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Going swimming? Lookout for lion’s mane jellyfish, officials warn

If you see one of these, stay far away: their tentacles can grow up to 120 feet long.

A foot-wide Lion's Mane jellyfish floats in the water of Scituate Harbor as it passed in the current of high tide at Veteran's Memorial Bridge. Photo by John Tlumacki/Globe Staff

The heat wave is over, but Massachusetts beachgoers still thinking about taking a swim should double check for lion’s mane jellyfish.

In a June 9 advisory, the Department of Conservation and Recreation warned the public of the presence of lion’s mane jellyfish at the Nahant Beach Reservation, Wollaston Beach in Quincy, and Carson Beach in South Boston. Last week the department also issued a warning for Nantasket Beach in Hull.

Chris Doller, supervisor of changing exhibits at the New England Aquarium, told The Boston Globe smaller lion’s mane jellyfish have been spotted in the harbor outside the aquarium.

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Lion’s mane jellyfish, also known as giant jellyfish, are among the largest jelly species in the oceans. These jellyfish have a translucent bell-shaped top, and are commonly a dark red or yellow color. Tentacles can grow up to 120 feet long, and stings are painful and can even cause allergic reactions in humans.

Lion’s mane jellyfish found in New England typically have a top about a foot wide and tentacles up to 20 feet long, according to reporting from the Globe, but big ones have had tops several feet wide and tentacles up to 50 feet long.

Last summer the Globe reported an increase in the number of jellyfish sightings, which researchers could not explain.

Asked whether climate change could result in larger populations of the jellyfish, Doller told the Globe, “There’s no concrete evidence that it’s related or not. It is pretty cyclical.”

According to Oceana, a global nonprofit working to restore the oceans, research suggests lion’s mane jellyfish thrive in areas affected by human activity, such as overfishing, climate change, and pollution. These factors reduce their main predators, like birds and sea turtles, and increase prey, creating a favorable environment.

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