Grades 6–8 · Organisms
How can a jellyfish tentacle release a microscopic sting so quickly?
A jellyfish tentacle carries specialized cells called cnidocytes. Inside many of them is a capsule called a nematocyst. The capsule contains a tightly packed thread that can rapidly turn outward when the cell is activated.
In the diagramA prepared capsule releases a packed threadBefore activationAfter dischargenematocyst capsulethread turned outwardtrigger
Mechanical contact and chemical cues can help trigger discharge. The capsule opens, and stored pressure helps drive the thread outward. Parts of the thread may penetrate or entangle a target, depending on the type of stinging structure.
Some nematocysts deliver venom that affects prey or a threatening animal. The rapid movement does not require a tiny muscle to push the whole thread out slowly. Energy and structure were prepared before the triggering event.
A tentacle contains many such cells, making its surface an effective capture system. Individual discharged capsules are single-use and must be replaced by the animal. Jellyfish therefore combine a soft body with a microscopic mechanism capable of a very fast response.
About this explanation
Starting knowledge: Recognize that animals can have specialized cells and that a stored structure can be released rapidly after a trigger.
Sources
- Smithsonian Ocean — Jellyfish and Comb Jellies: The Stings