They are iridescent, bright blue and jellylike, with stinging tentacles that dangle from their flat, oval-shaped bodies. Just 3 to 4 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 passersby 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.…