Zebra Mussels: What You Should Know About This Invasive Species

Zebra mussels are small freshwater shellfish named for the contrasting stripes that decorate their shells. Native to the lakes and rivers that drain into the Caspian, Azov, and Black seas in eastern Europe and western Asia, these mussels are now widespread throughout Europe and the United States, usually traveling into new waterways attached to boats, as well as via water discharged from large ships (called ballast water).

Growing to be about an inch in size, each female zebra mussel can produce up to 1 million microscopic larvae, and the mollusks have spread rapidly across the eastern United States since their introduction in the 1980s, causing hundreds of millions of dollars in economic damage and altering ecosystems dramatically.

Zebra mussels are unique when compared to native freshwater bivalves in that they have byssal threads — strong, silky fibers, also called beards, that they use to attach to objects and remain stationary. Byssal threads allow zebra mussels to cover and incapacitate larger native mussel species, and also to accumulate on the surface of the shallow water, as well as inside pipes and all types of equipment, clogging them as more and more mussels grow inside. These mussels also have a unique reproduction capacity, releasing free-swimming larvae called veligers. Zebra mussels are an invasive species, and it is illegal to knowingly possess or transport them in the United States.

How Were Zebra Mussels Introduced to the United States?

Zebra mussels (Dreissena polymorpha) are a native of the Ponto-Caspian region, and began spreading across Europe along trade routes in the 1700s. It wasn't until the latter part of the 20th century that zebra mussels established a population in the United States. Researchers are not sure exactly when these mussels first arrived, but it is believed to have been in the mid to late 1980s, when a transatlantic cargo ship (or several) released ballast water containing zebra mussel larvae into the Great Lakes.

This mussel is unique compared to other freshwater bivalves, except perhaps Mytilopsis, because it produces veligers. It is often during this life phase that the species colonizes new environments, though zebra mussels can disperse during all life stages. Veligers are microscopic, and recreational boaters catching bait fish, swimming, and moving their vessels between different rivers and lakes, also began transferring zebra mussels into other parts of the Great Lakes system after their initial introduction.

Eventually, they were present in most navigable waterways in the eastern United States, crossing 23 states in around 15 years. While there is an established population of zebra mussels in the Colorado River and its tributaries, the bulk of the western states have yet to see an explosion of zebra mussels. The threat of their economic and environmental impact has led some states to take preventive actions, working to raise public awareness an invest in watercraft inspections and decontaminations to stop the mussel's spread.

Like many invasive species with a rapidly expanding population, zebra mussels have several characteristics that distinguish them from native freshwater mussels and allow them to exploit an "empty niche" in North American freshwater ecosystems. They reproduce prolifically, and their larvae require several weeks of development, during which they can be widely dispersed by winds and currents. Their byssal threads are also an advantage, allowing them to attach to mussels and other surfaces. Their ability to rapidly consume primarily phytoplankton, which serves as an important part of the food chain, also helps them thrive.

Problems Caused By Zebra Mussels

Alteration of Food Webs

Zebra mussels form dense mats that can filter massive amounts of water. In parts of the Hudson River, their densities can reach over 100,000 individual mussels per square meter, and they are capable of filtering all of the water in the freshwater portion of the river every two to four days. Before zebra mussels arrived in the Hudson, native mussels filtered the water every two to three months. The phytoplankton, small zooplankton, large bacteria, and organic detritus that zebra mussels eat as they filter the water, straining out the edible material, form the base of the aquatic food web, leading scientists to fear cascading effects throughout the food chain as reductions of plankton in the biomass may cause increased competition, decreased survival, and decreased biomass of fish that also rely on the tiny organisms for food.

Biofouling

Biofouling occurs when organisms accumulate in unwanted areas, commonly seen with barnacles and algae. Zebra mussels colonize pipes at hydroelectric and nuclear power plants, public water supply plants, and industrial facilities, constricting flow and reducing the intake in heat exchangers, condensers, fire fighting equipment, and air conditioning and cooling systems. They also negatively impact navigational and recreational boating, increasing drag due to attached mussels. Small mussels can get into engine cooling systems, causing overheating and damage, and navigational buoys have been sunk under the weight of attached zebra mussels. Long-term attachment of these mussels also causes corrosion of steel and concrete as well as deterioration of dock pilings.

Zebra mussels will form large exposed mats on shorelines and in shallow water, decreasing opportunities for recreation in those areas, as beach-goers need protective shoes to avoid being cut by the shells. In a survey of power and water companies across the mussel's range, over 37% of surveyed facilities reported finding zebra mussels and 45% had initiated preventive measures to keep zebra mussels from entering the facility operations. Almost all surveyed facilities with zebra mussels had used control or mitigation alternatives to remove or control zebra mussels, with an estimated 36% of surveyed facilities experiencing an economic impact, estimated at $267 million total.

Harm to Native Species of Mussels

Zebra mussels harm native mussel species in many ways, including attaching via their beards and impeding valve operation, causing shell deformity, smothering siphons (long tubes that exchange water and air), competing for food, impairing movement, and depositing metabolic waste.

According to research by the U.S. Geological Survey, survival rates of native unionid (a family of freshwater mussels) in the Mississippi River in Minnesota have been shown to decline significantly with an increase in zebra mussel colonization, and unionidae have been completely eliminated from Lake St. Clair and almost extirpated in western Lake Erie.

Efforts to Curb Environmental Damage

Because zebra mussels reproduce prolifically and their larvae are microscopic, it's difficult to eradicate an established population, leading most officials to encourage the general public to be educated about how zebra mussels can spread and how to stop that from happening. Zebra mussels can easily be accidentally transferred from water in bait buckets, or attached to different parts of boats, meaning that carefully cleaning boats, trailers, and gear, can help a lot to reduce their movement.

In recent years, scientists have been working to sequence the genome of this mussel, in the hopes that a chemical or biological tool can be developed to specifically target and kill this species without harming other organisms. As it stands, there are a variety of poisons that officials have used to kill the mussels with varying degrees of success, but of course any poison released into the water could also have an impact on other species present.

Perhaps the most interesting (and ironic) development in zebra mussel-infested waterways has been the arrival of the quagga mussel (Dreissena bugensis), an invasive cousin of the zebra mussel that has displaced the earlier-arriving species in some shallow waterways. Zebra mussels continue to dominate in faster-moving waterways, something researchers are tentatively attributing to a stronger byssal thread attachment. New management strategies are looking at solutions for both of these invasive species and hoping to stop further damage to aquatic ecosystems and water infrastructure.