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Freshwater Mussels In Tennessee

Mussels

Freshwater mussels, collectively called shellfish, clams, bivalves and unionids, belong to an important group of animals known as molluscs. Mussels occur in a wide variety of aquatic habitats, from small ponds, 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 water borne 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.

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 mussels resource is an ultimate goal and primary directive of the Tennessee Wildlife Resources Agency

Morphology

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).

Life History

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.
 

Commercial Use

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.

Commercial Musseling Regulations

Annual commercial musseling licenses are available to both residents and non-residents of Tennessee with one exception. Persons from states that do not permit the sale of non-resident licenses to Tennesseans are prohibited from working as a commercial musseler here. A synopsis of the regulations governing commercial musseling can be obtained by contacting the TWRA Fish Division.

There are currently 10 species of freshwater mussels that can be harvested commercially in Tennessee and only those individuals which will not pass through a special ring with an inside diameter specified for that species can be taken. All other mussels must be returned immediately to the bed from which they came.

Management of the Mussel Resource

Proper management is one way to enhance and protect the mussels in Tennessee. This is partially accomplished by understanding the general life history of the animals (how they live, feed, reproduce and grow). Since most mussels take several years to become reproductively mature, Tennessee requires that all undersized and non-commercial mussels be immediately returned to the bed or area from where they were taken. This helps to protect the reproducing individuals and to improve the chances of more juveniles reaching maturity.

Mussel Diver     TWRA Divers Survey the Mussel Population