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Endangered Mussel Recovery

Status of Freshwater Mussels in America

View a fact sheet on Genoa National Fish Hatchery and Native Mussel Restoration. (352 KB Adobe pdf document)

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Freshwater mussels, or "clams" as many people call them, have a long history on the North American continent. These animals have been found in the fossil record throughout the continent. These quiet, bottom dwelling animals fill a much larger role in their aquatic world than most casual observers would guess from a quick glance into their habitats.

Mussels directly provide food for many other aquatic as well as terrestrial animals anywhere they are found. When they are young and very small, they provide food for aquatic insects, small fishes, water birds, and a myriad of tiny predators that form the basis for many food webs. As they become larger they serve as food for larger fishes, waterfowl, and important fur bearers such as muskrats, raccoons, and otters. Mussels are filter feeders, which is to say that they remove small particles of food such as algae, detritus, or small animals from the water around them to provide energy for their growth and metabolism. This energy is in turn recycled up the food chain when they are eaten by other predators until in many instances it is utilized by a terminal predator such as man or another fish eating mammal or bird.

Besides providing an important strand in the complicated food web of the river, stream, or lake that they dwell in, clams are very important indicators of the health of an aquatic ecosystem. As filter feeders, mussels are exposed to toxins or other deleterious environmental conditions at a more acute level than many higher trophic level organisms that we as humans may be familiar with. Minute levels of some toxins, or chronic environmental stresses such as siltation, low oxygen, or high ammonia levels can cause catastrophic losses in mussel communities long before they are noticed in higher fish populations.

Genoa NFH has been working with Upper Mississippi River mussels since 1996. The hatchery has served as host to several refugia trials during the late 1990's, and was the first National Fish Hatchery in recent times to act in this capacity. The program then evolved in 1999 to include propagation of the Federally Endangered Higgins-Eye Pearly mussel. This species of mussel has been on the federal protection list since 1976 and was one of the first aquatic invertebrates to be afforded protective status. The hatchery propagated and released an estimated 1.1 million Higgins-eye during the first four years of the project into the Upper Mississippi River watershed. The facility has expanded into other State listed threatened and endangered species of mussels, and is currently propagating host fish and/or mussels for three Midwestern states.

Freshwater mussels have long played an important role to humankind in North America. The first Americans utilized the varied species of clams found in what is now the United States for food, tools, ornamentation, and trade for thousands of years prior to European settlement. During the late 1800's and early 1900's a huge shell button industry centered around the upper Mississippi River drainage. This industry provided millions of shell buttons annually for clothiers around the world until the advent of plastic buttons in the early 1940's.

From the 1970's until the present time the United States has exported tons of freshwater mussel shell to Asian markets to support the marine cultured pearl industry. The process for the artificial culture of almost all of the world's marine pearls required a small "seed" pearl, constructed in most cases from the shell of a freshwater mussel. This seed pearl is inserted into a pearl oyster and after a relatively short time period the finished pearl is removed. Until the late 1990's the majority of seed pearls were constructed from American freshwater mussel shells.

Most recently the bio-medical research community has shown interest in freshwater mussels for promising insights that they may provide to cancer research. The importance that these animals may have for future research activities is impossible to gauge, but must be considered significant, given increased insights into their complicated biology and reproductive adaptations that science is uncovering.

North America is home to nearly one third of the world's known species of freshwater mussels. Of the approximately 1,000 known species described worldwide, 297 are found in the United States. In North America, freshwater mussels represent the most widely endangered group of any animal currently monitored. Of the nearly 300 species known to have lived in the U.S., 18 are believed to be extinct, and 60 are currently listed as federally endangered or threatened. The majority of losses to the U.S. mussel community have been through habitat destruction and, most recently, invasive species. In some river systems more than 70% of native mussel species are being impacted or are in serious decline.

Biology

The life cycle of most freshwater mussels is a complex process that involves the parasitism of specific fish or amphibian hosts. A typical reproductive cycle, as represented by the Butterfly Mussel,(Ellipsaria lineolata) is described in the following illustration.

Typical life cycle of freshwater mussel.

Male mussels release sperm directly into the stream, river or lake where the female mussel draws it into her body during normal respirations. The eggs, which are being held in modified "pouches" along the females' gills, are fertilized and held for periods ranging from several weeks to months while they develop into larvae called "glochidia."

After the eggs have matured to this "glochidial" stage, the mussel must deliver these tiny larvae to an appropriate host to complete their development into free-living juvenile mussels. Different mussel species have developed a myriad of strategies to ensure the appropriate host organism comes into contact with these small parasitic larvae. Some mussels have developed modified fleshy protuberances called "palps" which mimic small prey items to predacious fishes. The mussel goes as far as to rhythmically move these "lures" to attract a strike from predatory or territorial fish. As the fish strikes, the mussel releases thousands of larvae, which the unwitting fish breathes in where some may attach to the exposed gill surface. After a period of days to months, depending upon the species, the larva continues its metamorphosis into a free living juvenile mussel. After the mussel has completed its transformation, it falls off the host and begins its independent life, which may last for decades if conditions are favorable.

Other mussel species have adopted a less aggressive strategy whereas they release their larvae in "packets" called conglutinates. These conglutinates come in many sizes and shapes, all of which have one thing in common: they contain from dozens to thousands of larval mussels waiting for a hungry fish or amphibian to mistake the packet for something edible. Once the predator has bitten, the conglutinate membrane containing the larvae ruptures, releasing the parasites directly in the host's mouth where they encyst on the gills.

U.S. Fish and Wildlife Service