The zebra mussel is a small shellfish. Color patterns could vary, having 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. They are currently a major problem in Northern America's Great Lakes region because of a fast reproduction time. Zebra mussels are notorious for their biofouling capabilities by colonizing water supply pipes of hydroelectric and nuclear power plants, public water supply plants, and industrial facilities. 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. They have no relation to zebras.