These are marine sea slugs from a Japanese vessel from Iwate Prefecture, washed ashore in Oregon in April 2015. Photo: John Chapman

A 9.0 magnitude earthquake rocked Japan on March 11, 2011, causing a massive tsunami that rose 125 feet over the country’s coast on the same day.

Six years later, scientists are still finding all types of debris washed along the shores of Hawaii and the U.S. west coast. The debris--including fishing vessels, docks, buoys and pieces of plastic--acted as makeshift rafts for hundreds of Japanese marine species to cling to and hitchhike across the Pacific Ocean, a first in recorded history.

Marine biologists from the Smithsonian Environmental Research Center, Williams College, and other institutions report in Science this week that at least 289 living marine species native to the Japanese coast traveled 4,800 miles to the U.S.

Groups of animals have been documented as far north as Southwest Alaska, and as far east as Central California. A total of 635 debris objects were found with animals living onboard. The first species were identified in 2012, and although findings have become more infrequent, the researchers reported finding species clinging to debris in 2017.

The team believes they may have found just a fraction of the species that actually washed ashore along the U.S., since many pieces of debris were cleared away before any analysis or census began.

The nearly 300 invasive species identified included mollusks (which were most commonly found out of all the invertebrate groups), fish, barnacles, sea slugs, crabs, clams and sponges, among others. The fish discovered miraculously survived in water-filled troughs on fishing boats that somehow didn’t overturn during the lengthy trek.

Nearly two-thirds of the species had never been spotted on the U.S. west coast before. And many of the pieces of debris collected were classified as “high-richness arrivals”--meaning they carried more than 20 different species.

"This has turned out to be one of the biggest, unplanned, natural experiments in marine biology, perhaps in history," said co-author John Chapman, of Oregon State University.

The researchers believe that synthetic, non-biodegradable materials like plastics and fiberglass provided an unprecedented opportunity for the marine species to survive the years-long, open ocean journey. According to a 2015 Science study, more than 10 million tons of plastic waste enter the ocean each year. By 2025, that number could be 10 times as much. 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," said Greg Ruiz, a co-author and 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."

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. The debris moved only 1 to 2 knots, compared to 20 or more knots typical of a commercial ship. This slow speed could have also made it easier for some species to reproduce and attach their larvae to the debris.

The team reported that as of now, none of the invasive species have colonized the west coast as a direct result of the 2011 tsunami. But they do note it can take years to detect an established population of a non-native species.

The findings do highlight the power and potential of invasive species, however. And if natural disasters and storms continue to become more severe in the future as many climate models suggest, and plastic waste continues to increase, this phenomenon could be observed more often.

Researcher John Chapman inspects a Japanese vessel that washed ashore on Long Beach, Wash. Photo: Russ Lewis via AP