Some beaches in the northeastern United States are dealing with more than the threat of COVID-19 this holiday weekend. They have to contend with an unwelcome visitor: the Lion's Mane jellyfish.
The fish, which can reach sizes of four feet in diameter with tentacles over a hundred feet long, have been spotted from Maine to Massachusetts.
"The ones that we're seeing here are probably not much bigger than maybe dinner plate, " says Steve Spina, Assistant Curator of Fishes at the New England Aquarium. "Which is big enough."
Because of their size, Lion's Mane jellyfish are particularly dangerous to swimmers. Their long tentacles can sting a person whose guard is down because the fish looks like it's feet away.
Officials in the area are warning residents and visitors to be on the lookout and to stay safe.
Beaches are flying purple flags to indicate "the presence of dangerous marine animals," said the Massachusetts Department of Conservation and Recreation in a tweet.
Hingham and Scituate beaches, both popular with tourists for the Fourth of July weekend, both put out public health statements advising caution when swimming in areas where the fish may be.
"The abundance of them this year is what's unusual. There's an awful lot," says Spina.
The jellyfish frequently appear in deep water in the North Atlantic in late spring and early summer. But this year, they're washing up on shore later-and in more significant numbers- than before.
The growth and development of the fish are triggered by temperature and seasonal changes in ocean water, which may explain why these jellies are showing up in new places.
At the "polyp" stage of a Lion's Mane's life cycle, the animal may "strobilate", or spawn, more jellyfish than is usual if temperatures are unstable, says Amy Arnold, Senior Aquarist in Acquisitions and Quarantine at the Georgia Aquarium.
"This could then cause a larger number of jellies seen than usual, and later in the season, then we are accustomed," she says.
In a year that has seen hundred-degree temperatures in Siberia, Lion's Mane jellyfish may be the latest sign of a warming planet.
"The short answer to all of this: global warming," says Arnold. "A one-degree temperature difference in the ocean can affect all of this."
However, it can be difficult to point to a clear explanation for year-to-year fluctuations. Jellyfish populations, says Spina, are "very unpredictable."
"It's tempting to blame a warming climate and warming waters, but it's hard to really make that statement without having a lot of good data to back it up," he says. "You really need some numbers and some time to sort of start drawing conclusions."
This is what beachgoers should do to stay safe
Lion's Mane jellyfish grow biggest in ice-cold water, so it's unlikely that people will see record-breaking fish over their holiday weekend. However, the tentacles require caution as they can float yards away from the fish, carried by the current, and still sting.
"It is also important to remember," says Arnold, "that if you see them washed up onshore and out of the water, they still have the ability to sting." This is particularly important to keep in mind in order to keep dogs and children safe.
While serious stings have been reported this year, none have required medical attention and the Lion's Mane sting is rarely fatal. Beachgoers are advised to stay out of the water where the fish have been spotted and to treat stings with sting kits.