Groton — Moise Solomon has been stung by jellyfish many times while swimming in local waters, experiencing some mild discomfort that fades quickly.

But the sensations Solomon experienced on Sunday after swimming in the eelgrass beds at Mumford Cove with his daughter were nothing like that.

“This was completely different. It started out as intense and built in intensity,” said Solomon, 50, a part-time resident of the neighborhood near the cove. “I had a huge welt on my shoulder that swelled up, muscle pain, a constricted chest, and generalized, stabbing pain.”

After the sting, Solomon went to Lawrence + Memorial Hospital’s Pequot Health Center for treatment, where doctors gave him steroids, Claritin and an intravenous antacid and antihistamine to reduce inflammation, then transferred him to Yale-New Haven Hospital for further treatment. He was released a few hours later.

“I’m feeling better now, though I still have some pain,” he said Monday. He’s planning to stay out of the eelgrass beds, and warned his neighbors to do the same through an email list.

Based on his symptoms and the fact that he was in an eelgrass bed when he was stung, marine biology experts have concluded that he probably encountered a clinging jellyfish, a small and somewhat mysterious invasive species from the western Pacific Ocean that packs a venomous punch.

Annette Govindarajan, research specialist at the Woods Hole Oceanographic Institution in Woods Hole, Mass., said the species, which goes by the Latin name Gonionemus vertens, has been recorded in Groton waters as far back as 1894, and continued to be found in eelgrass beds there through the 1930s but was not known to be venomous. When the eelgrass beds declined, the clinging jellyfish went with it.

Now, after regrowth of the eelgrass beds in Mumford Cove over the last two decades — a result of improved water quality after a sewer outfall pipe discharging into the cove was moved in 1987 — the clinging jellyfish have returned, but this time with a powerful sting. Govindarajan said Solomon’s case is the first known report of someone in Connecticut getting stung by a clinging jellyfish.

“The symptoms can last a few days, but there haven’t been any fatalities,” she said. “Most of the stinging reports we’ve had since the 1990s have been on Cape Cod and Martha’s Vineyard. Eelgrass researchers are at the highest risk.”

The clinging jellyfish favor sheltered coves with little wave action, so swimmers at sandy beaches on open waters aren’t at risk, she said. But anyone going into areas with eelgrass should be wary, she said.

Populations tend to be “patchy,” she said, and individual jellyfish are small — from the size of a dime to a quarter. If they’re seen at all, it’s usually too late — when they’re dislodged from the eelgrass they’ve been hanging onto, and apt to sting the cause of the disturbance.

Paul Bologna, director of the Marine Biology and Coastal Sciences Program at Montclair State University in New Jersey, has been tracking recent reports of clinging jellyfish. They were found in some of the salt ponds in Rhode Island last year and are showing up in several areas on the Jersey Shore this summer, he said. Thus far there has been one confirmed sting there, he added.

“It’s the excruciating pain and cramping that escalates that distinguishes them,” he said. “It can cause cognitive impairment and slurred speech, because their venom is a neurotoxin they use to paralyze their prey.”

Research thus far indicates the earlier, nonvenomous variety of clinging jellyfish came from the eastern Pacific, while the current ones are a cousin from Asian waters of the western Pacific. Clinging jellyfish polyps can cling to rocks in sheltered coves for many years, then suddenly bloom into adults due to triggers that aren’t well understood, Bologna said. In New Jersey, clinging jellyfish populations are being controlled by sea nettles, another invasive jellyfish that eats them but is not common in Long Island Sound.

“If you’ve seen one, there are likely to be a lot more,” said Bologna, who plans to sample Mumford Cove for clinging jellyfish next weekend, hoping to understand how many are there. He advised people to stay out of eelgrass beds or to wear a wet suit while paddleboarding, clamming or wading in these areas. Anyone who does get stung should seek medical treatment.

“They’re so small, it’s hard to see them,” he said.

j.benson@theday.com