They are iridescent, bright blue and jellylike, with stinging tentacles that dangle from their flat, oval-shaped bodies. Just three to four inches long, these alien-looking creatures use a short, clear flap of skin to catch gusts of wind, propelling them through the ocean.

That flap, or sail, is how they got their nickname, “By-the-Wind Sailors,” but it is less helpful when the creatures are stranded by the thousands on California beaches, as they are now.

In the past few days, nature aficionados and everyday passers-by alike have reported the phenomenon, posting photos of the invasion on social media and on biodiversity-spotting websites.

The marine carnivores, whose scientific name is Velella velella, are not quite jellyfish but are closely related. They appear every spring and have been doing so for “millions of years,” said Steven Haddock, a marine biologist at the Monterey Bay Aquarium Research Institute. But every few years (including this one), when especially strong winds blow in the right direction, millions of them can wash ashore in droves, “spanning from Baja California to Alaska,” he said.

This year, they’ve been spotted along the coasts of Washington and Oregon, although a majority of sightings so far have been in California.

Despite the velella’s stingers, those who stumble upon them have no need to worry, Mr. Haddock said. They’re not likely to sting a human who picks them up by their sail.

“People may have never seen that many jellylike things washing up on the beach, but it’s really not cause for alarm,” he said.

Usually, they live near the ocean surface, where they can bunch together by the thousands. Mr. Haddock said that at times, they accumulate in such a way that if you’re on a boat and see a colony, it would appear as if you could step out and walk across them.

They are prey for larger animals like sea turtles and ocean sunfish, he said. The velella, whose name is derived from a Latin word meaning “little sail,” usually feed on plankton, using their dangling stingers to capture them.

And for those who might feel drawn to help them return to their ocean home, there’s really no point, Mr. Haddock said. By the time the creatures are blanketing the beaches, they’re normally already toward the end of their life cycle.

“The thing that you see washed up on the beach is actually what you would normally find attached to a rock or something on the sea floor,” he said. “They’re pretty much on the way out.”

Soon enough, their blue and purple hues will fade to white, and then all that will be left of them are the brittle husks of their sails, looking more like cellophane candy wrappers or translucent potato chips.

“Within a couple weeks, they will probably be desiccated and just blow away, like a piece of rice paper,” he said.