A tasty Japanese seaweed is taking root in Monterey Bay, the first time the nonnative species has invaded the Northern California coast, and city officials fear it could someday choke marine life in one of the richest underwater gardens in the world.

Dozens of volunteer divers and dockside workers are trying to eradicate the culprit by physically plucking the foreign vegetation from its new home.

If left alone, the ribbon-shaped seaweed called Undaria pinnatifida, a common ingredient in miso soup, could carpet the bay floor and displace the native giant kelp that shelters fish and predators in a habitat abounding with endangered creatures.

"When you add things to the ecosystem, you are going to do things to it, and we don't know if that's benign or not," said Steve Lonhart, a marine ecologist with the Monterey Bay National Marine Sanctuary, which is overseeing the removal effort. "We don't want to wait and see if there's a disaster."

Mike Graham, a kelp ecologist at Moss Landing Marine Laboratories, warned that removing the seaweed may hamper scientists' ability to study how the plant spreads.

Graham said he is not too worried about the eight-foot-high Undaria replacing the 150-foot-tall giant kelp, which has been living on the California coast for 3 million years.

"It's like weeds in the sequoia forest choking the redwoods," Graham said. "It could turn out to be a bad thing or not very bad at all."

The Monterey City Harbor Division thinks the foreign vegetation hitched a ride on vessels from Japan two years ago and settled in Los Angeles and Santa Barbara harbors, where the plants flourished.

The microscopic spores of the brown seaweed migrated north more than a year ago on the hulls of some of the more than 6,000 fishing boats or small vessels that enter Monterey Harbor every year.

Undaria has already caused trouble in other aquatic neighborhoods.

In Australia, the immigrant plant has edged out native competitors in Tasmania, and New Zealand has been unable to pull up the seaweed that has blanketed some harbors completely. Listed among the top 100 worst invasive species on a global database, Undaria also carpeted 10,000 acres of the Mediterranean Sea in the 1980s, suffocating sea grasses, corals and sponges.

Because of environmental protections, marine life is often more threatened by nonnative organisms than by man-made pollutants such as oil spills or toxic sewage. State and federal laws have banned oil drilling in the bay, and the federal Clean Water Act curtails sewage pollution.

But natural menaces such as the Chinese mitten crabs in the San Francisco Bay have jammed water intake pipes for the Central Valley Project. Resistant to local diseases, zebra mussels have pushed aside native clams and fish in the Great Lakes region. A study by the San Francisco Estuary Institute found more than 234 species of alien plants, mammals and other creatures around San Francisco Bay.

Scientists are still teasing out the ecological effects of Undaria. Monterey Bay marine ecologists said in the worst-case scenario, Undaria could spread beyond the harbor into the Pacific and proliferate enough to displace the giant kelp forest that provides a canopy for marine snails, sea stars and rockfish that are gobbled up by sea lions and the threatened population of sea otters.

The seaweed, which withers during Japan's hot summers, may flourish in the more temperate California climate. It is also naturally abundant in the winter when the native population of giant kelp, or Macrocystis pyrifera, is dwindling.

"If the giant kelp forest is eliminated, fish might move on," Lonhart said.

Preventing invisible seaweed spores from entering the busy Monterey harbor is nearly impossible, as boats are not inspected and cleanings are not required, said Scott Pryor of the Monterey City Harbor Division.

So far, yanking the seaweed from the deep has been a labor-intensive task, though divers are making progress. Since the city and the Monterey Bay Sanctuary launched the eradication effort in January, 35 volunteers using their own tanks and scuba gear have dredged up more than 1,000 pounds from the docks and the deep water -- as much as three-quarters of the hardy plants identified at the outset. But just like garden weeds, Undaria is constantly regenerating.

"It's like hair," Lonhart said. "I can shave three-fourths of my scalp, and it grows back."

If the volunteer crew is unable to exterminate the plants, the city is exploring whether to sell some of the seaweed to abalone farms and use the funds to employ divers.

Arthur Seavey, co-owner of the Monterey Abalone Co., said his giant marine mollusks would find Undaria a delicious alternative to other kelp. "If there is a way we can help, we'd be happy to do that," Seavey said. But it is unclear whether abalone munching on it in their harbor farms would disperse more seeds. "We don't want to make it worse."