How do jellyfish sting?
Quick answer
Jellyfish sting using thousands of stinging cells called cnidocytes on their tentacles. Each holds a capsule, the nematocyst, containing a coiled, harpoon-like thread. When something touches a trigger hair, the thread fires out in a fraction of a microsecond, pierces the skin and injects venom to stun prey.
One of the fastest actions in nature
A nematocyst builds up very high internal pressure. When triggered by touch or chemicals, its lid flips open and the thread turns inside out, shooting outward with an acceleration millions of times that of gravity. It’s one of the fastest movements known in biology.
Each nematocyst can fire only once; the cell then has to be replaced.
Who has stinging cells?
Nematocysts belong to a group of animals called cnidarians (the “c” is silent), from the Greek word for nettle.
| Cnidarian | Notes |
|---|---|
| Jellyfish | Use stings to catch fish and plankton |
| Sea anemones | Sting prey that touches their tentacles |
| Corals | Tiny polyps sting plankton at night; see the largest coral reef |
| Portuguese man o’ war | Not a true jellyfish, but a colony of organisms with long stinging tentacles |
| Hydras | Tiny freshwater cnidarians |
The most dangerous ones
| Species | Where | Why it’s feared |
|---|---|---|
| Box jellyfish (Chironex fleckeri) | Northern Australia, Indo-Pacific | Venom can affect the heart within minutes |
| Irukandji jellyfish | Australia | Tiny, about 1 cm, but can cause severe illness |
| Portuguese man o’ war | Warm oceans worldwide | Very painful stings |
Most jellyfish stings are only mildly painful. On beaches with dangerous species, follow lifeguard warnings, swim in netted areas when advised, and get medical help quickly for any serious sting.
Jellyfish facts
- Jellyfish have no brain, heart or bones and are about 95% water.
- They’ve existed for over 500 million years, older than dinosaurs.
- The “immortal jellyfish” (Turritopsis dohrnii) can revert to a young stage instead of dying.
Some fish live safely among stinging tentacles; see why clownfish live in sea anemones.
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