Tsunami carried Japanese animals to US

Researchers have recorded 289 separate species which they say survived the trip from Japan to Hawaii or the US West coast.

Among those species were fish, mussels, snails, worms, crabs and algae. Some of them survived up to six years at sea. Many reproduced during the voyage.

The species were discovered on more than 600 separate "objects" surveyed on the US Pacific coast by a team led by James Carlton of Williams College.

The objects included anything from small pieces of plastic to entire ships and even structures from harbors and marinas, such as a dock that washed up on the coast of the US state of Oregon.

How did they get there? Once the tsunami's waters had pulled these objects into the ocean, currents sent them along on a subsequent journey of 7,000 kilometers or more.

One of the participating researchers, John Chapman from Oregon State University, described a sense of "shock" upon seeing so many species survive the journey - but also worry.

"The crustaceans and bivalves are of particular concern because they could introduce new diseases, and compete with, displace or otherwise affect our oyster or mussel populations," he has said of the new arrivals.

Not a one-off event

At this point it remains unclear whether any of the Japanese species have found permanent homes in their new marine environments in the US. Such a process could already be underway but would take decades to document.

Species are still arriving, but most are now doing so on pieces of plastic. That's because wooden "floats," which decompose rapidly on open ocean waters, stopped arriving after 2014.

Still, are such events limited to the kind of mega-tsunamis that occur after earthquakes of 9+ on the Richter scale?

The scientists suspect two factors will increase these cross-oceanic transfers of species in the coming years.

The first is the increasing accumulation of plastic "transporters" in our oceans. Ten million tons of plastic reach the ocean yearly, with the number expected to rise in the near future.

The second is due to climate change, which is expected increase the strength - and possibly the frequency - of extreme weather events, like hurricanes or tropical storms. These storms are capable of pushing local species into open waters, where the current takes them away.

The article was published in the US journal "Science."