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Tropical snorkelers, be advised.

Unlucky divers and swimmers have experienced “stinging water,” a mysterious occurrence when people are stung when they get near, but don’t touch, Cassiopeia, a type of jellyfish that spends its time hanging out upside-down.

What causes this frustrating phenomenon? Scientists say it’s a malicious mucus emitted by the jellyfish.

After years of research, scientists published a study Thursday in Communications Biology, a journal from Nature Research, that revealed that toxic cells from Cassiopeia are shot into the surrounding water.

“The answer was right under our noses,” Cheryl Ames, a marine biologist and lead author of the study, said in an e-mail.

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Sparked by their own encounters with stinging water, a team led by scientists from the Smithsonian’s National Museum of Natural History set out to find the culprit. Examining samples of the jellyfish mucus under a microscope, researchers saw small balls spinning in the slime.

“We thought it might be caused by bits of jellyfish tentacles, a larval stage of jellyfish, or sea anemones, or some other unexplained phenomenon,” Ames said.

The bumpy little balls, called cassiosomes, turned out to be hollow spheres of cells that were likely filled with the same substance that gives jellyfish their structure, the Smithsonian said in a statement. Discovered for the first time, the cassiosomes were found to be fringed with thousands of stinging cells called nematocysts. Usually found in tentacles, nematocysts are filled with toxins.

When they provoked the jellyfish, researchers observed the self-propelled cassiosomes slowly breaking away and leaving the jellyfish’s arms and mingling with the animal’s mucus. The mucus then disperses in the surrounding waters, waiting for victims.

Angel Yanagihara, an associate researcher at the University of Hawaii who studies jellyfish venom and was not involved with the study, told the New York Times that the study is “paradigm-shifting,” changing the way scientists will think about jellyfish feeding and stinging going forward.

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Snorkelers need not be afraid, however, researchers said. No serious injuries or deaths have been caused by “stinging water.” To prevent such attacks, Ames encourages people to wear protective clothing in areas where there may be Cassiopeia.

US Navy scientists have been curious about the phenomenon for years because of its effect on their divers, Gary Vora, a coauthor of the study and a deputy laboratory head at the Naval Research Laboratory in Washington, D.C., told the Times.

When Navy divers return from the waters, sometimes they’re “lit up like a Christmas tree,” he told the Times. “You have evidence of a sting, but you never saw what stung you.”

Because of the findings, scientists have a better understanding of Cassiopeia’s feeding habits. It “uses the edge of its umbrella to push water over its frilly feeding structures,” sucking in a net of mucus filled with prey, such as tiny shrimp and plankton, Ames said.

“We now know that the prey are captured after being stunned and killed on contact with the motile cassiosomes in the mucus,” Ames said.


Material from Globe wire services was used in this report. Matt Berg can be reached at matthew.berg@globe.com. Follow him on Twitter @mattberg33.