How upside-down jellyfish create stinging water without being touched
In short: Most jellyfish require physical contact to sting, but the upside-down jellyfish uses a different tactic. By jettisoning microscopic, autonomous 'grenades' into a sticky mucus, these creatures create a zone of stinging water that can catch prey and irritate divers from a distance.
The upside-down jellyfish, belonging to the genus Cassiopea, lives a sedentary life belly-up on coastal seafloors from Florida to the Red Sea. To hunt, they do not rely on long, trailing tentacles. Instead, they release a thick, sticky mucus that acts like a spider's web to trap prey like brine shrimp. Within this goo float tiny, hollow balls of stinging cells known as cassiosomes.
These cassiosomes are remarkably autonomous. Measuring up to half a millimetre wide, each orb is equipped with 60 to 100 hair-like cilia that allow them to paddle through the mucus in circular motions. This movement helps them bump into prey, which they can kill in under a minute. In laboratory settings, these structures have been observed surviving for up to 10 days outside the jellyfish.
The mechanism is so powerful that the stinging structures can discharge with an acceleration of up to 5 million g's, making it one of the fastest movements in the animal kingdom. While the sting is generally considered an irritation—causing an itchy or burning sensation in divers—it is a highly efficient evolutionary novelty. This 'stinging snot' allows the jellyfish to expand its reach far beyond its physical body.
Interestingly, these jellyfish also host algae inside them. By resting upside down on the ocean floor, they expose these photosynthetic plant cells to the sun, providing a supplemental energy source that likely fuels the production of these complex stinging structures.
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Sources
- Summary by Intellect · source: How Upside-Down Jellies Sting You Without Touching You
- Also: This is how jellyfish can sting you without even touching you | New Scientist
- Also: These jellyfish can sting without touching you, thanks to 'mucus grenades'
- Also: Team of Scientists Uncover Mystery Behind “Stinging Water”
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