TOKYO — The towering tsunami that devastated Japan six years ago also unleashed a very different sort of threat onto the distant coastline of North America: a massive invasion of marine life from across the Pacific Ocean.

Hundreds of species from the coastal waters of Japan — mostly invertebrates like mussels, sea anemones and crabs — were carried across the Pacific on huge amounts of floating debris generated by the disaster, according to a study published Thursday in Science. Less than a year and a half after the enormous earthquake and tsunami on March 11, 2011, left more than 18,000 dead or missing in Japan, the first pieces of wreckage began washing up on the shores of Canada and the United States.

To the surprise of scientists, the debris was covered with sea creatures that had survived crossings that in some cases had taken years.