Baby zebra mussels, no bigger than a grain of rice, provoked an emergency response across the country in early March, and now state and federal officials are contemplating changes to protect the Northwest from an invasive species that some people have unknowingly invited into their fish tanks.
Nothing official has been proposed, but experts are looking at possible procedures and import restrictions on certain brands of aquarium “moss balls,” which have been found to harbor invasive zebra mussels. Wyoming has already imposed a ban on imports of moss balls, and Arizona (PDF 371 kb) requires an inspection to certify that the fluffy green balls are free of aquatic hitchhikers.
The threat from the tiny zebra mussels has been given a top priority, said Capt. Eric Anderson, aquatic invasive species enforcement manager with the Washington Department of Fish and Wildlife. If the invaders were ever to become established, they could perturb the entire food web and destroy the habitat of many aquatic species, including salmon, he said.

Extensive efforts to keep the invaders out of Northwest waters has so far been successful. The Columbia River and its tributaries is the only river system in the U.S. that has not fallen victim to the mussels, which grow in three dimensional clusters in freshwater. The mussels have the potential to take over salmon-spawning streams and wreak havoc on the piping networks of dams, water systems and sewers.
Anderson told me that the emergency triggered by the infested moss balls could be viewed in a positive light, as it showed how a nationwide network can mobilize quickly to deal with a sudden threat. Pet stores have always been a potential source of invasive species, he said, but their response to the problem with zebra mussels raises confidence that threats can be managed.
As soon as stores were alerted to the problem, the two major pet store chains — Petsmart and Petco — immediately pulled the suspected moss balls off the shelves and sent out alerts to their customers. The West Coast distributor also halted all shipments. As of last week, 171 aquarium-supply stores across Washington state had been alerted to the problem.
“On the bright side …, this opens up a dialog with the pet industry; it’s a channel of communications that did not exist,” Anderson said. “I look at this as a really good opportunity to get cooperation going like it has never gone before.”
The initial discovery was made in February when a Seattle Petco employee reported that multiple shipments of Betta Buddy Marimo Balls contained the tiny mussels lodged within the filamentous green algae. The employee posted his observation on the U.S. Geological Survey’s database of Nonindigenous Aquatic Species, and the zebra mussels were confirmed by an agency expert, setting off a nationwide hunt for the infested moss balls.
It turns out that the specific brands of imported moss balls containing the zebra mussels were taken from a natural source — probably a lake — located somewhere in Ukraine. (I’m hoping that the U.S. Fish and Wildlife Service can eventually provide more details.) Some brands of moss balls on the market are harvested from tanks in aquaculture facilities and do not contain zebra mussels, according to Anderson. Some of those products are still being sold, and that’s OK, he said, but it adds some confusion to the story.
Before import, most of the water was removed from the Ukraine moss ball containers. After their arrival, water is added back before distribution. The California distributor, which had shipped many of the infested moss balls along the West Coast, apparently intermingled natural and cultivated products in tanks, Anderson said. Consequently, the distributor ended up destroying much of the supply before thoroughly disinfecting the tanks.
The distributor reported shipping about 100,000 packages of moss balls every two weeks, and it appears shipments were going out for months, Anderson said. It isn’t clear what percentage of the mussels were still alive when they reached people’s aquarium tanks, but the potential threat could be very real.
Since the initial discovery, other distributors have been identified in Florida and states in the Northeast, but the total amount of product and where all the moss balls might be now is a matter still under investigation. At last count, zebra mussels have been found in moss balls in 32 states and five Canadian provinces.
Aquarium owners are advised to carefully destroy any suspected moss balls that might contain zebra mussels and avoid dumping potentially contaminated water down the drain, since the mussels might grow within the pipes or get into larger sewage systems. See info from USFWS. Methods of disposal and disinfection also are described in a news release from WDFW and in a video from the Oregon Department of Fish and Wildlife shown on this page, starting at 2:11.
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Questions of potential quarantine for all moss balls and other control measures are being considered at the regional level by the Columbia River Basin Team and affiliated Aquatic Invasive Species Network, which are under the purview of the Pacific States Marine Fisheries Commission. (See Northwest strategy for invasive mussels [PDF 1.6 mb]). These issues are scheduled to be discussed this week with the team, which includes representatives from Washington, Oregon, Montana, Idaho, Wyoming and British Columbia.
Because this issue involves federally recognized invasive species, the U.S. Fish and Wildlife Service is leading the investigation into the source and distribution. U.S. Department of Agriculture and customs officials also play a role in the import of vegetation, and either could develop new inspection or treatment procedures.
Some chemicals have been identified that can kill mollusks — including zebra mussels and snails — without harming the vegetation. One idea would be to treat all imported moss balls — and perhaps other aquarium vegetation — before sale to aquarium owners. The treatment described by one internet retailer involves keeping the moss exposed to a chemical for at least two weeks.
If treatment to kill zebra mussels becomes a legal requirement for import, then minimum protocols would need to be approved, Anderson noted. The type and concentration of chemical, length of exposure and other factors would need to be tested for effectiveness in a laboratory.
Experts are still brainstorming other issues, such as whether moss balls can be imported safely and how to strengthen the invasive species network for future invaders that could come through pet stores and get loose in the wild.
Invasive species are not really invasive if they are kept out of the environment, yet the threat is considered so severe that a person merely possessing a listed species could be charged criminally with a Class C felony. It all seems like overkill until one considers the potential for damage — a lesson we should have learned during the COVID-19 pandemic, which threw some countries into a health and economic spiral while others avoided major problems.
If zebra mussels were to grow out of control in Washington, as they have in many states, it could easily cost Washington taxpayers hundreds of millions of dollars each year, according to Justin Bush, executive director of the Washington Invasive Species Council.
As with all invasive species, the strategy begins with prevention and early detection, Bush said, followed by rapid response and control actions. A major focus in Washington state has been to intercept pleasure craft coming here from out of state that might be carrying zebra mussels or other aquatic invaders on their hulls. Washington state is even using a trained dog to sniff out the mussels, as described in a news release from WDFW.
Other areas of the country, including the Great Lakes, have seen major economic damages after the mussels get into water-distribution systems for drinking water, sewage and irrigation. Still, the Northwest could face even greater problems with its numerous dams and ongoing struggles to save salmon from extinction.
“We have the most to lose,” said Anderson, explaining four major threats that zebra mussels pose when considering salmon alone:

1. Disruption of the food web: Zebra mussels consume huge amounts of algae, a critical food supply for juvenile salmon as well as prey for adult salmon and other species.
2. Water-quality changes: In consuming algae, Zebra mussels increase the transparency and visibility of the water, changing predator-prey relationships and increasing the growth of vegetation.
3. Damage to dams: Zebra mussels growing on fish ladders can scrape and injure adult salmon that already have trouble passing over the huge impediments. Juvenile salmon, moving downstream, can be literally shredded “like in a Cuisinart” as they pass through encrusted pipes.
4. Habitat destruction: Zebra mussels can colonize stream and river beds, turning loose gravel and rocks into crusty substrate no longer suitable for spawning.

Concerns about the threats of invasive species that might be released by individuals is so great that the U.S. Fish and Wildlife Service, in concert with the Invasive Species Action Network and state agencies, has developed a specific campaign of prevention called Don’t Let it Loose. Check out specific campaigns designed for Washington state, including programs by the Washington Invasive Species Council.