TOKYO (NYTIMES) - The towering tsunami that devastated Japan six years ago also unleashed a very different sort of threat onto the distant coastline of North America: a massive invasion of marine life from across the Pacific Ocean.
Hundreds of species from the coastal waters of Japan - mostly invertebrates such as mussels, sea anemones and crabs - were carried across the Pacific on huge amounts of floating debris generated by the disaster, according to a study published on Thursday (Sept 28) in academic journal Science.
Less than a year and a half after the enormous earthquake and tsunami on March 11, 2011, left more than 18,000 people dead or missing in Japan, the first pieces of wreckage began washing up on the shores of Canada and the United States.
To the surprise of scientists, the debris was covered with sea creatures that had survived crossings that in some cases had taken years.
The study's authors say it is too early to tell how many of these tiny invaders have gained a foothold in North American waters, where they could challenge or even displace native species. While such "rafting" of animals across oceans happened in the past, the authors say the Japanese tsunami is unprecedented because of the sheer number of organisms that it sent across the world's largest ocean.
And this points to one of the main findings of the study: that this mass migration was the result of not just the huge natural disaster, but changes in human behaviour. Such large numbers of marine animals were able to cross the Pacific because they rode on debris - made of materials such as plastic and fibreglass - that proved durable enough to drift thousands of miles.
These synthetics, the use of which has taken off around the world, can stay afloat for years or even decades. The debris that was dragged out to sea by the 2011 tsunami formed an unsinkable flotilla capable of transporting a large population of organisms across the world's largest ocean.
"We have created a new ecological process, the process of mega-rafting," said professor of biology Steven Chown at Monash University in Australia, who was not involved in the report, but wrote a commentary that also appeared in Science.
"The development of materials that can float for ages, and the rising levels of seas due to climate change, make the possibility of these events larger and larger," he said.
This flotsam ranged in size from coolers and motorcycle helmets to entire fishing boats and even larger objects, teeming with living sea animals that were native to the coastal waters of Japan, but foreign to North America.
The larger the object, the more animals it carried. One of the first pieces of tsunami debris that appeared was a 180-ton floating dock that washed ashore in Oregon in June 2012. It was carrying a diverse mini-ecosystem of more than 120 different species.
"This was our first head's up, that this was the vanguard of what might be coming from Japan," said one of the report's co-authors, professor emeritus James Carlton of marine sciences at Williams College. "After that, we got a steady stream of reports of boats, buoys and other debris, all with Japanese markings, and all carrying an amazing cross section of Japanese sea life."
Prof Carlton called it remarkable that such a wide range of species - which also included barnacles, worms and tiny filter-feeders called bryozoans - could survive the journey across the northern Pacific.
In many cases, these passages took years, longer than the life spans of the individual organisms. The authors concluded that not only did these creatures adapt to an open ocean where food was scarcer than in rich coastal waters, they were also able to reproduce, in some cases for at least three generations, before reaching the North American coast.
"We found that hundreds of species could survive for multiple generations at sea," said Prof Carlton, who is a former director of William's Maritime Studies Program in Mystic Seaport, Connecticut. "They could do this so long as their rafts did not dissolve or sink."
To conduct the study, the authors relied on more than 200 volunteers, including state park rangers and beachcombers, to find and examine some 634 pieces of debris that washed ashore from 2012 to earlier this year.
While there was concern in the early days that some debris might have been contaminated from the nuclear accident at Fukushima that was caused by the tsunami, Prof Carlton said such worries quickly eased after tests showed no traces of radioactive contamination.
The washed up objects were found to carry 289 invasive species from the western Pacific. While most were invertebrates, a few vertebrates also survived the long journey, including a small number of emaciated fish that were trapped inside the water-filled hulls of half-sunken fishing boats.
All told, thousands of pieces of debris from Japan washed up on North American coasts from Sitka, Alaska, to Monterey, California, and as far afield as Hawaii. Since the authors and volunteers were only able to inspect a fraction of these objects, Prof Carlton said he believes hundreds more species likely made the crossing.