No, dear beachgoer, it’s not your imagination. It seems to be a bumper season for brainless, bloodless, boneless and heartless jellies at some East Coast beaches.

There have been reports and warnings all summer of their increased presence at beaches and their stings to beachgoers. Some of the creatures are also behaving unusually, with moon jellyfish — not typically known to sting humans — doing so on Cape Cod in Massachusetts, and a swarm of jellyfish forcing a shutdown of a nuclear power plant in France.

But there’s nothing extraordinary about any of this, scientists say.

There cannot be a jellyfish census, and there isn’t even a consensus on how many species of jellyfish exist, but there are at least 1,000. So it’s impossible to say whether there is a surge in the population of jellyfish, which have survived for over 500 million years.

“I don’t think it’s time to push the panic button yet,” said Chris Doller, an expert at the New England Aquarium in Boston.

A group of researchers looked at several hundred years of historical records, finding that there appeared to be a cyclical pattern in the occurrence of big jellyfish blooms over a period of several decades.

That pattern, overlaid with steadily warming ocean waters in many parts of the world, especially along the coast, may be helping jellyfish expand their range, according to one of those researchers, Larry Madin.

“Certainly, that’s true here in the Northeast, in the Gulf of Maine,” said Dr. Madin, deputy director emeritus at the Woods Hole Oceanographic Institution in Woods Hole, Mass.

“It’s true in the Gulf of Mexico, it is true in a lot of coastal areas, and warm waters enable a lot of things to grow faster. So that may be a factor which is accelerating some of this growth of the jellyfish population.”

The warm waters create more favorable breeding conditions for many jellyfish species, and can extend their reproductive seasons. But the warming water can also disrupt their food sources, like krill, copepods and fish eggs.

The jellyfish may also be increasing because of the dwindling population of predators like sea turtles and tuna. There are also more polluted low-oxygen “dead zones” where jellyfish can survive; and the building of infrastructure, like docks, marinas and pipelines for jellyfish polyps to latch onto.

The authorities in Maine and Massachusetts have issued warnings this summer to beachgoers about the increase in jellyfish, after stings were reported.

In June, a nearly five-foot-wide lion’s mane jellyfish washed ashore, officials in South Portland, Maine, warned. The moon jellyfish on Cape Cod have been stinging people and have grown to a larger size, researchers have reported, though they are not sure why.

“We do have the molecular tools now to try to sort this out, and people are working on it, but I don’t think they have the answers,” Dr. Madin said.

While the sting of a jellyfish can ruin a beach trip, Mr. Doller said that the risk of serious injury or death is negligible.

Still, some towns have temporarily closed beaches out of an abundance of caution, he said.

“They’re very difficult to predict and nearly impossible to quantify,” he said of jellyfish. “You can’t see what is five feet underneath the surface. They drift with the tides, they drift with the winds, and the technology does not exist to track jellyfish yet.”

People have tried to do so by using mechanical devices, but that technology has not been fine-tuned, he said.

The New England Aquarium sits on Boston Harbor, and Mr. Doller said that some summers, though not this one, he has looked out his window and there have been so many moon jellies that “you feel like you could walk across them, the water is so thick with them.”

In Delaware, beach patrol captains reported a steep rise in jellyfish activity and stings in July, the most they’ve seen in recent memory. The lifeguards keep bottles of white vinegar with water to pour on stings and to relieve the pain, which feels like tiny hypodermic needles shooting into skin.

Mr. Doller, too, has been stung, despite wearing protective equipment while at work, and said it felt like “a really bad sunburn.”

“I always tell people that jellies have no brain,” he said. “So they’re not actually swimming after the swimmers. People going into the water have to sort of avoid them.”