Color patterns vary, with only dark or colored shells or no stripes. They are typically found attached to objects, surfaces, or each other by threads underneath the shells. Although similar in appearance to the quagga mussel, the two species can be easily distinguished. When placed on a surface zebra mussels are stable on their flattened underside while quagga mussels, lacking a flat underside, will fall over.
These musssels spread rapidly by sticking to boats. They spread by this passive method to many places where they have no natural predators. That explains their huge growth in numbers.
Zebra mussels are a problem in Northern America's Great Lakes region because they reproduce fast. They colonize water supply pipes of hydroelectric and nuclear power plants, public water supply plants, and industrial facilities. As a result, they clog water sources. They colonize pipes constricting flow, therefore reducing the intake in heat exchangers, condensers, fire fighting equipment, and air conditioning and cooling systems. It is estimated that zebra mussels will continue to populate the Great Lakes and even enter the Mississippi River region.