Adult zebra mussels have a pair of roughly triangular shells connected by an elastic hinge, the outside of the shells are usually brown with stripes that give the species its common name in English, but patterns and the darkness of color varies. They grow to about 5cm maximum length. The ventral side of the mussel is flattened, so much so that the mussel shell will stand on a flat surface. In life they attach to substrate with a glue they secrete that forms fibers called byssal threads.
Zebra mussels have a free-swimming larval stage. This stage in molluscs is called a veliger. Veliger larvae are small enough that they can easily drift in the water, only about 100 micrometers long (0.1 mm) in diameter. Zebra mussel veligers have a tiny shell, and a curved sheet of skin called a velum, that is covered with tiny hairs that beat in the water. This helps them swim, and also draws food particles in for them to eat. (Nalepa and Schloesser, 1993; U.S. Geological Survey, 2008)
Zebra mussels were originally found in the rivers and lakes that connect to the Black Sea and Caspian Sea, in eastern Europe and western Asia. Shipping and canal construction in the 1800's allowed them to spread west into most European rivers and lakes. In the late 20th century they were accidentally brought to North America, probably in ballast water of large ships. They now occur in the Great Lakes basin, most of the Mississippi River drainage, the Hudson River and many other eastern North American rivers. (Nalepa and Schloesser, 1993; Neumann and Jenner, 1992; U.S. Geological Survey, 2008)
Zebra mussels live in still or slow-moving freshwater, and attach themselves to any hard surface under water, natural or man-made, including rocks, submerged wood, boat hulls, buoys, docks, and water intake pipes. They need water will dissolved calcium to live, and they grow best if the water is not acidic at all.
Most zebra mussels live in water that is 2 to 12 meters deep. In shallower water waves and ice make it harder for them to survive. They can live in much deeper water -- some zebra mussels have been found as deep as 60 meters, and they can probably live deeper than that.
They stop growing at about 3°C, and increase their growth and feeding rates as they warm to 20-25°C. Above that they slow down and again, and start to die at 30°C. (Nalepa and Schloesser, 1993; U.S. Geological Survey, 2008)
There are three stages in the life of a zebra mussel. The speed of development depends on temperature -- warmer mussels grow faster. About 3-5 days after fertilization, a tiny larva that emerges from the egg. This stage is called the veliger. It can swim upward (but not strong enough to swim against a current), and has only a tiny start of a shell. It grows, and when it develops several internal organs (including a muscular foot), and peak (called an umbo) at the hinge of its shell, it is in the next stage, the postveliger. The postveliger continues to grow, and after about a month, it settles onto a hard surface. It uses its foot to move slowly, and when it finds a suitable spot, glue itself to the surface, where it will stay for the rest of its life. Then it metamorphoses into the adult stage. (Nalepa and Schloesser, 1993)
Adult zebra mussels start to reproduce in the spring, when water temperatures rise to about 12°C. In habitats where they water stays warm year round, they may reproduce continuously. Females release eggs into the water, and males release sperm, and fertilization occurs after they are released. Females grow and release eggs in batches of up to 40,000, up to four times during the breeding season, which lasts as long as the water stays warm enough. Each can release as many as 1 million eggs each year.
The only parental investment is in the production of eggs and sperm.
Zebra mussels life span varies. Most live 3-9 years. (U.S. Geological Survey, 2008)
Zebra mussel larvae drift in the water, feeding and growing and then settle down on to a hard surface. Once they attach, they do not move again. When they are abundant, they often completely cover any exposed rock or other surface. Even if there are many close together, they do not seem to have any interaction with each other, except for reproduction. (Nalepa and Schloesser, 1993; U.S. Geological Survey, 2008)
Zebra mussels have no heads, and no eyes. However, they are sensitive to chemicals in the water, and can detect gravity, touch and temperature. If disturbed they will close their shells. (U.S. Geological Survey, 2008)
Zebra mussels filter their food from the water. They eat mainly single-cell organisms, such as bacteria, blue-green algae, small green algae, and protozoans. They also consume very fine detritus particles. (Nalepa and Schloesser, 1993; U.S. Geological Survey, 2008)
Zebra mussel larvae have no special defense against predators, but they are so small that only small predators and filter-feeders eat them. The larvae are part of the zooplankton in the water, and pretty much any predator that eats zooplankton eats them. This includes many small fish (including the young of large fish), other zooplankton such as copepods, freshwater Cnidaria like hydras, even freshwater sponges.
Most fish can't eat zebra mussels because they can't crush the shells. A few fish species have specialized teeth and jaws that are strong enough to break the shells of mollusks, and some of them do eat zebra mussels. In Europe the roach, is a major predator of zebra mussels, along with bream, and silver bream. Round gobies and common carp, native to Eurasia, have been introduced to North America, and eat zebra mussels where they occur. The black carp is an east Asian species that has been introduced to Europe, and eats zebra mussels there. The pumpkinseed sunfish has been introduced to Europe from North America, and eats zebra mussels on both continents. Besides pumpkinseeds, the several other North American fish eat zebra mussels, including freshwater drums, redhorse suckers, river carpsuckers and smallmouth buffalos.
Some species of waterbirds are important predators of zebra mussels too. These are mostly diving ducks. Species known to feed significantly on zebra mussels include greater scaups, lesser scaups, pochards, tufted ducks, buffleheads, goldeneyes, common coots oldsquaws, herring gulls, and white-winged scoters.
Blue crabs (Callinectes sapidus) consumed many zebra mussels during a study in the Hudson River. Crayfish, including the northern clearwater crayfish, Orconectes propinquus, may prey on small zebra mussels. (Molloy, et al., 1997; Nalepa and Schloesser, 1993; U.S. Geological Survey, 2008)
Zebra mussels can be very important in freshwater ecosystems. If they are enough of them, they can filter an enormous amount of plankton out of the water. This changes the flow of energy in the foodweb -- the energy in the phytoplankton goes to the bottom, to the mussels and the animals that eat them, instead of swimming plankton predators like zooplankton and fish.
Also, if zebra mussels clear the water, sunlight can penetrate deeper into the water, allowing more aquatic plants to grow. These plants provide food and hiding places for fish and invertebrates.
Zebra mussels attach to the outside of North American freshwater mussels. They slow the larger mussel down, interfere with its growth, sometime jam the shell open, and prevent the large mussel from feeding and pumping water in and out of its shell. Where zebra mussels have moved into the Great Lakes basin, native mussels have been wiped out. (Great Lakes Information Network, 2008; U.S. Geological Survey, 2008)
The introduction of zebra mussels into many areas of the world has created major economic problems.
The mussels grow on all kinds of man-made structures in the water, include water intake pipes for drinking water plants and power plants. So many grow there that they clog the pipes. Businesses and governments spend hundreds of millions of dollars every year to clear out the mussels and keep the pipes open. Mussels also grow on navigational buoys, sometimes sinking them, and on locks and dams, interfering with their operation. They grow on hulls of boats and ships, slowing them down and clogging engine intakes.
The ecological impacts of zebra mussels are still happening, and not all the effects are known. They eat phytoplankton faster than zooplankton in the water does. This means zooplankton and the fish that live in the open water (like walleye, salmon, and lake trout) have less to eat. Also, zebra mussels don't like to eat certain kinds of toxic blue-green algae. When zebra mussels have spread to inland lakes in North America, the amount of this toxic type of algae increases.
See the references for more information on the many ecological effects of zebra mussels, especially in North America.
Zebra Mussels were added to freshwater lakes in the Netherlands to help make the water more transparent (they eat the phytoplankton that makes the waters cloudy). Other cities in other countries have done the same. (Nalepa and Schloesser, 1993; Neumann and Jenner, 1992)
Zebra mussels are still common and abundant in their original range, and have spread far beyond it. They are not considered to be in any need of special conservation efforts. (Great Lakes Information Network, 2008; U.S. Geological Survey, 2008)
Tiffany Murphy (author), University of Michigan-Ann Arbor.
Great Lakes Information Network, 2008. "Zebra Mussels in the Great Lakes Region" (On-line). Invasive Species in the Great Lakes Region. Accessed December 17, 2008 at http://www.great-lakes.net/envt/flora-fauna/invasive/zebra.html.
Molloy, D., A. Karatayev, L. Burlakova, D. Kurandina, F. Laruelle. 1997. Natural enemies of zebra mussels: predators, parasites, and ecological competitors. Reviews in Fisheries Science, 5/1: 27-97. Accessed December 17, 2008 at http://www.sgnis.org/publicat/rfs27.htm.
Nalepa, T., D. Schloesser. 1993. Zebra Mussels: Biology, Impact, and Control. Boca Raton, Florida, USA: Lewis Publishers.
Neumann, D., H. Jenner. 1992. The Zebra mussel Dreissena polymorpha : ecology, biological monitoring and first applications in the water quality management. New York City, New York, USA: G. Fischer.
U.S. Department of Agriculture, 2008. "Zebra Mussel Species Profile" (On-line). National Invasive Species Information Center. Accessed December 17, 2008 at http://www.invasivespeciesinfo.gov/aquatics/zebramussel.shtml.
U.S. Geological Survey, 2008. "Zebra and Quagga Mussel Page" (On-line). Non-indigenous Aquatic Species. Accessed December 17, 2008 at http://nas.er.usgs.gov/taxgroup/mollusks/zebramussel/.