In recent weeks, about a billion jellyfish-like "purple sailors" have washed up on West coast beaches of the United States. The animals—known as "by-the-wind sailors" or Velella velella—founder on the shore and pile up like a carpet of deflated blue and purple balloons.

The jellies started washing up on Oregon and Washington State beaches four to six weeks ago, says Kevin Raskoff, a marine biologist at Monterey Peninsula College in central California. Then they started showing up in California.

The animals usually float on the surface in the open ocean, riding wind and water currents in search of food using a hardened, triangular "sail." But in years when the wind changes direction, they are pushed toward shore—and almost certain death. (See stunning portraits of the Portuguese man-of-war, another open ocean "sailor.")

The mass strandings aren't unusual, Raskoff says. They happen about every three to six years.

Raskoff estimates that as many as a billion Velella have piled up on beaches along the West coast. But no matter how pretty they are—the biologist says their dried out sails look like transparent Pringles—people shouldn't collect them. It's against California law to collect marine life without the proper permits.

Since these animals are related to jellyfish, they can also sting. Velella are predators and hunt microscopic plankton on the ocean's surface. (See more pictures of diaphanous, sometimes deadly, jellyfish.)

If you touch them, you might not feel any pain in your fingers, Raskoff says. But if you rub your eyes or touch a more sensitive part of your skin, "you're going to feel it." In other words, the rule is: don’t touch.