Every spring, "pulses" of floating garbage from the Pacific trash vortex arrive on the US West Coast—and every year since 2011, more unwanted visitors from Japan arrive.

Researchers say that the devastating Japanese tsunami sent an armada of debris across the ocean, with "mega-rafts" carrying a horde of sea creatures to new territory, the Australian Broadcasting Corporation reports.

Researchers say they have detected at least 289 species transported 4,800 miles across the ocean by tsunami debris, including invasive species that could badly damage West Coast oyster and mussel fisheries.

They say it is too early to tell how many of the species will take root and push out native species. "It's a bit of what we call ecological roulette," says James Carlton, lead author of a study published in the journal Science.

His team searched beaches from Alaska to California for Japanese species. "The diversity was somewhat jaw-dropping," he tells the AP. "Mollusks, sea anemones, corals, crabs, just a wide variety of species, really a cross-section of Japanese fauna." He says a big part of the problem is that unlike in earlier ages, when tsunami debris would have been wood and other organic materials, much of the debris is plastic and has not degraded, allowing the invaders to survive for years and spawn new generations at sea.

The researchers call it an "extraordinary transoceanic biological rafting event with no known historical precedent" and "the longest documented transoceanic survival and dispersal of coastal species by rafting." (An entire dock washed ashore in Oregon 15 months after the tsunami.)

This article originally appeared on Newser: In Wake of Tsunami, the Unprecedented Happened in Pacific