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Creatures raft across Pacific, thanks to tsunami

Hundreds of species hitched a ride on debris from the 2011 Japanese tsunami for a 4,400-mile journey across the Pacific Ocean.

These are marine sea slugs from a derelict Japanese vessel that washed ashore in Oregon in April 2015. Image via John Chapman.

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The 2011 Japanese tsunami set the stage for something unprecedented, say marine biologists. For the first time in recorded history, scientists have detected entire communities of coastal species crossing the ocean by floating on makeshift rafts. According to a study published in the journal Science on September 29, 2017, nearly 300 species have appeared on the shores of Hawaii and the U.S. West Coast attached to tsunami debris.

The March 11, 2011 tsunami swept millions of objects along the Japanese coast out to sea, from small pieces of plastic to fishing boats and docks. These kinds of objects, scientists said, helped the species attached to them complete the transoceanic journey.

An Asian amur sea star found on the Oregon coast. Image via Oregon State University.

Greg Ruiz is a study co-author and marine biologist at the Smithsonian Environmental Research Center. Ruiz said in a statement:

I didn’t think that most of these coastal organisms could survive at sea for long periods of time. 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.

Scientists began finding tsunami debris with living organisms still attached washing up in Hawaii and western North America in 2012. So far, they’ve detected 289 living species on tsunami debris – including buoys, crates, vessels and docks- originating from Japan, and they suspect there are far more that escaped their notice. While the arrivals have slowed down, they haven’t stopped. The team was still finding new species when the study period ended in 2017.

Pelagic gooseneck barnacles cling to a boat that got washed out to sea after the 2011 tsunami in Japan and floated to the U.S. coast. Image via ScienceNews.

Of all invertebrate groups, mollusks such as mussels were found most frequently. Not far behind were worms, hydroids (sea anemone and jellyfish relatives), crustaceans and bryozoans that form branch-like underwater colonies. Nearly two-thirds of the species had never been seen on the U.S. West Coast. According to the study:

None of the species had been known — or expected – 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, the slower speed of ocean rafts (1 or 2 knots, compared to 20 or more knots for commercial ships) may have allowed species to gradually adjust to their new environments. The sluggish pace of these “floating islands” may also have made it easier for some species to reproduce and for their larvae to attach to the debris.

The increase in marine plastics and other more durable debris also made survival easier, according to the researchers. 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.

So far, no new species are known to have colonized the West Coast directly due to the 2011 tsunami. However, it can take years after a nonnative species first arrives to detect a newly established population. This provides a window of time for action, although the full consequences of the tsunami-driven rafting are still uncertain.

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Bottom line: Hundreds of species crossed the Pacific Ocean on debris from the 2011 Japanese tsunami.

Read more from the Smithsonian Marine Research Center

Eleanor Imster

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