Tiny algae and a whale native to the Pacific have crossed a thawing Arctic Ocean in what may portend a marine invasion threatening Atlantic fish stocks, scientists said on Sunday.

The Pacific algae, absent from the North Atlantic for 800,000 years according to fossil records, apparently returned after climate change thawed sea ice and currents carried the microscopic plants across the Arctic Ocean, they said.

And a gray whale spotted in the Mediterranean in 2010 -- 300 years after the species was hunted to extinction in the Atlantic region -- is believed to have swum from the Pacific through newly ice-free waters in the Arctic Ocean in summertime.

"It's a Pandora's Box," said professor Chris Reid, from the Sir Alister Harvey Foundation for Ocean Science in Britain who said the algae had now drifted almost as far south as New York.

"We can expect more species to come through from the Pacific," he told Reuters. Upheavals to life in the seas have been documented by European scientists from 17 marine institutions in 10 nations in a project called CLAMER.