Jellyfish Have Been Stinging Swimmers Worldwide With "Mucus Grenades"

Three Cassiopea, or upside-down jellyfish, Allen Collins and Cheryl Ames

Rachael Funnell 13 Feb 2020, 17:00

If you’ve ever been swimming in the sea and suddenly found yourself being stung all over, the identity of your attacker may have finally been discovered. Patches of "stinging water" have long confused scientists, but the culprit has been revealed thanks to a study published in the journal Communications Biology, which found a species of jellyfish that's been releasing gelatinous balls of stinging cells into the ocean. Rude. 

The upside-down jellyfish, Cassiopea, has long been avoided by seasoned swimmers who were aware that swimming near them was a bad idea. Many reported a stinging sensation followed by irritation even when the jellies were a considerable distance away, but before now scientists had been unable to work out why. 

A team led by researchers at the Smithsonian's National Museum of Natural History, the University of Kansas, and the US Naval Research Laboratory have been working together to crack the mystery for years. It all began when Cheryl Ames, museum research associate and associate professor at Tohoku University, talked with her colleagues about the discomfort they had all experienced firsthand after swimming near upside-down jellyfish. However, they weren't sure the "stinging water" actually had anything to do with the jellyfish, as existing explanations blamed the sensation on severed jellyfish tentacles, anemones, and “sea lice”.

The first clue that Cassiopea was to blame came when the team noticed that the jellyfish in the museum’s aquarium-room tanks released clouds of mucus when they were fed or agitated. Microscopic investigation of the mucus revealed it contained small bumpy balls that were spinning around in the fluid. Using sophisticated imaging methods, they discovered these blobs were hollow spheres with an outer layer of stinging cells known as nematocysts. The blobs also had cilia – small hair-like filaments – which pulsed, enabling them to move around the mucus. They called these mucus-gyrating balls of stinging cells (as if they needed a better name?) cassiosomes. 

Armed with this knowledge, they took another look at the upside-down jellyfish and found cassiosomes were clustered into spoon-like structures on the jellies’ arms, which would shed when the creatures were agitated. These cassiosomes would slough off gradually until thousands were mixed within the animal’s mucus and shed into the water. These stinging grenades were found to be efficient killers of brine shrimp, who would succumb quickly to the venomous spheres as seen in the video above. 

"This discovery was both a surprise and a long-awaited resolution to the mystery of stinging water," said Ames in a statement. "We can now let swimmers know that stinging water is caused by upside-down jellyfish, despite their general reputation as a mild stinger."

The exact purpose of these gelatinous weapons is not yet known, but Ames hypothesizes that it could be a secondary source of nutrients for the jellyfish, which are known to subsist off the photosynthesizing algae that live inside them. By keeping incapacitated prey close at hand, the jellyfish can guarantee a reliable source of food in all conditions. Soon after their discovery, the team identified four other closely related species of jellyfish that also produce cassiosomes and are now keen to explore if other marine life is also packing these gooey balls of pain.

Think these mucus balls are scary? Wait till you hear about the terrifying sperm of box jellyfish.

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