STUDENT INSTRUCTIONS

      First read the information below on your own. Then discuss your answers with your neighbors and ask them to help you with any questions you may have. Be prepared to share your answers and questions with the whole class.

      One of the most dramatic examples of a successful invasion by an introduced species is the zebra mussel, Dreissena polymorpha. This native to southern Russia was introduced to the Great Lakes in 1985 or 1986 via ballast water from foreign ships. Since then, it has spread rapidly throughout the Great Lakes, into the Hudson River in NY state, and into many other freshwater habitats in the eastern U.S. (Figure 1a).

      Note that ships travel from the open ocean into the Great Lakes via the St. Lawrence River in Quebec, Canada (Figure 1a). To ride low in the open ocean big ships take in water (called ballast water) after they unload heavy cargo. After taking in ballast water in large rivers in southern Russia, the vessels traveled across the Atlantic Ocean and into the Great Lakes, where the "Russian" ballast water was then discharged when the ships took on more cargo. Any organism that survived this trip became potential Great Lake "invaders".

      It is predicted to invade most lakes and rivers in the nation in about 20 years. Zebra mussels grow in incredible densities of up to 700,000 individuals/m² (try to picture that!). Any hard surface in water like a car or a shopping cart will be completely covered in mussels in a few months. The ability of these mussels to cake onto surfaces has created havoc for utilities in the Great Lakes and elsewhere because pipes that take in and discharge water become quickly clogged. Estimates of the cost to remove mussels (by chlorine and other chemicals and hot water) are as high as $100 million per year. Some organisms are very successful at invading and growing in places in the world where they are not native. Many others are not. The zebra mussel’s biology and ecology predisposes it to be a successful introduced species.

      Use the list of characteristics below to explain D. polymorpha’s ability to flourish in the U.S.

Characteristics of the zebra mussel:

• Lifespan is typically three to five years,


• Shells are usually 25-35 mm in length,


• They reach sexually maturity in the second year, sometimes in the first year,


• Sexes are separate, gametes released into water for external fertilization,


• Individual females can release more than one million eggs in a spawning event; eggs are released 2-5 times a year,


• Fertilization requires water of at least 12 degrees C; eggs can be fertilized 2.5-5 hours after release and sperm last up to 22 hours,


• The free-swimming larvae, called veligers, are spread by currents; a later stage settles onto surfaces and firmly attaches itself to a surface with byssal threads which it excretes,


• Adults spread via boats they are attached to,


• The mussel can live in fresh and brackish water; it does not tolerate low pH or low oxygen,


• Adults grow in huge concentrations on top of one another,


• Zebra mussels are filter feeders and they can filter very large volumes of water ­ often the whole volume of a lake or reservoir in a few months,


• U.S. lakes and rivers with high densities of zebra mussels become very clear and the chlorophyll concentration in the water drops dramatically.