Tsunami drives species 'army' across Pacific to US coast
Scientists have detected hundreds of Japanese marine species on US coasts, swept across the Pacific by the deadly 2011 tsunami.
Mussels, starfish and dozens of other creatures great and small travelled across the waters, often on pieces of plastic debris.
Researchers were surprised that so many survived the long crossing, with new species still washing up in 2017.
The study is published in the journal Science.
The powerful earthquake that shook north-eastern Japan in March 2011 triggered a huge tsunami that reached almost 39m in height on the Tōhoku coast of Honshu.
The towering waves washed hundreds of objects out to sea, ranging in size from tiny pieces of plastic to fishing boats and docks.
A year later, scientists began finding tsunami debris with living creatures still attached, washing up on the shores of Hawaii and the western US coast from Alaska down to California.
"Many hundreds of thousands of individuals were transported and arrived in North America and the Hawaiian islands - most of those species were never before on our radar as being transported across the ocean on marine debris," lead author Prof James Carlton, from Williams College and Mystic Seaport, told BBC News.
"Much of the debris is still out there and it could be that some of these Japanese species will still arrive. I wouldn't be surprised if a small Japanese fishing boat lost in 2011 was to show up 10 years after the event."
The research team has detected 289 different species so far. Mussels were the most common, but there were also crabs, clams, sea anemones and star fish.
So common were findings that new species were still being discovered even as the study drew to a close in 2017, six years after the tsunami.
The scientists say that many other species have likely made the journey and so far escaped detection. No colonies of invaders have so far been established but the research team believes that this is likely to happen.
"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," said co-author John Chapman from Oregon State University.
"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 key element that has made this possible according to all the scientists involved is the ubiquitous presence of plastic, fibre glass and other products that do not decompose.
"The wood generated by the tsunami lasted a short time compared with the enduring nature of the plastic," said Prof Carlton.
"For aeons if a plant or animal was to raft across the oceans, their boat was literally dissolving underneath them. What we have done now is provide these species with rather permanent rafts; we have changed the nature of their boats."
Moving much more slowly than ships, the plastic or fibre glass rafts gave the species time to gradually adjust to their new environment, making it easier for them to reproduce and their larvae attach to the debris.
The researchers are concerned that with so much plastic in our oceans, and with climate change making cyclones and storms more intense, the threat of invasive marine species has never been greater. The tsunami research shows just how much of an impact this route can have.
"There's nothing comparable in the scale of what we've seen before in the history of marine science," said Prof Carlton.
"The thousands of kilometres travelled, the sheer diversity of the community combined with how long this has been going on - so this has really reset the stage for the role of marine debris and its potential dispersal of invasive species."