Groton — Last week, Annette Govindarajan collected 40 clinging jellyfish in just five minutes of surveying the waters of Mumford Cove, a protected inlet on Fishers Island Sound between Bluff Point and Groton Long Point.

“Last year when I looked I just found them in one small patch,” Govindarajan, a researcher at the Woods Hole Oceanographic Institute in Woods Hole, Mass., said Thursday. “This year we found them everywhere we looked in the eelgrass beds. The jellyfish are exceptionally abundant and widespread in Mumford Cove this year.”

People stung by clinging jellyfish, an invasive species native to the North Pacific, can have reactions that range from mild to so severe they require hospitalization. Respiratory and neurological symptoms from the stings can last from three to five days.

Govindarajan wanted to get the word out to local residents to be aware of the risks if they’re swimming or boating in Mumford Cove and surrounding waters, including those off nearby Pine Island and Groton Long Point.

She didn’t have to look far. Just a couple of miles west along the coast, at the Avery Point campus of the University of Connecticut, are the offices of Connecticut Sea Grant, which funded her research. Staff at Sea Grant let Ledge Light Health District and representatives of the Mumford Cove Homeowners Association know about the threat.

“We told Sea Grant we’d be happy to get the word out,” said Stephen Mansfield, director of health for Ledge Light, the public health agency for Groton and six other towns.

Mansfield said Ledge Light has asked the Mumford Cove association for permission to post warning notices in the neighborhood. Thus far he has not been informed about anyone with a severe sting reaction this summer, though one man was hospitalized after being stung there last summer.

Rich Langer, secretary of the Mumford Cove board of directors, said the group sent an email notice Tuesday to homeowners along with a fact sheet on the jellyfish, and is preparing a mailing. He’s also contacted the Groton Long Point Association. Anyone who plans to go swimming, paddleboarding, kayaking, water skiing or tubing in Mumford Cove also should be aware of the jellyfish risk, he added.

The jellyfish have been found in the waters of Cape Cod, Connecticut and other areas of the East Coast since the late 1800s, but they did not attract attention until the 1990s, when their numbers began climbing along with the toxicity of their stings. Whether this is due to a new strain of these jellyfish or more people with particular sensitivity to the stings being exposed is unclear, Govindarajan said.

“We’re also trying to understand how they’re getting around,” Govindarajan said. “They spend most of their time hanging out on the bottom, clinging to the eelgrass, and you don’t see them unless you disturb them.”

The jellyfish are chiefly found in calm, quiet marine waters, rather than in areas with strong waves. They also prefer eelgrass habitats, which recently have been restored in Mumford Cove and other areas as pollution sources have been eliminated and water quality has improved.

“Eelgrass making a comeback is a good thing, because it creates habitat for juvenile fish,” Govindarajan said.

Named clinging jellyfish because of sticky pads on their tentacles that allow them to adhere to seagrasses and seaweeds, they are about the size of a quarter, with transparent bodies with an orange-brown cross through their center.

They have been found in recent years off the New Jersey coast to Maine. Govindarajan said she surveyed local waters between Avery Point and Barn Island in Stonington, and found smaller populations of clinging jellyfish off Pine Island and Dodges Island in Stonington in addition to the proliferation in Mumford Cove.

“We don’t know why all of a sudden there seems to be a new wave of them coming in,” Govindarajan said.

Anyone who sees a clinging jellyfish is advised to avoid contact with it, and not to handle or collect it. If stung, people are advised to seek medical treatment if their reaction becomes severe.

j.benson@theday.com