Upside-down Jellyfish Create 'Stinging Water' That Kills Prey by Launching Mucus 'Grenades'

Scientists say they have unraveled the mystery of the unusual "stinging water" phenomenon long reported by swimmers and snorkelers who have strayed close to upside-down jellyfish—the creatures launch toxic mucus filled with tiny "grenades" of stinging cells.

Individuals who have experienced stinging water say it feels like being stung by a jellyfish, despite not having had any contact with the animals. Several hypotheses have been proposed to explain the phenomenon—including severed jellyfish tentacles, sea lice, anemones or other stinging marine animals—however, the exact cause has remained elusive.

But now, a study published in the journal Communications Biology, reveals what may be the real culprit.

"[This study] began when I and other marine biologists were concerned about the source of 'stinging water'—an irritating sensation that occurred while in the mangrove forest waters studying upside-down jellyfish, and working together with aquarists at major public aquariums," Cheryl Ames, an author of the study from Tohoku University, Japan, and the Smithsonian's National Museum of Natural History, told Newsweek.

"There were several theories exchanged by fellow marine biologists, and comments posted online by people after experiencing stinging water during snorkeling or swimming in those areas. We wanted to find out the scientific explanation behind the long-standing stinging water puzzle," she said.

Ames and colleagues investigated a jellyfish from the genus, or group of species, Cassiopea—which are commonly referred to as "upside-down jellyfish." These animals are found in warm coastal waters, such as mangroves, bays and lagoons, in Australia, Bermuda, Fiji, the Florida Keys, the Caribbean Islands, the Hawaiian Islands, Indonesia, Palau, Panama, Papua, New Guinea, and the Red Sea, as well as invasively in the Mediterranean Sea near Turkey.

"Cassiopea, like its common name upside-down jellyfish suggests, is found facing upward on the bottom of shallow coastal waters in bays, mangroves and lagoons—pulsing rhythmically in groups of hundreds to thousands of individuals," Ames said.

"Like all jellyfish, Cassiopea is a carnivore, but different from many jellyfish, it also has single-cell algae living in its cells. This symbiotic relationship allows Cassiopea to get nutrients through the alga's photosynthetic activity—much like a plant makes its own food," she said.

When these jellyfish feed they release clouds of mucus which they use to catch prey like a net. They then suck in the mucus filled with prey—such as shrimp and other plankton—using their frilly feeding structures to consume the meal.

This image shows three upside-down jellyfish in a lab at the Smithsonian's National Museum of Natural History. Allen Collins/Cheryl Ames

The researchers decided to analyze this mucus in the lab, suspecting that it could be responsible for the stinging water sensation. Using advanced microscopic techniques they were able to identify tiny masses of stinging cells called "cassiosomes," which the jellyfish use almost like "mobile grenades" to trap and kill prey. These structures are able to move independently due to tiny hair-like filaments known as cilia.

"Stinging water is caused by people coming in contact with the mucus of upside-down jellyfish, without actually touching the jellyfish," Ames said. "We found that the mucus contains tiny moving clusters of cells—that are sent out remotely from the jellyfish into its mucus, and which sting prey.

"We called these self-propelled cell masses cassiosomes. Using high-tech microscopy methods, our team discovered that the cassiosome outer layer is lined with thousands of jellyfish stinging capsules called nematocysts. Nematocysts are toxin-filled capsules normally found in the tentacles. The center is jelly-filled, and also contains symbiotic single celled algae that matches the type found living in the jellyfish," she said.

The scientists say that this stinging strategy has never been identified before. However, the team also found cassiosomes in several other related jellyfish species that cause stinging water symptoms.

According to the researchers, most of the jellyfish's nutrients come from the symbiotic algae living inside it. However, the cassiosome-packed toxic mucus may help the animal to acquire additional food from prey when needed.

"Venoms in jellyfish are poorly understood in general, and this research takes our knowledge one step closer to exploring how jellyfish use their venom in interesting and novel ways," Anna Klompen, another author of the study said in a statement.

While the venom of upside-down jellyfish is not particularly powerful, there are potential health impacts for humans.

"The sting is not known to be really dangerous. No deaths or serious injury have been reported from direct contact with the jellyfish," Ames said. "However, when scientists studied the pure venom, extracted from the stinging capsules—nematocysts—they found that the toxins can destroy cells.

"Additionally, Cassiopea generated stinging water, which we now know is caused by the cassiosomes in the jellyfish mucus, causes a sensation that is itchy-to-burning and—depending on the person—can cause enough discomfort to make them to want to get out of the water."