Hordes of bright blue, jellyfish-like sea creatures are washing up on beaches across California.
As of Monday, thousands of the critters — called velella velella or by-the-wind sailors — have been sighted along the California coastline since the start of March, according to iNaturalist, an online platform for sharing biodiversity information.
What are the mysterious critters? Are they harmful to humans or pets?
Here’s what to know:
Although the transparent blue blobs resemble jellyfish, velella velella aren’t actually jellies, according to the National Park Service.
They’re hydrozoans — a species related to jellyfish, anemones and corals, the park service said.
The invertebrate, oval-shaped creatures typically measure 3 to 4 inches long, and have translucent, triangular sails.
The marine animals use their stinging tentacles to prey on plankton and “float freely far out on the open ocean’s surface,” Cayucos photographer Danna Dykstra Coy said in an April 10 Facebook post. “Unlike jellyfish, which drift and swim in ocean currents, velella are at the mercy of the wind.”
Storms and strong winds can knock by-the-wind sailors onto beaches, where “they often are stranded on beaches in great numbers,” the U.S. National Park Service said.
Tiny, electric-blue blobs have been spotted on beaches across the Golden State, with sightings recorded as far north as Humboldt County and as south as San Diego County, according to iNaturalist.
On the Central Coast, Morro Bay Harbor Patrol officer Jeremiah Jacobs said, strong northwest winds blow by-the-wind sailors into Morro Bay every spring.
Dykstra Coy reported spotting the critters on the beach by Cayucos Pier in early April.
“Their appearance usually means spring is on the way!” she said in her post.
In an April 10 post, Reddit user Aloysius1955 wrote that Hendry’s Beach in Santa Barbara was “covered” with the jellyfish-like organisms.
“I was just up in Monterey whale watching and we saw literally tens of thousands of these out in the bay,” Reddit user Rip_Dirtbag commented on the post. “They form like little islands together in the ocean and it’s a sight to behold.”
The “bizarre blue sea creatures” have also been spotted around the San Francisco Bay Area and on sandy stretches of shoreline in La Jolla, Pacific Beach and Coronado, The Mercury News and San Diego television station KSWB-TV previously reported.
By-the-wind sailors most frequently turn up on California beaches during the spring or early summer months, the National Park Service said.
“As the velella dries out on the beach, it becomes brittle and transparent, looking like a cellophane candy wrapper,” before fully decomposing, the park service said.
Velella velella have several blue tentacles that they use to sting their food, but they rarely pose a risk to humans, according to the National Park Service.
If you touch one, however, you should wash your hands before touching your mouth or eyes, Jacobs said.
Dog owners should make sure that their pets don’t chomp on the dried-out jellies.
The poisonous sea creatures won’t permanently damage dogs who eat them, but they can cause nasty side effects, The Bee reported in 2024.
By-the-wind sailors’ small stingers can upset a dog’s mouth and stomach if consumed, according to previous reporting from The Sacramento Bee.
If you think your dog has bitten or ingested a velella, you need to act quickly, giving your dog “ice-cold water for a stinging mouth” and bland foods for an upset stomach, Dr. Brieana Sarvis, hospital director of San Diego Humane Society’s Escondido Campus, told ABC 10 in 2024.
If your dog is vomiting continuously, take your pet to the vet as soon as possible, The Bee reported.