Zebra mussel

The zebra mussel, known scientifically as *Dreissena polymorpha*, is a small freshwater bivalve mollusc.

These organisms are considered one of the most invasive aquatic species in the world. They originate from the lakes of southeastern Europe and southwestern Russia. However, they have been accidentally introduced to many new regions across the Northern Hemisphere. This movement disrupts entire ecosystems and causes significant economic damage. They are particularly problematic because they can change the physical and biological makeup of the water they inhabit.

Zebra mussels function as highly efficient filter feeders. They remove tiny particles from the water column to find food.

A single mussel can process up to one liter of water every day. During this process, they consume certain particles and deposit waste on the lake floor. They also produce pseudofaeces, which are clumps of mucus and non-food matter.

This constant filtering increases water clarity. In Lake Erie, clarity has increased from 2 meters to as much as 8 meters in some areas. This allows sunlight to reach much deeper depths.

This increased light penetration triggers a chain reaction in the environment. Sunlight enables the growth of submerged macrophytes, which are large underwater plants.

When these plants die and decay, they wash up on shorelines. This can foul beaches and create water-quality problems. While the mussels enrich the lake floor with nutrients, they also create new challenges for shoreline management. The shift in plant growth changes the entire habitat for other living things.

The physical structure of the zebra mussel is quite distinct. They possess D-shaped shells that often feature a striped pattern.

Most individuals are about the size of a fingernail. They use strong byssal fibres to attach themselves to a substrate.

These fibres emerge from the umbo on the dorsal side of the shell. They can attach to sand, silt, or much harder surfaces like rocks. Juvenile mussels typically prefer harder, rockier substrates for their initial attachment.

History shows how quickly these mussels can spread. German zoologist Peter Simon Pallas first described the species in 1769. He identified them in the Ural, Volga, and Dnieper Rivers.

In North America, they were first detected in the Great Lakes in 1988. Scientists believe they arrived via the ballast water of ocean-going ships. These ships carry water in large tanks to stay stable. When they move from the ocean to freshwater, they may release microscopic larvae called veligers. These larvae drift in the water for several weeks before settling on a hard surface.

The impact on native species is often devastating. Zebra mussels frequently form massive colonies on native unionid clams.

This prevents the native clams from moving, feeding, or breeding. In Lake St. Clair, this led to the near extinction of unionid clams. A similar pattern occurred in Ireland, where mussels were found in the River Shannon in 1997. However, the mussels also provide food for some animals. The yellow perch population in Lake St. Clair increased five-fold following the invasion.

Many birds and fish, including 15 species of fish and 36 species of birds, prey on them.

Managing zebra mussels is a difficult and expensive task. They often clog water-treatment plants and power plant pipes. In England, Anglian Water spends an estimated £500,000 every year to remove them.

Because they can survive out of water for about seven days, they can also travel in boat equipment. Scientists have tested various methods to control them, such as using liquid fertilizer or biopesticides like zequanox. Some researchers also look at natural predators, like crayfish, to help manage their density. Despite these efforts, zebra mussels remain a major global concern for freshwater ecosystems.