How Do Jellyfish Sting? Inside the Nematocyst

A copepod, a crustacean smaller than a grain of rice, bumps into a jellyfish tentacle. In less time than it takes a camera shutter to twitch, thousands of microscopic harpoons have fired into it. Each harpoon was packed inside a single cell, coiled under enormous pressure, and each one can be used exactly once.

That sting is one of the fastest things any living cell can do, and it runs with no brain choosing the target. The machinery behind it explains how a soft, slow drifter has managed to hunt successfully for more than half a billion years.

The short answer

Jellyfish sting with nematocysts, pressurised capsules built inside specialised cells called cnidocytes that line the tentacles. Each capsule holds a hollow thread coiled tightly inside it. When a trigger on the cell surface detects the right combination of touch and prey chemistry, the capsule bursts open and the thread turns itself inside out as it erupts, punching into the target and delivering venom in well under a microsecond. Every capsule fires once and is then replaced by a new cell.

A weapon built inside one cell

Two names get used for the stinging apparatus, and they refer to different things. The nematocyst is the capsule itself, the loaded weapon. The cnidocyte is the living cell that manufactures that capsule, holds it and fires it. The same cell turns up in corals, sea anemones and hydras. It is the signature of their whole animal group, the phylum Cnidaria, a name taken from the Greek word for nettle.

A single tentacle carries these cells by the thousand, clustered in dense patches called batteries. When a copepod or a fish larva brushes past, it touches not one weapon but a whole bank of them at once, and many fire together.

How the thread fires: turning inside out

Each cnidocyte carries a hair-fine trigger sticking out from its surface. When it is set off, the capsule opens and the stored pressure is released all at once. The coiled tubule does not simply shoot out like an arrow from a tube. It turns inside out as it goes, with the tip leading and the rest of the thread unrolling behind it, the way a sock peels outward when you push its toe through from the inside. Biologists call this motion eversion. Because the thread lengthens from its own tip, the leading point drives forward into whatever it meets.

How fast is a jellyfish sting?

Fast enough to rank among the quickest cellular events ever recorded. High-speed measurements on a hydra, a small freshwater relative of jellyfish, caught the first phase of discharge taking roughly 700 nanoseconds, about seven ten-millionths of a second. The distances involved are only a few micrometres, but the numbers that describe them are startling.

• Discharge time for the first phase: roughly 700 nanoseconds.
• Acceleration of the thread: more than five million times the force of Earth's gravity.
• Estimated pressure at the point of impact: above seven gigapascals, a force concentrated enough to punch through the tough outer shell of small prey.

Those measurements come from the study "Nanosecond-scale kinetics of nematocyst discharge," published in Current Biology in 2006.

Why jellyfish do not sting sand and bubbles

A weapon that fires once is too valuable to waste. Seawater is full of sand grains, air bubbles and drifting debris, and a cell that discharged at every bump would empty its tentacles for nothing. So the trigger is gated. Mechanical contact usually has to arrive together with chemical cues that signal living prey. In effect, the cnidocyte answers a sharper question than "was I touched?" It asks whether the right kind of target touched it.

Single use, constantly reloaded

Once a nematocyst has fired, the cell has no way to recoil the thread and use it again. The spent capsule is finished. New cnidocytes mature and move in to replace the ones that have discharged, so a tentacle is really a belt of disposable weapons that is always being restocked.

Not every capsule does the same job, either. One design pierces and injects venom. Another wraps around the bristles or legs of prey and holds tight. A third is sticky and glues itself to the target's surface. A jellyfish can carry several of these types on a single tentacle, giving it a mixed toolkit for grabbing, holding and subduing.

From thousands of tiny shots to a meal

Venom works at the scale of the prey it evolved for. A copepod can kick itself loose in an instant, so the venom has to act first. A small fish poses a different threat: its thrashing can rip the jellyfish's fragile tissue, so it has to be stilled fast. Once the prey stops moving, the tentacles contract or hand it along to the oral arms, the frilled structures around the mouth, and digestion begins in the stomach.

Nothing in the animal decides to attack. Each cnidocyte reacts to its own patch of contact. The tentacle's muscles and the jellyfish's nerve net then turn thousands of separate local events into one coordinated capture.

Why only some jellyfish are dangerous to people

Every jellyfish stings, but only a small fraction of species pose a serious risk to humans. Most venoms are tuned for copepods, larvae and small fish, and a human swimmer is far outside that target range. The dangerous exceptions carry venom systems shaped by different prey and different hunting styles. Across every species the core design holds: stored pressure, a thread that turns inside out in one direction only, and venom placed precisely where the prey was touched.

Key terms

Cnidocyte

The specialised cell that builds, stores and fires a stinging capsule. It is found only in cnidarians.

Nematocyst

The pressurised capsule inside a cnidocyte, holding a coiled hollow thread that delivers venom or grips prey.

Eversion

The inside-out unrolling of the thread during discharge, which drives its tip forward into the target.

Cnidaria

The animal group that includes jellyfish, corals, sea anemones and hydras, defined by their stinging cells.

Copepod

A tiny crustacean that drifts in the plankton and is one of the most common meals for jellyfish.

Gigapascal

A unit of pressure equal to one billion pascals, used here to describe the force concentrated at the thread's point.

Related field notes

For how this same stinging cell fits into the rest of a jellyfish's brainless, boneless body, explore more Jellyfish Decoded field notes.

This article is adapted from "The Cell That Fires Like A Harpoon," a chapter of Jellyfish Decoded, which follows the nematocyst from its coiled capsule to the moment a meal reaches the stomach.