Every year the damage wrought by aquatic invaders in the United States and the cost of controlling them is estimated at $9 billion, according to a 2003 study by a Cornell University professor, David Pimentel, whose research is considered the most comprehensive. The bill for controlling two closely-related invasive mussels  the zebra and the quagga  in the Great Lakes alone is $30 million annually, says the United States Federal Aquatic Nuisance Species Task Force.

Many scientists say that San Francisco Bay has more than 250 nonnative species, like European green crab, Asian zooplankton and other creatures and plants that outcompete native species for food, space and sunlight.

“Here you’ve got a veritable smorgasbord of habitats from shallow and muddy to deep water,” said Lars Anderson, a lead scientist with the United States Agriculture Department. The Oakland port ranks as the fourth busiest in the nation, and ships bring in tiny hitchhikers from across the globe to take up residence in the bay.

Most invasive aquatic species arrive stuck to hulls or as stowaways in ballast water. Wakame first arrived at the ports of Los Angeles and Long Beach in 2000, Dr. Zabin and other scientists said. A year later it had moved south into Baja California and north as far as Monterey Bay, where scientists in scuba suits yanked it off boat hulls and marina moorings.

Image The broad-leaf kelp is used in miso soup. Credit... Heidi Schumann for The New York Times

“It’s just like gardening, you can pull out all the weeds you want, but there will always be that little dandelion seed that will sprout and recolonize,” said Steve Lonhart, senior scientist at the Monterey Bay National Marine Sanctuary. The kelp, which can grow an inch a day, could spread as far north as Canada before the water becomes too cold to sustain it, Dr. Lonhart said.