By Courtney Sherwood

PORTLAND Ore. (Reuters) - Millions of jellyfish-like creatures have washed up on beaches along the U.S. West Coast over the past month, giving the shoreline a purple gleam and, at times, an unpleasant odor, ocean experts said on Thursday.

Though not poisonous to most people, beachgoers should avoid the animals because their venom can cause stinging in the eyes and mouth, said Steve Rumrill, an expert at the Oregon Department of Fish and Wildlife.

Known as Velella velella to scientists, and more informally as "by-the-wind sailors," the creatures regularly cluster offshore each spring. But it is unusual for so many to wash ashore at once, especially this late in the summer, he said.

In addition to the millions that have been spotted on beaches from Southern California to Washington, millions more are floating near the ocean surface offshore, Rumrill added.

Ocean experts do not know why more by-the-wind sailors are washing up this year, or why they are arriving later than usual, said Erin Paxton, spokeswoman for the Oregon Coast Aquarium.

Climate change may be a factor, but it is impossible to be certain, Rumrill said. "This is a wind-driven event, and winds are unusual this year," he said.

Though most people think the animals are jellyfish, they are in fact colonies of much smaller creatures known as hydrozoans, Rumrill said.

Hundreds of tiny organisms cluster together to create a gleaming purple body and a translucent sail-like protrusion that looks like a single animal.

(Editing by Daniel Wallis and Eric Walsh)