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How do jellyfish sting?

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Quick answer

Jellyfish sting using nematocysts, tiny capsules inside stinging cells along their tentacles. When skin or prey touches a trigger hair, the capsule's lid pops open and a coiled, hollow thread shoots out under high pressure, piercing the surface and injecting venom. Thousands of capsules fire together, which is why one brush can sting widely.

Jellyfish sting by firing thousands of microscopic venom capsules, called nematocysts, from cells lining their tentacles when something brushes against them. Each capsule bursts open under high internal pressure, shooting a coiled hollow thread into the skin and pumping venom through it in a fraction of a millisecond.

Step by step

  1. A swimmer's leg or a small fish brushes against a tentacle lined with thousands of stinging cells.
  2. The trigger hair on each touched cell senses the contact along with chemical cues from the skin or prey.
  3. The capsule's lid snaps open, and water floods in because of the high pressure built up inside.
  4. The coiled thread shoots out and turns inside out, piercing the outer layer of skin like a tiny harpoon.
  5. Venom flows through the hollow thread into the tissue, causing pain, redness or, in prey, paralysis.

Key things to understand

  1. Jellyfish tentacles are packed with stinging cells called cnidocytes, each holding one sealed capsule known as a nematocyst.
  2. Inside every capsule a long hollow thread lies coiled tightly, bathed in fluid kept at very high osmotic pressure.
  3. A tiny hair-like trigger on the cell surface responds to touch and to chemicals found on prey or skin.
  4. When triggered, the lid flips open, water rushes in, and the thread turns inside out at enormous speed into the target.
  5. Venom travels down the thread into the tissue, and each fired capsule is used once before the animal grows a replacement.

Labelled diagram

  1. Trigger hair

    A tiny bristle on the cell surface that senses touch and chemical signals.

  2. Capsule lid

    A hinged cap sealing the nematocyst until the trigger fires.

  3. Coiled thread

    A hollow tube wound inside the capsule, ready to turn inside out.

  4. Venom

    Toxic fluid stored in the capsule and pushed through the thread into tissue.

  5. Fired thread

    The everted tube lodged in skin, delivering venom before the cell is replaced.

Inside a jellyfish stinging cell as it fires

Real-world examples

  • Swimmers in northern Australia are warned about box jellyfish, whose stings can be life-threatening and need emergency treatment.
  • During the monsoon, blue bottles wash up on Mumbai beaches such as Juhu and can sting people walking barefoot.
  • Moon jellyfish common in aquariums have such weak stingers that most people feel little or nothing when touching them.
  • Beachgoers can be stung by tentacle fragments drifting in the surf even when no whole jellyfish is visible.

Common mistake

✗ Myth: Urinating on a jellyfish sting neutralises the venom and stops the pain.

✓ Fact: Urine does not neutralise venom and can even trigger unfired capsules to discharge. Advice depends on the species, so follow local lifeguard guidance and see a doctor for severe stings.

People also ask

How does a jellyfish sting work?
A jellyfish tentacle is covered with stinging cells, each holding a pressurised capsule with a coiled thread inside. When something touches the cell's trigger hair, the capsule bursts open and the thread shoots out, piercing the skin and injecting venom. This happens in well under a millisecond, among the fastest movements in nature.
Why do jellyfish stings hurt so much?
Thousands of capsules fire at once along every centimetre of tentacle that touches you, each injecting a dose of venom. The venom contains proteins that damage cell membranes and activate pain nerves, causing burning and red welts. Some species, such as box jellyfish, have far stronger venom than others.
Can jellyfish sting you out of the water?
Yes. A jellyfish washed up on the sand, or a tentacle torn off by waves, still carries loaded stinging capsules. Because firing is a mechanical reaction rather than a deliberate action, these capsules can discharge when stepped on or touched long after the animal has died.
Can a dead jellyfish still sting?
Yes. Nematocysts fire mechanically, so capsules on a jellyfish stranded on the beach, or on a broken tentacle drifting in the water, can still discharge for hours or days after the animal itself has died.
Why do some jellyfish stings hurt more than others?
Species differ in how far their threads penetrate and in the chemistry of their venom. Many common moon jellyfish barely pierce human skin, while box jellyfish carry potent venom that can affect the heart and needs urgent medical care.
Do jellyfish sting on purpose?
No. A jellyfish has no brain and does not choose to attack. Each stinging cell fires on its own when touch and chemical cues meet its threshold, which is why swimmers get stung simply by drifting into tentacles.
Why do jellyfish need stingers at all?
Jellyfish are slow swimmers that cannot chase food. Stinging cells let them paralyse small fish, shrimp and plankton that bump into their tentacles, and the same cells also discourage predators from taking a bite.

Part of the guide: Nature & Earth: Living Things and Rocks →