Giant jellyfish trailing long, thick clusters of tentacles that can cause painful stings are showing up in increasing numbers at Massachusetts beaches, and researchers say they aren’t sure why.
The blobby, bell-shaped creatures are called lion’s mane jellyfish, and officials are warning swimmers to be on the lookout for them.
“They’re not uncommon jellies to see in New England waters, but for some reason this summer there’s a lot of them, and they’re getting really big,” said Chris Doller, supervisor of changing exhibits at the New England Aquarium.
The giant jellyfish, which is the largest species of the marine invertebrates, have been seen off the coast of Maine, along Cape Cod, and in Rhode Island, Doller added.
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In New England waters, a typical lion’s mane jellyfish has a translucent, saucer-shaped bell that measures about 10 to 12 inches wide, with tentacles trailing up to 20 feet long.
But even larger specimens have been washing up on local shores lately. Some have bells that measure several feet wide with tentacles of up to 40 to 50 feet, he said.
Their tentacles are so long that they could be 10 to 20 feet away from you and still sting you, he said.
After months of being cooped up with few places to go, residents eager to enjoy the pleasures of summer have yet another concern: encountering the otherworldly visitors at their favorite beach.
“They’re considered one of the more stingy jellyfish,” Doller said. “They can really sting you, even if they’re washed up on the beach.”
Their sting is painful but not usually fatal. It typically causes a burning sensation that can be treated with vinegar, which helps neutralize the sting.
Lion’s mane jellyfish sightings have been reported throughout Hingham Harbor and at beaches in Scituate. The jellies, which pulse balletically through the shallow waters, have also been seen off Nahant.
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One unlucky Hingham resident was recently stung by one, and “experienced something similar to a brief electric shock followed by a stinging sensation,” according to the Hingham harbormaster. She treated it with a jellyfish sting kit and vinegar.
On June 17, the Duxbury Police Department tweeted out a stark warning to beachgoers.
“BE ADVISED: We are seeing a large amount of Lion’s Mane Jellyfish in the water, both ocean and bay and some are washing up,” Duxbury police tweeted. “DO NOT TOUCH THEM. If you happen to come in contact with one, please alert a lifeguard or a Beach Ranger.”
It’s not clear why there seem to be more of the jellyfish in local waters.
“It’s the question everyone’s asking, and we don’t really know the answer right now,” said Doller.
It could be the currents, the weather, or an increase of food in the water, he said.
“We just don’t know,” he said.
Keith Ellenbogen, an underwater photographer and MIT Sea Grant visiting artist, received an Ernest F. Hollings Ocean Awareness Award to capture images of marine life within Stellwagen Bank National Marine Sanctuary. He has also noticed more lion’s mane jellyfish in the water.
“They’re large ... and they have this red and yellow hue to them,” he said. “They’re absolutely beautiful animals.”
Ellenbogen said one of the things that amazes him is how some species of fish take shelter in the jellyfish’s stinging tentacles. He has photographed juvenile haddock swimming alongside the tentacles of a lion’s mane jellyfish, using them for protection.
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Anne Smrcina, the education and outreach coordinator at the Stellwagen Bank National Marine Sanctuary, said she was at Scituate Harbor on Wednesday and counted a dozen lion’s mane jellyfish right around the sanctuary dock.
“They’re amazingly beautiful creatures, and they’re an important part of the food web,” she said.
Smrcina said as the number of sightings has increased, people should be aware.
“It’s more than we’ve seen in the past,” she said. “They’re out there.”
Emily Sweeney can be reached at emily.sweeney@globe.com. Follow her on Twitter @emilysweeney and on Instagram @emilysweeney22.