3. Another example of a different kind of animal that has recently moved to another continent is the zebra mussel. This small shellfish was first discovered in the great Lakes of North America in 1986. It may have come over from Russia on a cargo ship. In a very few years, zebra mussels had spread over all the great lakes and into many important rivers. They have grown into thick masses, covering many areas of takes or river bottoms. They have also covered and closed up pipes of power stations and water treatment centers. Government officials say that the mussels have caused many millions of dollars worth of damage.