In an effort to curb the environmental degradation caused by exotic species, the U.S. government today will begin regulating the water that cargo ships dump when they enter U.S. ports. Ships will be asked to empty out their ballast water at least 200 miles off the U.S. coast to prevent invasive animals, microorganisms, and bacteria from disrupting U.S. ecosystems. Exotic species can have serious economic as well as ecological costs: The zebra mussel, accidentally imported from the Black Sea to the Great Lakes, now causes an estimated $3 billion a year in damage.