Aug. 4, 2006

Spreading faster than a river runs, more powerful than a native mollusk, and able to leap onto the hull of any wandering ship. Look! … It's a shellfish. It's a freshwater mollusk. It's a zebra mussel. And this super-nuisance is coming to a waterway near you.

Adult zebra mussels cover a rock. (J. Ellen Marsden, Lake Michigan Biological Station.)

The fingernail-sized, soft-bodied zebra mussel dwells in a black-and-white striped shell that typically spans less than 5 centimeters. But don't let this petite pest fool you: The little mussel is having a huge effect on North American ecosystems. Biologists, in conjunction with the U.S. Army Corps of Engineers, the U.S. Fish & Wildlife Service and the U.S. Geological Survey, are trying to prevent the further spread of the super-mussel. Unfortunately, kryptonite is not an option.

Zebra mussels (Dreissena polymorpha) originated in the former Soviet Union and are believed to be native to the drainage basins of the Black, Caspian and Aral seas. Europe has been battling its spread since the late 18th century. They first appeared in Great Britain in 1824 and have moved on to inhabit Denmark, Sweden, Finland, Ireland, Italy and the rest of western Europe.

The mussels attach en masse to the bottoms of traveling ships or are carried in freshwater ballast that is discharged into other bodies of water. Once introduced at major ports, zebra mussels spread through local waterways by attaching to recreational boats, which can move from lake to lake, as well as through canal systems.

Their rapid invasion in North America began in 1988 after they were discovered in Canada's Lake St. Clair, which connects to Lake Huron and Lake Erie. By the early '90s, the mussels had escaped the Great Lakes basin and had invaded all five Great Lakes, the St. Lawrence River, the Illinois and Hudson rivers, the Finger Lakes region of New York, and the Mississippi River basin.

Clustering hitchhikers

Crayfish covered with zebra mussels (Ontario Ministry of Natural Resources )

Within a decade, they were thriving in 23 states and were established in most of the large navigable rivers in the eastern United States. The mussels since have made their way throughout the Midwest and have been found as far south as Alabama and Louisiana. They have hitchhiked up the Arkansas River as far west as southcentral Kansas and up the Missouri River to northeastern Nebraska. Slowly but surely, they are heading for the Pacific Coast.

In 2004, zebra mussels made a brief appearance in Lake Mead and were found on the Washington-Idaho border. The stampede, however, may be slowing. Californian Department of Water Resources officials recently reported that the state is mussel-free. "We only have found zebra mussels on boats being trailered into California. These boats are quarantined and cleaned before launching," DWR's Tanya Veldhuizen says. The DWR is working to educate the public and have prepared a draft plan on how to deal with the mussels if they do arrive.

But Mother Nature seems to be on the zebra mussel's side. Stopping the invasion has grown into a monumental headache for water officials and biologists. Few natural predators exist to keep the mussel population in check, with the exception of migratory ducks, lake sturgeon, yellow perch, catfish and sunfish. Once attached, the zebra mussel has a life span of 4 to 9 years and is sexually mature within a year. The female can lay up to 1 million eggs during a single breeding season. Spawning usually occurs in the summer and lasts for two to five months.

Adult mussels secrete durable elastic strands, called byssal fibers, which attach to almost anything that floats, such as boat hulls, crayfish, or turtles. They can form a barnacle-like crust on any hard surface, including other mussels, and once attached, it is almost impossible to kill them.

Zebra mussels also are transported by waterfowl, in bait buckets, bilge water, or even in boat engine cooling water. These durable creatures can survive out of water for up to five days. The mussels can tolerate only slight salinity, but they can adapt to brackish waters. They prefer high calcium, so some waterways are more at risk for infestation.

Freshwater nuisance

The full effects of the zebra mussel infestation may not be understood for decades, but ecosystems already are showing the strain. Large mussel clusters are impairing the natural balance in waterways, regardless of size. Each mussel is capable of filtering a liter of water a day and consumes massive amounts of phytoplankton. When millions of mussels are present, the food chain is directly impaired as other organisms, such as native fish, mollusks and birds, compete for nourishment.

"Our best hope is that zebra mussels will eventually come into a balance with all the other organisms before any more impacts occur." Amy J. Benson, USGS

As a result, native mussel species are experiencing a significant decline. These endangered species are being wiped out by zebra mussels that attach to their shells, preventing reproduction and eventually suffocating them. When a mollusk is coated by a colony of zebra mussels, it cannot open its shell to eat. As a result, 90% of the native mussels in Lake Erie have been decimated.

Humans also are feeling the direct effects as massive colonies clog lake and river intake pipes. The mussels migrate into water pipes and constrict the water supply to hydroelectric and nuclear power plants. They invade public water supply plants and industrial facilities, compromising heat exchangers, condensers, and air conditioning and cooling systems. The mussels have been known to cause pump failures, which can lead to water outages. Plus, the required cleanup is expensive and time-consuming.

Infested riverbeds and beaches become decomposing mussel graveyards. Littered with dangerously sharp shells, shorelines are covered with mussel remains that can easily cut human flesh. The weight of a colony of mussels has been known to sink a navigational buoy at sea, and encrustation on dock pilings can render them unstable. Large clusters can even affect the structural integrity of steel after continued corrosion. As for recreational boats, smaller mussels can make their way into engine cooling systems.

Zebra mussels clog a 2-inch pipe. (Don Schloesser, Great Lakes Science Center, National Biological Services)

Once zebra mussels set up house, it seems as though they are there to stay. No effective and inexpensive way to eradicate them has been found, so far; in the meantime, research and public education continue in an effort to slow the spread. Experts have tried a variety of remedies with some success, including hot water (above 140° F), oxygen deprivation, thermal steam injections, electrical current, acoustic vibration, CO₂ injections and exposure to ultraviolet light. Without viable options, however, most officials rely on biocides (such as chlorine) for mussel management. But these chemicals are nonselective and may be toxic to other organisms.

Rock-solid remedy

Filters and screen mesh can be used to impede the assault of adult mussels on drinking water systems, but veligers pass through easily. Intake and distribution flow rates can be increased to prevent the mussels from attaching, but ultimately, scraping off the mussels may be the most effective method.

"Our best hope is that zebra mussels will eventually come into a balance with all the other organisms before anymore impacts occur," Amy J. Benson with the U.S. Geological Survey explains. "In a small closed system, such as a reservoir, drawdowns can strand zebra mussels on the banks. In cold winter climates, they can be frozen with a drawdown."

In 2002, zebra mussels were discovered in an abandoned 12-acre rock quarry in Virginia. After a 3½-year battle, the Virginia Department of Game and Inland Fisheries confirmed the first successful eradication of zebra mussels from a large, open body of water in North America. The solution: potassium. The quarry was injected with 174,000 gallons of potassium chloride during a three-week period. The concentration was far below the level that would harm the environment or humans, but more than double the minimum level needed to kill the zebra mussels.

"The quarry was used for recreational diving and may have been contaminated by SCUBA gear that was used at an infested lake," Benson says. "It is also possible that the mussels were intentionally introduced to clear the water for better diver visibility."

Adult zebra mussels engulf a rock. (Minnesota DNR)

Benson also reports that promising research is in the works in England with the development of a potassium chloride capsule, called the "BioBullet." This edible particle dissolves within the zebra mussel's digestive tract, releasing potassium chloride. Researchers say that this does not pollute the environment because the capsules rapidly degrade and disperse in water.

Research pays off

On the home front, Daniel Molloy, New York Sea Grant researcher with the University of the State of New York, has been leading zebra mussel research efforts since 1991. Molloy and his colleagues have developed a bacterial control method that kills selectively because of a naturally occurring compound. The strain does not harm untargeted species, such as fish and native mussels, and is actually dead before it is fed to the zebra mussels. "Currently, there is nothing available for controlling zebra mussels in open waters, and our bacterium is the only promising candidate. But first, we want to succeed inside power plants and other infested infrastructures," Molloy says.

"Currently, there is nothing available for controlling zebra mussels in such open waters, and our bacterium is the only promising candidate. But first, we want to succeed inside power plants and other infested infrastructures." Daniel Molloy N.Y. Sea Grant researcher, University of the State of New York

Molloy and his team have been working for years to identify the natural toxin. They are trying to isolate the gene that produces the toxin as part of a project funded by the USDOE National Energy Technology Laboratory. "Once we succeed at that, we should be able to identify the toxin and have the gene produce even greater quantities of this natural product so we can obtain even higher mussel kills with our dead cells," Molloy explains. "We want to be able to consistently kill over 95% of the mussels following a single six-hour treatment. Now, we can consistently kill above 70% of the zebra mussels in pipes after such a short treatment." By comparison, chlorination often is done continuously for weeks.

The research team's efforts to date have been aimed at controlling zebra mussels in infested pipes, but Molloy believes that because the bacteria is selective, it may be used in open waters, such as lakes and rivers. "Our research includes the successful commercial development of another strain of bacteria that is applied to streams and rivers to control another aquatic pest, the black fly, so treating mussels in lakes and rivers is not such a far-fetched idea," he says.

Recently, the project received funding from the National Science Foundation, which supports commercializing biotechnology products. The details of the program are provided in the "Biological Control of Zebra Mussels with Pseudomonas fluorescens: An Overview." [PDF]

What every boat owner should know

Biological invasions are the second leading cause of extinction, after habitat destruction. "Public support in the battle against invasive species is crucial. The more educated the public is, the easier it will be to slow the spread of biological disasters like the zebra mussel," Benson says.

Private boat owners may be the best weapon to stop the mussel invasion. Environmental groups are working to educate boaters and fishermen so that mussel larvae is not transferred from one water body to another. Proper cleaning of a boat after exposure to an infested waterway is crucial. Experts suggest that the boat be rinsed with hot water and kept out of the water for several days in high heat. The runoff also must be prevented from making its way into a storm drain or any other drain.

On the frontlines Several institutions have been instrumental in the battle to minimize zebra mussel migration. Among the best known is the NOAA National Sea Grant College Program, with offices in each coastal and Great Lakes state, including The Great Lakes Sea Grant Network and The New York Sea Grant office. The National Biological Service is also a great resource for nontechnical information. The USGS also offers a geo-referenced database of zebra mussel occurrences online: • Time Series Map • Distribution in U.S. Lakes Map • Query By State

Even though the mussels cannot be seen by the naked eye in the larval stage, any area where water may have accumulated should be drained before transporting, including the engine, trailer and wheels, bilge, live wells and bait buckets. Anything that has made contact with the infested water needs to be thoroughly cleaned, and all screens, water intakes and drain pipes should be inspected. Hulls also can be protected with anti-fouling paint to prevent mussels from attaching to a boat.

Nationwide, environmental management agencies are working to protect endangered native species. Many states have added zebra mussels to their prohibited species list, making it illegal to bring in live specimens. New laws include The Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990, which was subsequently amended by The National Invasive Species Act of 1996. The National Aquatic Invasive Species Act of 2005 [PDF] reauthorized and amended the 1990 law.

"The new amendments are targeting ship ballast water as a pathway for many introductions, marine (seaports) and fresh water (Great Lake ports)," Benson explains."It does not take a lot of effort from an individual to make a difference," she says. "We just need more people to care and make the effort."

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U.S. Army Corps of Engineers: Zebra Mussel Research Program

U.S. Army Corps of Engineers: Zebra Mussel Information System

USDOE National Energy Technology Laboratory Project Fact Sheet [PDF]

USGS Zebra Mussel Page

Virginia Department of Game and Inland Fisheries

Wayne State University Zebra Mussel Page