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. |