Foreign marine species, from bivalves to fish, have taken hold across the U.S. Over the past 150 years, 300 alien species have dramatically altered the biodiversity of San Francisco Bay. In the Chesapeake Bay, millions of snails from Japan prey on the region's shellfish. And in the Great Lakes, the long tails of spiny water fleas, originally from Northern Europe, tangle fishing lines. America spends more than $1 billion a year trying to control such invasions both in the sea and on land. Still, hundreds of invasive species survive to flourish in their new homes.