The towering tsunami, triggered by a 9.0 magnitude quake on the afternoon of 11 March 2011, generated five million tonnes of debris from the three prefectures of Iwate, Miyagi and Fukushima.
The species include crustaceans, sea slugs and sea worms.
Most of the creatures found in the U.S. were attached to buoys, boats, crate and other items made from plastic, fibreglass and other materials that do not decompose.
Almost two-thirds of the species had never been seen on the west coast of the U.S., the study said.
Six years later, scientists are still finding all types of debris washed along the shores of Hawaii and the USA west coast.
The researchers from the Smithsonian Environment research center and other institutions have found out 289 marine species of Japanese origin in the United States coastal region.
"This has turned out to be one of the biggest unplanned natural experiments in marine biology - perhaps in history", said John Chapman, an expert at Oregon State University who co-authored a study of the creatures published this week in the journal Science.
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Almost 20 per cent of the newly arrived species were capable of reproduction and still new species continued to arrive this year, explained marine ecologist Jessica Miller, adding: "We were able to not only identify this unique suite of species but, in some cases, examine their growth and ability to reproduce which provides useful information on how they fared during their transoceanic voyage". With more arriving all the time scientists are concerned.
Numerous creatures were attached to plastic. Plastics and microplastics can easily survive years at sea, unlike natural materials like wood.
"I didn't think that most of these coastal organisms could survive at sea for long periods of time", says one of the researchers, Greg Ruiz, from the Smithsonian Environmental Research Centre. Since many species were just recorded on one piece of debris, however, the authors estimate that the real number of species that hitched a ride across the Pacific is much higher. "Now plastic can combine with tsunami and storm events to create that opportunity on a large scale".
The slow travel speed of the debris may have also allowed for a more hospitable environment, and enabled the animals to adapt to their new surroundings.
In total, they detected 289 living species on tsunami debris originating from Japan, and they suspect there are far more that escaped their notice.
"There is huge potential for the amount of marine debris in the oceans to increase significantly", said lead author James Carlton, an invasive species expert with the Maritime Studies Program of Williams College and Mystic Seaport in CT.
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