These Mysterious Creatures Are Washing Up On SF's Beaches. Here's What To Know

SAN FRANCISCO, CA — No, those aren't jellyfish scattered across San Francisco's beaches.

Masses of blue translucent creatures known as Velella velella have been washing ashore across San Francisco's coastline in recent days, including at Baker Beach and Ocean Beach.

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The Velella velella, commonly referred to as "by-the-wind sailors" due to the sail-like structure that makes up the top of their body, are not jellyfish despite their close resemblance. However, they pack a similar punch.

Experts say the creature's triangular-shaped sail is diagonal to the soft tissue that surrounds it like a sailboat, allowing the animal to effortlessly glide along the surface of the ocean.

Wind directions will at times send the creature toward the coastline, stranding it, according to the San Francisco Chronicle, which first reported the recent mass appearance of these creatures along the city's beaches.

"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," according to the Point Reyes National Seashore website.

The Vellela have a lifespan of one year, during which they go through two phases: a polyp stage, where most people see them in their sail-like phase, and a medusa stage, where they look like a jellyfish.

But despite having a medusae stage, it is not considered a "true" sea jelly because Velellas are colonial hydroids with individual male and female polyps that serve different roles within their colony.

Experts say unsuspecting swimmers, surfers or people walking along the sand might often confuse their sting with that of a jellyfish.

Although the Velella possesses stinging cells that help it feed in open water and protect itself, just like jellyfish and other cnidarians, they're not powerful enough to penetrate human skin.

Still, the sting can cause mild skin irritation that can spread by touch if not washed off with a mild soap.

"They can sting more when they are floating in the water and less so when they've been thrashed in the surf before beaching onto the coarse sand," Roundhouse Aquarium's Program and Operations Director Marissa Wu previously told Patch. "Then the Velella velella tend to be worn out and quite harmless by the time folks discover them among the beach wrack."