Over time, various organisms become extinct due to natural occurrences such as disease or an increase in predators. The numbers of freshwater mollusks within the Ohio River basin have drastically declined within the past century. This is not due to natural causes; it is due to over harvesting, toxic chemical spills, channelization, dams, siltation from runoff, urbanization, mining, and competition with alien species for nutrients.
The freshwater mussels are in a phylum known as Mollusca. The Mollusca is the largest animal phylum next to the Arthropoda, in that there are approximately 50,000 living species and some 35,000 fossil species (Cleveland, et. al., 1993). The name Mollusca indicates one of their most distinctive characteristics, their soft bodies (Cleveland, et. al., 1993). The name mussels is given to the aquatic molluscs of the class Lamellibranchia or Bivalvia.
Uses for the Freshwater Mussels
Freshwater mollusks have been harvested throughout the years for a variety of uses. Historically, freshwater mussels were used as a source of food (Helfrich, et. al., 1997) . Not only did humans use the mollusks as a food source, but we also used the shells for decorations, jewelry, and for making pottery and tools.
After the turn of the century, the shell's shiny pearly appearance prompted the production of pearl buttons (Helfrich, et. al., 1997) . Millions of freshwater mussels were harvested throughout the rivers of North America to produce the pearl buttons (Helfrich, et. al., 1997). The pearl buttons were drilled out of the freshwater mussel shells, trimmed, ground, and polished to produce the buttons. They were popular within the clothing industry, but with the production of more durable plastic buttons, the pearl button industry died (Helfrich, et. al., 1997). However since the 1950's a new demand for mussel shell has evolved, supplying the Japanese cultured pearl industry (Helfrich, et. al., 1997) .
Today in the United States, wild mussels can still be harvested in some states, but other states forbid the commercial harvest of freshwater mussels (Helfrich, et. al., 1997). Each year, thousands of tons of live mussels are harvested and steamed open to remove the mussel, and the shells are packaged and exported to Japan (Helfrich, et. al., 1997). There, the shells are cut and ground to produce round beads that are then placed within the Japanese pearl oyster to produce a commercialized pearl. Globally, the cultured pearl industry has become a multi-million dollar industry (Helfrich, Louis A., et. al., 1997) .
Why are they declining in numbers?
Freshwater mussels are considered to be "environmental monitors" of rivers, streams, ponds, and lakes. They indicate the water quality in terms of their numbers within their particular niche. Since freshwater mussels are towards the bottom of the food chain, toxic chemical spills, chemical runoff from fields, and other chemical pollutants will be present within their tissue. Predation of the polluted freshwater mussels could eventually lead to biomagnification of the pollutant within the predator thus leading to the death of the predator. Other pollutants in the water could affect a host fish needed for the reproduction cycle of a particular mussel species. If the pollutants affect the host fish to the point where the host fish becomes extinct, the particular freshwater mussel that needed that host fish to reproduce will follow its host fish into extinction.
Siltation "smothers" the mussels with silt, thus decreasing respiration. The added silt could also decrease the dissolved oxygen present within the the water. This would not only affect the mussel community but also other aquatic organism that need the oxygen to survive.
Impoundments increases the depths of the water for boats and other watercrafts to pass over shallow waterways but it also decreases the water temperature within the benthic region. Freshwater mussel reproduction is very dependent on the temperature of the water. This temperature decrease will inhibit reproduction cycle of the various mussel species. Either the reproduction cycle will not take place or the reproduction cycle will take place but later on in the year, when the benthic regions water temperature increases. This decreases the amount of time the young juvenile needs to develop into an adult, thus threatening the survival of the juveniles through the winter.
Stream and river mining affects the mussel populations by crushing or burying the mussel population present within the substrate being mined. The mining will also change the particular niche needed for the specialized mussel to reproduce and survive thus decreasing the population of that particular species.
Various experiments and projects are being done to better understand how freshwater mollusks live and interact with their environment. Some of the projects that are being done consist of studying a host fish and matching it up with the particular mussel that needs that host fish to carry its glochidia. Other experiments include studying possible substitutes to take the place of a rare host, and also the correlation of the temperature and glochidia dispersal.
More work needs to be done within this zoological field to understand the freshwater mussel. If we don't act now, the population of the freshwater mussel may become extinct and the only time that you'll see a freshwater mussel is in a book or museum collection.
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