A massive tsunami sparked by the huge Tōhoku earthquake in 2011 sent nearly 300 living Japanese coastal marine species on a six-year journey across the Pacific Ocean, leading to a transoceanic migration that has no known historical precedent.

These species, which include mollusks, fish, parasites, and more, have been found residing in Hawaii, Midway Atoll, and parts of the West Coast, from Alaska to California. Some may eventually disrupt pre-existing ecosystems, raising concerns over invasive species.

While the tsunami was a natural event, the migration has been aided by humans, since these species were able to hitch a ride across the ocean on non-biodegradable debris, such as plastics.

A study detailing the migration, which was published Thursday in the journal Science, examined the biological ripple effects from the magnitude 9.0 earthquake and the subsequent tsunami that struck northern Japan on March 11, 2011.

The tsunami, which reached a height of about 126 feet, sent millions of pieces of debris into the ocean. Many of these pieces, including boats, docks, and buoys, already supported marine life living on them. Other objects picked up marine species once they were out at sea.

Remarkably, many of these species not only survived in the open ocean, where food is typically more scarce than in coastal regions, but even thrived and reproduced, the study found.

“The thing that’s remarkable about the study, is the sheer number of coastal organisms that survived the transit," Greg Ruiz, a senior scientist at the Smithsonian Environmental Research Center in Maryland and a coauthor of the new study, said in an interview.

“They survived it in the open ocean which is remarkable."

According to the study, species from Japan continued to show up, alive, along the West Coast of the U.S. six years after the tsunami, which was four or more years longer than previously documented cases in which species "rafted" across an ocean.

Some of the hardiest species that survived the journey include mussels, barnacles, limpets, sea anemones, and amphipods.

Before you freak out about some kind of Japanese marine species invasion, though, consider that at least 35 percent of species that migrated as a result of the tsunami had been previously been found along the West Coast of North America.

And not all the new arrivals are likely to establish themselves in their new homes, since the climate needs to be suitable, the nutrients present in the water, and the broader ecosystem needs to support the species.

The study says that scientists do not yet know if non-native species have established themselves in, say, the Pacific Northwest, which has seen a high concentration of arriving Japanese marine species. There is typically a lag time of several years between a species' first arrival in a new area and the detection of a harmful, invasive species.

Ruiz says marine scientists should have their guard up, though.

“Our view is that many of the species that came across to North America especially have the potential to colonize, to establish self-sustaining populations in part because they were in good condition when they arrived,” Ruiz said.

James Carlton, a scientist at Williams College and the lead author of the new study, echoed this sentiment, but went a step further in his comments via a press release.

"When we first saw species from Japan arriving in Oregon, we were shocked. We never thought they could live that long, under such harsh conditions," he said in the statement.

"It would not surprise me if there were species from Japan that are out there living along the Oregon coast. In fact, it would surprise me if there weren't."

The researchers attribute the unprecedented nature of this species migration to the widespread use of plastic materials in the latter half of the 20th century, which was not present for previous major tsunamis that struck the same area in Japan during the late 19th and early 20th centuries.

“This was a flotilla of debris objects that were colonized by a marine community,” Ruiz said. “Those plastic materials enabled transport that likely didn’t happen historically”

The widespread use of plastics, buildup of coastal megacities, plus the potential for climate change-related shifts to oceanic storms such as typhoons and floods means that there will be more opportunities in the future for long-term transoceanic dispersal events.

"Given that more than 10 million tons of plastic waste from nearly 200 countries can enter the ocean every year -- an amount predicted to increase by an order of magnitude by 2025 -- and given that hurricanes and typhoons that could sweep large amounts of debris into the oceans are predicted to increase due to global climate change, there is huge potential for the amount of marine debris in the oceans to increase significantly," Carlton said.

Plastics have already led to the establishment of giant garbage patches in parts of the world's oceans, including the Pacific, where ocean currents and trade winds trap biodegradable debris.

According to Ruiz, plastics need to be looked at as even more problematic than previously thought, given their potential to spread new species thousands of miles across the world's largest ocean.

“I think this is another dimension of plastics that we haven’t given much thought too. They potentially play a really important role in dispersal of organisms,” he said.

Remarkably, debris from the tsunami, along with its cargo of marine species, are still arriving in the U.S., and may continue to do so for years to come.