These Jellyfish Sting with “Mucus Grenades”

Researchers have discovered that seemingly benign upside-down jellyfish utilize a unique, previously unrecognized weapon to capture prey: “Stinging mucus grenades.” It explains the “stinging water” pain that divers, snorkelers and waders sometimes experience without coming into actual contact with the jellies. Upside-down jellyfish have been recognized for some 200 years, but nobody knew this until now.

TO ALL APPEARANCES, UPSIDE-DOWN JELLYFISH LIKE CASSIOPEA XAMACHANA spend their days resting on the shallow bottoms of reefs and mangrove forests, content to lie still and soak up tropical sun rays.

From my companion’s hand into the water: My Cassiopea xamachana.

That’s unless someone reaches down and picks one up to show how harmless these jellies with short tentacles are. That’s what happened with one a fellow kayaker and I spotted on a mangrove tour in Bonaire several years ago. We were standing in four feet of water in a sandy channel between dense mangrove groves.

Holding the five-inch-wide jelly on its back in his palm, he explained that these creatures live on nutrients provided by photosynthetic algae embedded in their tissues, not on stinging tentacles. Then he dropped it and let it descend back to the sand as I shot photos.


As it turns out, he was wrong. New research has proven upside-down jellyfish to be deadly predators of small marine life, with a unique, previously unrecognized hunting technique: It lobs “stinging mucus grenades” into the water to paralyze and capture passing prey, like brine shrimp.

The mysterious “stinging water” sometimes encountered by divers and snorkelers without contacting jellies won’t kill us humans but it can cause enough pain to make us want to leave the water.

My fellow kayaker and I were lucky. We escaped without any problems. But they issue has long been an issue for many people, and a mystery.


upside-down-jellyfish-mucus grenades
A side view as it drifts toward the sandy bottom. 

To be clear, the typical M.O. of the 200 species of sea jellies found in the oceans worldwide is to drift in the open waters as the currents take them, not to lie around passively on sandy seafloors. And they drift with long stinging tentacles trailing behind (often beneath) them, intent on paralyzing and capturing any prey unfortunate enough to drift close by.

The nine or so species in Family Cassiopeidae, on the other hand, are content to be couch potatoes of the shallow-bottom kind. Considered mild stingers, they’ve long been known to be equipped with zooxanthellae, photosynthetic algae embedded within their tissues. They lie upside down to maximize access to the solar rays that feed their photosynthesis habits.

Jellies as a whole, members of Class Scyphozoa (“cup shaped,” more or less) inhabit every type of ocean environment, tropical and cold, shallow and deep. Cassiopeidae species tend toward shallow, tropical coastal regions. They’re found in mangrove forests, mudflats and turtle grass flats throughout the Caribbean, the Florida Keys and the South Pacific.



The new research, published in the journal Nature Communications Biology earlier this month, found something different. The upside-down jellies throw spitballs of mucus – “stinging mucus grenades,” the researchers call them – into the water to capture prey. The research team involved scientists from the Smithsonian Institution’s National Museum of Natural History, the University of Kansas and the U.S. Naval Research Laboratory.

The researchers observed that upside-down jellies in the museum’s aquarium-room tanks released clouds of mucus when they were agitated or feeding. Under a microscope, they found bumpy little balls spinning and circulating in it, which further investigation revealed to be hollow spheres of cells, probably filled with the same jelly-like substance that gives jellyfish their structure. They called these spinning balls of stinging cells “cassiosomes.”

upside-down jellyfish
An upside-down jelly seen from the side.


Most of the outer cells were stinging cells known as nematocytes. Other cells were present, too, including some with cilia—waving, hairlike filaments that propel the cassiosomes’ movements. Inside the jelly-filled center of each sphere was a bit of ochre-colored symbiotic algae—the same sort that lives inside the jellyfish itself.

On the jellyfish themselves, the team was able to detect cassiosomes clustered into small spoon-like structures on the creatures’ arms. When they gently provoked a jellyfish, they could see cassiosomes slowly break away, steadily leaving the appendages until thousands of them mingled with the animal’s mucus.

They found that these mucus grenades were efficient killers of lab-fed brine shrimp, and videos that the team produced show tiny crustaceans succumbing quickly to the venomous spheres in the lab. Molecular analyses identified three different toxins within the cassiosomes.


While its exact role is not yet clear, cassiosome-packed mucus may be an important part of upside-down jellyfishes’ feeding strategy, notes the study’s lead author, Cheryl Ames, a research associate at the museum and an associate professor at Tohoku University.

While the photosynthetic algae that live inside upside-down jellyfish provide most of the animals’ nutritional resources, the jellyfish likely need to supplement their diet when photosynthesis slows—and toxic mucus appears to keep incapacitated critters close at hand.

“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,” Klompen said.

Cassiopea xamachana
Coloration of C xamachana reflects the algae embedded in their tissues. A top view of three from Bonaire in a National Museum of Natural History lab tank.

Co-author Allen Collins said the discovery was especially exciting because Cassiopea jellies have been known for 200 years but cassiosomes have remained unknown until now. Already, the team has identified cassiosomes in four additional closely related jellyfish species, reared at the National Aquarium, and they are eager to learn whether they might be even more widespread.

PRINCIPAL SOURCES:  Cassiosomes are stinging-cell structures in the mucus of the upside-down jellyfish Cassiopea xamachana, Nature Communications Biology; Stinging Water Mystery Solved: Jellyfish Can Sting Swimmers, Prey With ‘Mucus Grenades’, Smithsonian Institution News Office; Jellyfish, Oceana; You Didn’t Touch These Jellyfish, but They Can Sting You With Tiny Grenades, New York Times; A Bizarre Jellyfish Species Can Sting You Without Touch by Hurling ‘Mucus Grenades’, ScienceAlert; Cassiopea, WikipediaAquarium and museum photos and some text used under Creative Commons license.