For the first time in recorded history, researchers have discovered that entire communities of coastal species have crossed thousands of miles of water floating on makeshift rafts.
Between 2012 and 2017 nearly 300 species of marine animals arrived alive in North America from Japan, having travelled on crates and other objects released into the Pacific following the Japanese earthquake and tsunami of 2011.
Although the natural disaster was an extreme case, scientists say it is likely that many non-native species are travelling across thousands of miles of water on ‘ocean rafts’ of marine plastic, carried by storm surges.
"I didn't think that most of these coastal organisms could survive at sea for long periods of time," said Greg Ruiz, marine biologist at the Smithsonian Environmental Research Center.
"But in many ways they just haven't had much opportunity in the past. Now, plastic can combine with tsunami and storm events to create that opportunity on a large scale.
Oregon State Universitymarine scientist John Chapman added: "This has turned out to be one of the biggest, unplanned, natural experiments in marine biology, perhaps in history."
The 2011 tsunami created a wave of up to 125 feet which swept millions of objects out to sea, from small pieces of plastic to entire fishing boats, and even docks.
Scientists began finding tsunami debris washing up in Hawaii and western North America in 2012, with living organisms still attached.
In total, they detected 289 living species on tsunami debris originating from Japan, and they believe there are far more. The team was still finding new species when the study period ended in 2017.
Mollusks such as mussels occurred most frequently of all invertebrate groups. Worms, crustaceans and bryozoans that form branch-like underwater colonies were close behind. Nearly two-thirds of the species had never been seen on the US West Coast.
None of the species were known to survive a transoceanic rafting voyage between continents, largely because the open ocean is considered to be a harsher environment for creatures used to more hospitable waters of the coasts.
However, scientists think the slower speed of ocean rafts - 1 or 2 knots - may have allowed species to gradually adjust to their new environments. The sluggish pace may also have made it easier for some species to reproduce and for their larvae to attach to the debris.
Much of the debris the scientists found rafted ashore was made of fiberglass or other plastic materials that do not decompose and could easily survive six or more years at sea.
"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 lead author James Carlton, an internationally known invasive species expert with the Maritime Studies Program of Williams College and Mystic Seaport.
"One thing this event has taught us is that some of these organisms can be extraordinarily resilient.”
More than 10 million tons of plastic waste enter the ocean each year, and that figure may increase 10 times by 2025.
Hurricanes and typhoons, which scientists also expect to become more frequent due to climate change, also can sweep debris out into the ocean.
The research was published in the journal Science.