The zebra mussel is a small freshwater bivalve mollusk that has become one of the most problematic aquatic invasive species in North America. The mussels first appeared in the Great Lakes in the late 1980s, likely transported across the Atlantic in the ballast water of commercial ships. Their rapid spread and ability to colonize new environments quickly have established them as a significant threat to freshwater ecosystems across the continent.
The Environmental and Economic Threat
The intense filter-feeding activity of large zebra mussel populations alters water bodies. A single mussel can filter up to a liter of water per day, and their collective action removes massive amounts of phytoplankton and suspended particles from the water column. This increased water clarity allows sunlight to penetrate deeper, encouraging the growth of submerged aquatic vegetation, which changes the habitat and can lead to toxic algal blooms. By consuming the base of the aquatic food web, zebra mussels starve native species that rely on phytoplankton for sustenance, severely disrupting the natural balance of the ecosystem.
Beyond the ecological damage, the mussels inflict substantial financial harm through biofouling, the process of attaching to submerged surfaces. Using strong, root-like structures called byssal threads, they colonize docks, boat hulls, and, critically, water intake pipes for power plants and municipal water treatment facilities. The resulting blockages reduce water flow, increase friction, and accelerate corrosion, leading to costly shutdowns for maintenance and cleaning. North American industries and municipalities spend hundreds of millions of dollars annually to manage and clear infrastructure encrusted with these invaders.
Chemical Eradication Methods
Chemical methods are often employed in closed systems, such as industrial pipelines and water treatment facilities, where the water flow can be controlled and treated with greater precision. Oxidizing agents, such as chlorine, are effective at preventing the settlement of microscopic larval mussels, known as veligers. However, established adult zebra mussels can detect the presence of chlorine and will tightly close their shells, allowing them to survive continuous exposure for up to three weeks. Because these agents are toxic to non-target aquatic life, any treated water discharged back into a natural waterway must first undergo a dechlorination process to neutralize the chemical.
Newer, targeted molluscicides offer a more environmentally specific approach, working by using a bacteria-derived toxin rather than broad-spectrum poisons. The product Zequanox, for instance, utilizes a dried form of the naturally occurring soil bacterium Pseudomonas fluorescens strain CL145A. When zebra mussels filter-feed, they ingest the bacterial cells, which release a toxin that selectively destroys the mussel’s digestive lining. This method is effective on all life stages, from veliger to adult, and is registered for use in both enclosed systems and large-scale, open-water applications without the need for post-treatment deactivation.
Potassium chloride (potash) is another non-oxidizing chemical used for mussel control, particularly in irrigation and industrial pipelines. Traditional application requires high concentrations and long contact times to be lethal, but a specialized delivery system has been developed to improve its efficacy. This method involves encapsulating the potassium chloride in tiny, edible micro-particles that trick the mussels into ingesting the poison while they are actively filter-feeding. This specialized delivery bypasses the mussel’s natural defense mechanism of shell closure, allowing the potassium to disrupt their physiological processes and achieve higher mortality rates with a lower overall chemical dose.
Physical and Mechanical Removal
For localized infestations and smaller watercraft, physical and mechanical removal techniques provide effective control without the introduction of chemicals.
Thermal Treatment
Thermal treatment, which involves using high-temperature water or steam, is one of the most effective methods for killing all life stages. Water applied at a temperature of at least 140°F for 10 seconds, or 120°F for two minutes, is generally lethal to adult mussels and veligers. This technique is especially useful for decontaminating boat engines, hulls, and dock structures.
Desiccation and Filtration
Desiccation, or drying, is a reliable method that exploits the mussel’s need for a moist environment. Adult mussels can survive out of water for several days by sealing their shells, but prolonged exposure to dry air is fatal. In hot, dry conditions, equipment must be dried for at least five to ten days to ensure complete mortality, though cooler or more humid weather may require 15 to 20 days. Another element is physical screening: industrial intake systems often use fine-mesh filtration to physically screen out the microscopic veligers, and high-pressure washing is employed to blast off mussels and their byssal threads from hard surfaces.
Biological and Natural Control Agents
Biological control involves utilizing natural predators to limit the zebra mussel population, though this approach has limitations in achieving widespread eradication. Several native species in North America have adapted to prey on the invaders, including the freshwater drum, which is one of the few fish with pharyngeal teeth strong enough to crush the shells of adult mussels. Large drum are documented to feed heavily on the mollusks, though their overall impact on entire lake populations remains relatively small.
Crayfish are also known to consume smaller zebra mussels, typically those less than 12 millimeters in size. An adult crayfish can consume a significant number of these smaller mussels daily, but this predation rate declines sharply if other, more appealing food sources are readily available. Diving ducks, such as scaup and canvasbacks, consume large quantities of mussels during migration, but their predation is seasonal and often limited to shallow waters. While these predators contribute to population control in specific locations, no single North American predator or combination of predators has been shown to effectively reduce a large-scale zebra mussel infestation to manageable levels.
Preventing Future Infestations
The most effective strategy for managing zebra mussels is to prevent their introduction in the first place, largely through the public adoption of the “Clean, Drain, Dry” protocol. This three-step process provides a clear set of actions for recreational water users to interrupt the spread of the invasive species.
The first step is to thoroughly clean the hull, trailer, and all recreational gear, removing all visible aquatic plants, mud, and attached zebra mussels. The next action is to drain all standing water from the boat and equipment before leaving the access area. This includes the bilge, live wells, ballast tanks, and motor cooling systems, as microscopic veligers can be transported in even small amounts of water. Finally, allowing all equipment to dry completely for a minimum of five days is the most reliable way to ensure that any remaining mussels or veligers are killed by desiccation. Following this procedure every time watercraft moves between water bodies is the primary defense against new infestations.