Estuary News

March 2021

Invasive Mussels Hide in Aquarium Moss Balls

A few weeks ago, someone working in a big-box pet store in the Seattle area informed the U.S. Geological Survey that they had seen suspicious mollusks in ornamental aquarium plants that were being offered for sale. Federal scientists confirmed the presence of zebra mussels tucked away in a clump of Aegagropila linnaei, a green alga marketed as moss balls or marimo balls, and issued a warning through the Nonindigenous Aquatic Species Alert System on March 2.

The national Aquatic Nuisance Species Task Force, co-chaired by the U.S. Fish and Wildlife Service and the National Oceanic and Atmospheric Administration, swung into action, bringing in regional networks and state wildlife agencies. On March 3, Martha Volkoff at the California Department of Fish and Wildlife in Sacramento, responsible for the detection and monitoring of invasive aquatic species, was notified that one of the most-watched invasives in the West, hitchhiking in aquarium plants being distributed out of Southern California, posed a threat to previously uncolonized areas.

Native to the Black, Caspian, and Azov Seas, the zebra mussel (Dreissena polymorpha) reached the North American Great Lakes in the ballast water of a cargo ship and is now widely established east of the Rockies, with outlying populations in Colorado, Utah, and San Justo Reservoir in San Benito County, California. The similar and closely related quagga mussel (D. bugensis) has been detected in Lake Mead and parts of Southern California. Both species have limited salt tolerance but can otherwise tolerate a wide range of water conditions.

Zebra mussels are tiny, but their mindboggling numbers can clog the pipes of power plants, water distribution systems, and industrial facilities, degrade dock pilings and other structures, and disrupt aquatic ecosystems. California and other states, organized in a Western Regional Panel, have been working to contain their further spread and last year finalized a Quagga and Zebra Mussel Action Plan 2.0 with emphasis on recreational boating.

“We had been very focused on watercraft and their movement as the primary vector,” says Volkoff. The aquarium trade as a potential pathway wasn’t on anyone’s radar. Federal and state authorities have dealt with invasive tropical fish and aquarium plants, eradicating the marine alga Caulerpa in Southern California, but hadn’t considered ornamental plants as a Trojan horse for zebra or quagga mussels.

The alga A. linnaei occurs naturally in cold northern freshwater lakes in Europe and Asia, from Iceland’s Lake Myvatn to Lake Akan on the Japanese island of Hokkaido. It‘s not known to be invasive. Volkoff says moss balls have been popular among home aquarists for several decades. A 2010 study identified their main source in the aquarium trade as the Shatsk lake region in northwestern Ukraine.

The algae have been shipped to 17 American importers in seven states, four of whom import directly from Ukraine. “They may change hands a couple of times,” Volkoff notes, and multi-country supply chains may be involved. Current import permits issued by the U.S. Department of Agriculture (USDA) require that the moss balls be free of living stowaways, but enforcement and tracking is challenging.

According to Volkoff, a single Southern California distributor — just one of several suppliers — provided moss balls for 2,800 retail outlets in 49 states, from large chains like PetCo and PetSmart to small independent retailers, under brand names like “Betta Buddy.” At press time, inspectors had found moss balls with zebra mussels in stores in 32 states, including California and Oregon; so far, no quagga mussels have been detected.

There are also internet sales. USDA’s Animal and Plant Health Inspection Service, which has regulatory jurisdiction over the moss balls, is working closely with the wildlife agencies. Law enforcement authorities are investigating potential violations of the federal Lacey Act, which prohibits importation of endangered or harmful animals and plants, and applicable state regulations like section 2301 of the California Fish and Game Code.

The chains quickly pulled the product when made aware of the zebra mussel problem, and the U.S. Fish and Wildlife Service and the Pet Industry Joint Advisory Council issued decontamination guidelines for home aquarists. While home aquaria may not be optimal habitat for the mollusks, wildlife managers are advising aquarists to kill any zebra mussels they find in their tanks. At a minimum they should quarantine and monitor their tanks for at least six months after disposing of moss balls.

Photo: Wikimedia Commons

“I think the response was pretty darn fast,” says Karen McDowell, senior environmental specialist with the San Francisco Estuary Partnership and an ex-officio member of the national Task Force. “Once the report had been verified, things happened very quickly.”

In a California Department of Fish and Wildlife press release, Volkoff appealed for cooperation: “It is imperative that pet suppliers and aquarists take action to prevent these mussels that have entered the aquarium trade from reaching our waterways.” The agency has an Invasive Mussel Hotline (866-440-9530) and website. Other states where the mussel-infested moss balls were found have also issued public advisories.

Invasive species are known to have reached California in many ways: ballast water, ships’ hulls, ornamental plants escaping from cultivation, discarded pets, ill-advised intentional introductions. But you never know what’s going to come out of left field.

Moss Ball Information CDFW

CDFW Invasive Mussel Watch

Prior Estuary News Stories

Invasive Overbite Clams Deplete Fish Food, August 2019

Pie-billed Grebes Biocontrol Red Swamp Crayfish, February 2019

Makeover for Delta Weeds Patch, September 2020

Clams Muddle Delta Restoration, April 2013

Top Photo: Home aquarium. Wikimedia Commons

About the author

Joe Eaton writes about endangered and invasive species, climate and ecosystem science, environmental history, and water issues for ESTUARY. He is also "a semi-obsessive birder" whose pursuit of rarities has taken him to many of California's shores, wetlands, and sewage plants.

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