WASHINGTON — Nearly 300 species of fish, mussels and other sea critters hitchhiked across the Pacific Ocean on debris from the 2011 Japanese tsunami, washing ashore alive in the United States, researchers reported Thursday.

It is the largest and longest marine migration ever documented, outside experts and the researchers said.

The scientists and colleagues combed the beaches of Washington, Oregon, California, British Columbia, Alaska and Hawaii and tracked the species to their Japanese origins.

Their arrival could be a problem if the critters take root, pushing out native species, the study authors said in the journal Science.

“It’s a bit of what we call ecological roulette,” said lead author James Carlton, a marine sciences professor at Williams College, in Williamstown, Mass.

It will be years before scientists know if the 289 Japanese species thrive in their new home and crowd out natives. The researchers estimated that a million creatures traveled 4,800 miles across the Pacific Ocean to reach the West Coast, including hundreds of thousands of mussels.

Invasive species are a major problem worldwide, with plants and animals thriving in areas where they don’t naturally live.

Marine invasions in the past have hurt native farmed shellfish, eroded the local ecosystem, caused economic losses and spread disease-carrying species, said Bella Galil, a marine biologist with the Steinhardt Museum of Natural History in Tel Aviv, Israel, who wasn’t part of the study.

A magnitude 9 earthquake off the coast of Japan triggered a tsunami on March 11, 2011, that swept boats, docks, buoys and other man-made materials into the Pacific. The debris drifted east with an armada of living creatures, some that gave birth to new generations while at sea.

“The diversity was somewhat jaw-dropping,” Carlton said. “Mollusks, sea anemones, corals, crabs, just a wide variety of species, really a cross-section of Japanese fauna.”