Invasion of the Critters; Seemingly harmless marine organisms are wreaking havoc on the world's coastal water ways, rivers and inland lakes.

Byline: Mac Margolis (With Tom Masland, Sudip Mazumdar And Liat Radcliffe)

Cleanup crews are used to thankless tasks. But when maintenance men at the Sao Paulo Electrical Co. (CESP) descended to the bowels of the huge Sergio Motta hydro- electric plant on the Parana River earlier this year, they couldn't believe what they saw. Or smelled. Like some nightmare still life, rotting shellfish were everywhere. And that was the good news. Limnoperna fortunei --better known as the golden mussel--is a tiny monster. Left untended, the fast-multiplying mussels would quickly clog the cooling tubes, causing the turbines to overheat and, conceivably, the plant to shut down. The Sergio Motta plant is one of the crown jewels of the regional power grid, which supplies electricity to six of 10 Sao Paulo residents. The only way to fight back is to drain the turbines and scrape off the mussels with water jets and pickaxes. "We hauled out trucks of the stuff," says engineering chief Luis Tadeu de Freitas. "The stink was unbearable."

Odor is the least of their problems. The golden mussel, originally from Asia, is a bioinvader. When it arrived in Brazil in the late 1990s no one gave the trifling bivalve organism much thought; it's a fraction of the size of a clam, and can't even swim. But Limnoperna fortunei is persistent. Sucked up into the ballast tanks of cargo ships, the golden mussel flourishes over transoceanic voyages, to be spit out in ports half a world away. From there, the trip inland is a cakewalk. Clinging like barnacles to the plumbing of ships, the mussels have hitchhiked their way deep into Brazil, Argentina, Paraguay and Uruguay. These organisms spread so quickly they can smother the aquatic food chain in lakes, rivers and wetlands; starve fisheries, and clog water and sewage-treatment plants. If it is not stopped, ecologists warn, this aquatic Anschluss could roll over the Pantanal, Brazil's teeming wetlands preserve, and eventually disrupt the delicate mosaic of plants, birds and fish in the Amazon Basin itself.

Wind, weather and ocean currents have long helped whisk organisms from one place on the planet to the next. In recent years, though, global commerce and travel have compounded the problem, nowhere more dramatically than on the high seas. Ocean cargo has increased tenfold in the past 50 years, according to the world's shipping records, and 90 percent of the world's goods are now ferried on the oceans.

The volume of shipments isn't the problem so much as a change in ballast. To keep an even keel, oceangoing vessels used to fill their holds with rocks, sand or steel. In recent years, though, ships have switched to seawater. They'll drink up ballast in one port to spit it out in the next. That makes each boat a biological Trojan horse, with up to 7,000 invasive species hidden away in 11 billion tonnes of ballast water on any given day. Every nine weeks, a new marine bioinvader is set loose. The International Maritime Organization (IMO), which watches over the oceans, calls shipborne invaders "one of the most serious threats to the health of the world oceans."

The global exchange of plant and animal species is a two-edged sword. It can replenish the world food supply (the Andean potato) or devastate a habitat (the Indian mongoose, which killed off 12 bird ...