These jellyfish can sting without touching you, thanks to 'mucus grenades'

Swim through the mangrove forest waters of the world, from the coasts of Florida to Micronesia, and you may encounter a jellyfish that stings despite having no tentacles. In fact, you needn’t even touch these jellies to get zapped.

How is that possible? According to a study published Thursday in Communications Biology, the spicy trick relies on clouds of mucus teeming with microscopic venom “grenades.”

These so-called upside-down jellyfish, named for the way they spend their lives belly-up on the ocean floor, have been the subject of study for more than a century. But no one had quite figured out how the jellies’ goo worked until now. Doing so can help explain why these jellies so often harm swimmers, even from a distance.

“We knew it had to be something in the mucus,” says Cheryl Ames, a marine biologist at the Smithsonian National Museum of Natural History and a co-lead author of the new study.

Upside-down jellyfish of the genus Cassiopea produce tons of sticky mucus that trap small prey, such as brine shrimp, almost like a spider’s web. Some fish even perish in the slime. What’s more, when human divers swim near the jellies, they can experience what’s known as “stinging water sensation” anywhere skin is exposed, despite never coming into contact with the invertebrates. The sensation is typically described as an annoying itch or burn, but laboratory tests on the venom suggest that excessive exposure could be detrimental.

When Ames and her colleagues looked at the mucus under a microscope at high magnification, they discovered something swimming inside the slime.

The scientists call the newly-described structures cassiosomes, but you can think of them as microscopic pieces of popcorn. Each grenade is made of a jelly-filled center, a number of stinging cells, called nematocysts, and 60 to 100 hair-like cilia that allow the cassiosomes to paddle through the muck.

“They were autonomous,” Ames says, “moving around like little Roomba vacuums and bumping into the brine shrimp that we fed them, just killing them on contact, and moving on to the next.”

The secret of the ooze

At first, the researchers believed the structures they’d found might be parasites. But after examining the globules with a host of new technologies, including DNA analysis and micro-sized arenas to observe the small, squishy blobs in three dimensions, the team revealed a more surprising origin. The cassiosomes were composed of the very same stinging cells and jelly material that make up the upside-down jellyfish’s body.

Looking wider, the team found cassiosomes in four other jellyfish species, suggesting the structures are not one-off oddities, but perhaps a common feature.

Many of the cassiosomes came with another surprise buried inside: algae. Certain algae species serve as symbionts for Cassiopea jellyfish, providing the animals with nutrients harvested from sunlight by way of photosynthesis. These algae give the upside-down jellies their rainbow array of pink, blue, and green hues.

“We really don’t know what the algae are doing [inside the cassiosomes],” says Anna Klompen, a jellyfish biologist completing a Ph.D. at the University of Kansas and a co-lead author on the study.

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These algae may act like solar-powered battery packs, since the scientists know the cassiosomes can survive and travel on their own for up to 10 days. “But we haven’t been able to confirm that with the techniques used so far,” Ames says.

While it’s clear the cassiosomes have some willpower, so to speak, it will take more research to learn if they can sense prey around them and move towards it, or if they are simply bumbling stinging cells set to autopilot.

“I don’t know if they can seek,” Klompen says, “but they can definitely destroy.”

“A remarkable adaptation”

For Angel Yanagihara, a biochemist and jellyfish expert at the University of Hawaii at Mānoa, the new findings lay to rest some long-standing questions about the stinging water sensation.

“The pat explanation that the mucus was just in and of itself somehow an allergen or allergenic didn't seem credible to me,” says Yanagihara, who was not part of the new study. “So this is quite satisfying to see such an elaborate description and breakdown of exactly what they're releasing into the water.”

The upside-down jellies are known as Medusazoans, Yanagihara says, which are traditionally thought of as the swimming jellies. But bizarrely, Cassiopea evolved to sit on the ocean floor, more like its distant cousins, the anemones.

This discovery of the cassiosomes takes us one step closer to understanding how these sessile creatures manage to capture prey. “This is a remarkable adaptation,” Yanagihara says.

The exact way upside-down jellyfish benefit from their lethal mucus has yet to be described by scientists, but Ames says their lab has seen hints. Feed one of the jellies a batch of brine shrimp and you can watch as its mucus turns into a “pink cloud” full of zapped crustaceans.

“Then, within 24 hours, the pink cloud will be gone,” Ames says.

Editor's Note: This article originally misstated the name of the study's journal. It is Communications Biology.