Jellyfish-Like Creatures Invade US West Coast
The creature is known as the "by-the-wind sailor" but its scientific name is Velella Velella.
"The numbers, if you extrapolate, are awe-inspiring," said Kevin Raskoff, a professor of Biology at Monterey Peninsula College told Southern California Public Radio.
"With some of my students we counted more than a thousand per metre," Mr Raskoff said.
"The numbers get astronomical pretty fast."
Sightings have been reported in California, Oregon and Washington states, as well as across the border on Canada's West Coast.
The creatures are carnivorous and sting, but usually not enough to harm humans.
They normally live several miles off the coast, and once they wash ashore, they die and decompose, becoming cellophane-like.
The National Geographic said experts have been trying to explain the cause of such "blooms".
Possible reasons range from ocean currents and winds to over-fishing, which deprives jellyfish of some of their favourite foods, it said.