Like other invasive species, zebra mussels are prolific breeders: A single adult female produces between 30,000 and 1,000,000 eggs per year. The tiny larvae live as planktonic creatures, meaning they drift suspended in waters rather than settle to the bottom. This characteristic helps them travel in currents, and once they reach a new body of water, the young or the adults are not choosy about the aquatic ecosystem they invade, usually with human help. Invasive species expert Jim Carlton told National Geographic in 2005, “Before humans started moving around, the rate of species movement was a geologic rate. Now we’re moving species faster and farther than they ever would or could have moved in nature.” Adult mussels attach to surfaces and then secrete a strong, plasticlike fiber that sticks to living and nonliving things. The mussels then get transported from one body of water to another on boats or anything else that picks them up in the water.
Zebra mussel invasion creates five main ecosystem problems. First, mussels efficiently filter water for food and remove almost all the water’s algae and phytoplankton. In other words, they act as exploitative competitors because their actions remove an entire component from a food chain. Second, with phytoplankton gone, crustaceans, small fish, larger fish, and higher predators in the food chain each lose their main food source. Third, clear algae-free water allows more aquatic plants to grow, which benefits some species such as dabbling ducks but harms other aquatic species by crowding them out and blocking sunlight. Fourth, the mussels concentrate toxic pollutants in their bodies then excrete the pollutants in their feces, endangering native plants and animals near the mussels. Invaders that produce toxins harmful to ecosystems are called interference competitors. Fifth, zebra mussels have decimated the Great Lakes population of American unionid clams by attaching to their shells and restricting their movement, breeding,
and feeding. The native clams are now near extinction in the Great Lakes, and it is likely that zebra mussels will cause similar harm to other native species.
Zebra mussels have also made a surprising contribution to the ecology of the upper Midwest. Some migratory duck populations have rebounded because they have adapted to eating zebra mussels to fortify themselves during
migration stopovers. Fish such as catfish, sunfish, sturgeon, salmon, and yellow perch have also adapted to eating the mussels. Finally, smallmouth bass numbers have risen because the young have abundant aquatic plants in
which to hide and grow.
Zebra mussels are marine and freshwater invasive species that provide an example of how globalization contributes to the movement of species around the world. These animals were brought to North America from Eurasia in cargo
ship ballast. Scientists have since learned that ship ballast is a major means of transport of invasive species. (ECHO, Lake Aquarium and Science Center, Leahy Center for Lake Champlain)
The zebra mussel invasion has put an economic burden on local businesses because of the cost of removing mussels from boats, ships, buoys, docks, and piers. The mussels have clogged the intake pipes of factories, power plants, water utilities, and other businesses that withdraw water from lakes and rivers. These places must now mussel-proof their pipes by installing strainers that keep out the intruders.
Jim Carlton summed up the issue of invasive species like the zebra mussel, which have migrated halfway around the world in ballast water: “Five thousand or more species could easily be in motion on any given day.” Though invasive species move around the planet today with ease, the problem is not new. Carlton explained in a 2005 National Geographic article, “Fouling is responsible for quite a few [in San Francisco Bay], especially from the gold rush, when so many old, heavily fouled wooden ships sailed in and were abandoned here. The oyster trade. Fish bait. Ballast water brought the rest.” New Zealand, Norway, and parts of the United States have now put legal restrictions on how and where ships may empty their ballast tanks, but that will not affect the zebra mussel invasion that has already spread.
