The Unionid Mussel Strike Force is a team of experts prepared to pounce on the next investigation, wherever it might take place unionid refers to a family of freshwater mussels common in the US. Based in Wisconsin, its core group of scientists stand ready to store and analyze samples fed to them by a nationwide network of biologists as die-offs crop up.
In , when Richard was just settling into his first postgrad job as a Fish and Wildlife Service biologist in the Southeast, the Strike Force had yet to assemble. But soon it would have a reason to form.
It sucks every time you see it. The following fall, and each one since, Richard and his colleagues fell into a routine. At the other end, veterinary epidemiologist Tony Goldberg and the newly formed Strike Force waited. While fungal, bacterial, and viral diseases have been documented for other aquatic invertebrates such as crayfish, oysters, and sea stars, potential freshwater mussel pathogens have only become a recent focus of scientific investigation.
Goldberg, who has studied infectious disease in polar bears, primates, and penguins, suspects mussels could be suffering from their own version of the white-nose syndrome threatening North American bat populations or the Tasmanian devil facial tumor disease first observed in the s.
In , members of the Strike Force compared healthy Wisconsin mussels from the upper Mississippi to dead pheasantshells from the Clinch River.
After incubating the samples, the scientists discovered 42 percent of the pheasantshells contained the bacterium Yokenella regensburgei , a very rare and poorly understood pathogen found in humans as well, and the same pathogen a separate team also documented in mussels during the peak of a recurring Alabama die-off. This is not the only research effort to point to bacteria in mussels sampled from a Clinch die-off. In an independent study of pheasantshells collected in , Virginia biologists noted an unidentified streptococci-like bacteria in one specimen.
The mussels may be suffering from other germs too. Last year, Goldberg and his colleagues reported the first known virus found in wild North American freshwater mussels, which they discovered by examining Wabash pigtoes from the Upper Mississippi River basin.
And there may be more. The ebony shell Fusconaia ebena and elephant ear Elliptio crassidens mussels dramatically examplify the impact of dams on mussel distribution. Skipjack herring is the primary fish host for these mussels. It was actually a migratory species in the upper Mississippi River watershed where it was known to spawn in Lake Pepin and probably used other areas in the watershed as well.
But after a hydropower dam was erected on the Mississippi River at Keokuk, Iowa in , the skipjack herring could no longer migrate north of the dam. Thus, this species of fish was extirpated from the upper Mississippi River north of the dam at Keokuk. Along with the extirpation of the skipjack, all or most reproduction of the ebony shell and elephant ear mussels, upstream of the dam, ceased.
Individual live specimens of these species are still found today in the upper Mississippi watershed because these mussels are so long-lived and were so abundant before the dam. However, reproduction of these species no longer occurs north of the dam at Keokuk. In addition to acting as barriers to fish movement, dams impact mussels by changing the flowing water environment of rivers.
The reservoir area upstream of a dam is a lakelike environment in a segment of river. In most cases, riverine kinds of mussels cannot survive in lakes. So all the mussel beds in the upstream reservoir area of the dam are eliminated.
Depending on the type of dam, releases from the dam may not mimic natural conditions, instead unnaturally high releases of water alternate with unnaturally low releases of water. The low water conditions are particularly hostile to mussels and most other kinds of wildlife in the river. Agricultural production causes eroding soil to run off into rivers and lakes. Bridges are also sites of high erosion and sedimentation. Large amounts of sediment entering streams and rivers can bury gravel and rocky bottoms and smother mussels.
Many kinds of mussels cannot live on muddy or unconsolidated sandy bottoms, they need the river bottoms to be rock, gravel, or firm sands. The sediment in runoff often carries pesticides with it, which further pollutes the water.
Other contaminants such as PCBs, mercury and lead are deposited in waterways from industrial plant discharges. A wide variety of other toxic substances are released from industrial sites. With navigation on many rivers and industrial complexes located on riverbanks, there is always a threat of an oil or toxic chemical spill. Also, many public and private septic systems empty into our waterways.
It was referred to as a mussel desert. Poor water quality was the reason that these areas could not support mussels. Since that time, sampling indicates that water quality is improving. Over several weeks, it begins to develop gills, a foot, and other internal structures to become a juvenile mussel.
The now fully transformed, but still microscopic, juvenile will drop off the fish and begin its life on the stream bottom. If the mussel is lucky enough to grow into an adult, it may live years or more depending on the species.
When settlers first arrived on these shores, they were astonished to discover rivers practically paved in freshwater mussels. Unfortunately, over the proceeding years, water pollution, dams, and introduction of exotic species has taken its toll on many species.
It is estimated that 70 percent of the mussel fauna in the U. Over seven percent have gone extinct, and another 50 percent are receiving special protection under the U. Endangered Species Act.
The situation does not get much better for Virginia. Of the 82 species in the Commonwealth, only 30 percent are considered stable with the remaining in decline. Given their high level of imperilment, a number of states and conservation organizations are trying to recover freshwater mussels by actively propagating them. The process begins with checking glochidia viability. A few glochidia are removed from the female mussel and salt grains, used to simulate the chloride found in fish blood, are placed alongside each other in a small water dish.
Mature or viable glochidia rapidly close, while immature glochidia will react slowly or not at all. If the glochidia are viable, they are removed from the female and placed into an aerated container containing the known host fish. Under natural conditions, the number of glochidia that infest is so low that it rarely causes damage to its host; however, care must be taken not to over infest, which can cause the fish undue stress or death in the laboratory.
Fish are infested over a short period and checked to determine the degree of infestation. The best control is to not introduce clams to the pond. Sometimes our management decisions have unintended consequences that undo positive things we are trying to achieve. This is especially true when we add new species to a system such as a recreational fish pond. In managing recreational largemouth bass and bream ponds, stocking additional forage species should only be done after careful consideration of the potential negative consequences.
While the intention might be to improve growth or numbers of a particular fish, stocking alternative forage can lead to a dramatic imbalance in the pond system, which leads to poor fish growth and other management problems. A good example of this is adding freshwater clams, particularly Corbicula or Asiatic clam Corbicula fluminea , to a pond to try to improve the growth of redear sunfish Lepomis microlophus.
Redear sunfish are also known as shellcrackers because they often eat snails and small clams. One might logically think that adding clams to a pond would be beneficial to shellcrackers; however, these mollusks can become so abundant in the pond that they remove much of the algae.
When this happens, the productivity of the entire pond goes down, including the growth and abundance of the fish. There are four groups of freshwater clams or mussels in North America: fingernail clams, freshwater pearly mussels in the order Unionidae and often referred to as unionids, zebra mussels Dreissena spp. Both fingernail clams and unionid mussels are groups native to North American streams and ponds, while zebra mussels and Corbicula are invasive species from Asia and Europe.
Fingernail clams Figure 1 are small mollusks commonly found in natural and human-made ponds. The largest species in this family may reach 1 one inch in diameter. Fingernail clams are common food for many fish, including redear sunfish. These small clams pose no threat to pond management, but it is likely that they are already in the pond and adding more will do little good. Unionid mussels Figure 2 are considered to be some of the most interesting water animals.
Some can grow to a very large size, sometimes exceeding 12 inches in diameter. Some larger unionids found typically in large streams make colorful pearls in an array of shapes.
While many species are quite common, native freshwater mussels include some of the rarest and most endangered species found in streams. Unionids have parasitic larvae called glochidia that attach to the gills and skin of fish and some frogs and salamanders.
The larvae live on the fish until they transform, drop off, and burrow into the pond or stream bottom to become free-living, filter-feeders. These mussels are most likely introduced into fish ponds while they are attached as larvae to fish. Unionids are not a significant food source for most pond fish because the mussels are buried when they are small and they are too large as adults. However, unionid mussels are food for some species of wildlife such as muskrats, beavers, racoons, and otters.
Piles of shell on the bank are an indication that the pond has both the mussel and a wildlife species that eats them. Only a few species of common unionid mussels thrive in pond conditions. Those that thrive can develop large beds, but they usually do little harm to a pond. It is possible, though, that they will reach high enough numbers to filter the pond water, reducing the algae and leading to reduced growth of the fish. To prevent unionid mussels from getting into a pond, do not stock the adult mussels or bring in wild fish from streams and reservoirs that may be infected with glochidia.
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