may be found in freshwater mussels , it is extremely rare to find a commercially valuable pearl (1 in 10,000 mussels). Usually these are just tiny fragments of nacre called "baroques."

The shells of native Tennessee mussels are ideal for the lucrative cultured pearl industry.   Sections from a mussel shell are taken, partitioned, rounded, polished and inserted into an oyster as the nuclei.    After a period of time (usually 2 to 6 years)a "pearl" results.   Approximately 80 percent of the mussel shells exported from the United States are harvested in Tennessee.   The demand for Tennessee mussel shells fluctuates from year to year.   During peak harvest years, the commercial mussel shell industry in Tennessee employs approximately 2,000 people and provides nearly $50 million to the Tennessee economy according to an industry spokesman.   However, lately, biological problems affecting the survival and production of Japan's pearl producing oysters combined with other factors affecting the cultured pearl industry have reduced the market demand for Tennessee's mussel shells.   Currently, two to five million pounds of mussel shells with a wholesale value of two to six million dollars are harvested annually.

Freshwater mussels have three basic characteristics which help distinguish them from other animals.   In a group of invertebrates called bivalve molluscs, freshwater mussels have (1) a two- valved shell, (2) a soft body and (3) a muscular foot.   The soft body includes respiratory, reproductive and digestive organs.   The foot, often seen extended from between the two valves, aids the mussel in locomotion as well as in burrowing and positioning in the substrate.   The two- valved shell is secreted by the mantle, a membrane-like tissue which surrounds the soft parts.   Food is filtered from the water as it is siphoned across two sets of gills.

The mussel's shell contains several diagnostic characteristics which are helpful in separating different species.   The shell is composed of calcium carbonate and protein.   The often white shiny layer seen inside the shell is called the nacre, or "mother of pearl."   The outer layer or periostracum is made of protein and serves mainly to protect the shell.   Shells have a variety of shapes-round, elongate, oval or tear-drop.   The exterior or outside of the shell can be described by its length, height, width, color and general appearance.   The beak the oldest part of the shell is used to distinguish the front or anterior and rear or posterior part of the mussel.   The end closest to the beak is the front.   The outer part of the shell may be smooth; bumpy; ridged, or have depressions, furrows and wings.   It may be shiny, dull, brightly colored or plain all of which assist in the identification of the many species.

The interior of the shell also displays characteristics that are used to identify mussels.   (However, mussels should not be killed solely for this purpose.)   The inside contains two sets of hinge teeth, both used to hold the two valves together (although a few species lack these teeth); a beak cavity; front and rear muscle scars (places where the muscle attaches firmly to the shell); a shiny nacre; and a parallel line (where the mantle attaches).

The life cycle of a mussel is quite complex. 

Fertilized eggs develop into larvae, called glochidia, within the gills of female mussels. 

Glochidia, when released from the female may come in contact with a passing fish and although harmless to their host attach to the gills, fins or body of that fish.   After a few days to several weeks, the glochidia free themselves from the host, drift to the bottom substrate and begin their lives as juvenile mussels. 

It may take several (2-9) years before juveniles mature and can reproduce as an adult.   

Adults may live 60 to 70 years if conditions are right.

Description:  

The zebra mussel, Dreissena polymorpha, is a freshwater bivalve (containing two halves) mollusk, which rarely exceeds 1.5 inches and has a triangular shape like the letter “D”.   The mussel resembles a small clam with alternating dark and light bands on the shell, resembling the stripes of a zebra.   However, some may be entirely dark or light in color.   The most distinguishing characteristic of the zebra mussel is the tuft of fibers called the byssal threads that grow from the foot and through the hinge. It produces a powerful glue for attachment to any hard surface. Zebra Mussel

History and  Distribution:  

The zebra mussel is native to parts of western Russia near the Caspian Sea. By the 1830s, the mussels had invaded most of Europe through a series of shipping canals. They entered North America through the release of ballast water from shipping vessels traveling from Europe. By the 1990s within the United States, they could be found throughout the Great Lakes, the Mississippi River, and its major tributaries, including the Cumberland and Tennessee Rivers. The expansion of the mussel range has been a function of natural water flow, wildlife activity, and a variety of human pursuits, including boating, fishing, and the bait industry. 

Life  Cycle:  

Zebra mussels typically live 3 to 5 years and are able to reproduce in their second year. A single zebra mussel may produce over 30,000 eggs per reproductive cycle, translating to more than one million eggs per spawning season when the water warms above 54 degrees Fahrenheit.  Fertilized eggs develop into swimming microscopic larvae called veligers. These veligers can remain suspended in the water for 3 to 4 weeks before they find a hard surface to attach to; those that don’t find a substrate will die.  Any hard surface is a suitable place for the zebra mussel to live, such as rocks, metal, wood, vinyl, glass, rubber, fiberglass, intake pipes, boats, plants, other mussels, and bodies of small moving animals like crustaceans and turtles.  Unlike most freshwater mussels, the zebra mussel grows in clusters containing numerous individuals. 

Impacts:

Zebra mussels are filter feeders and feed on phytoplankton (tiny plants) and suspended particulates in the water column. Plankton is an important food source for young fish, native mussels, and other aquatic organisms. A single adult has the ability to filter large amounts of water (1 liter/day), which increases water clarity. Increased light penetration allows submerged aquatic plants to grow in deeper water and spread to a larger area, which may cause problems for recreational boaters and anglers.

The economic impact of zebra mussels can be extreme, due to their need to attach to hard surfaces and the ability to layer upon themselves. Industrial water intake structures are prime locations for attachment, causing reductions in pumping capabilities and even blockages. They can also damage, cause costly repairs, and require frequent maintenance of boats and recreational motors.  Even when they die, their sharp shells wash up on beaches, creating foul odors and cutting the feet of swimmers, which may result in reduced recreation.

Prevention: 

Both the movement of adult zebra mussels and the larval form (veligers) can infect other bodies of water.  Adults can attach to boats and boating equipment that are moored in infested waters. They frequently attach to aquatic plants, which themselves may hitchhike a ride on boats, trailers, and other recreational equipment to new water bodies.  Adults are able to close their shells and survive for many days in cool/moist conditions.  Veligers can be transported in water being held in bilge areas, live wells, boat motors, bait buckets, and puddles of water on the deck of boats.

Research is ongoing to find ways to eliminate the presence of mussels in infested water bodies.  Winter water body drawdowns have had a positive effect in drying and killing the mussels.  Native fish species such as sturgeon, yellow perch, freshwater drum, catfish, and sunfish are known to consume them. Most management efforts are geared at preventing any further spread of zebra mussels. It is important to take the following steps to prevent their spread.

Before moving your boat from one water body to another

Inspect and remove aquatic plants, animals, and mud from your boat, trailer, and equipment. Scraping of adult mussels may be necessary to remove them.

Drain all water from the boat, motor, bilges, live wells, etc.

Dispose of unwanted bait on land, not in the water.

Rinse your boat and equipment with hot water (>140℉), use high pressure, particularly if moored in a water body for more than one day.

Dry your boat and equipment thoroughly (in the sun) for five days. If boats and trailers have been exposed to freezing temperatures for the winter, they are considered decontaminated.

OR Decontaminate after cleaning and draining, use one of the following approved cleaning solutions: