Thousands of bright-blue sea creatures known as Velella velella, commonly called “by-the-wind sailors,” washed up on Baker Beach, Crissy Field and Ocean Beach in San Francisco on Monday, covering stretches of sand along the city’s western and northern waterfront.
Despite their jellyfish-like appearance, Velella velella are not true jellyfish. The National Park Service describes them as free-floating hydrozoans, related to jellyfish, sea anemones and corals. They live at the ocean’s surface and drift with the wind, using a small, clear sail to move across the water.
That same sail is also what sends them ashore.
The immediate reason for the wash-up is wind direction: Strong onshore winds can push floating colonies toward land, and the creatures cannot swim against the current or steer themselves back out to sea. Once they are blown into the surf zone, waves strand them on the beach.
“When the prevailing winds shift, such as during a storm, the Velella are driven towards the coast, where they often are stranded on beaches in great numbers,” Point Reyes National Seashore says on its website.
Once stranded, they don’t last long. As they dry out, the park service says, they become brittle and transparent, “looking like a cellophane candy wrapper.”
The Bay Area has seen big Velella wash-ups before. Scientists and park officials have long said the events are hard to predict, though they are most common when seasonal wind patterns push floating colonies toward shore.
The phenomenon has been showing up elsewhere in California. There were recently large beachings on Central Coast shores, and vast numbers had been spotted offshore in the Santa Barbara Channel and on local beaches.
The creatures use stinging cells to catch plankton, but officials say they are generally not dangerous to people. Even so, beachgoers are advised to leave them alone and keep pets from eating them.
For anyone walking the coast, the display is vivid but brief. The blue soon fades. Within days, the creatures dry into pale, papery shells that are carried off by wind and waves.