Nematocysts & the Science of Sting

nematocysts sting
Lion’s mane jellyfish. Stingers very potent. Photographed at the New England Aquarium.

NEMATOCYSTS ARE OCEAN STINGERS’ SECRET WEAPONS. In fact, they are the stingers. The way in which nematocysts sting is a story of the sneaky, harpoon-like ordnance of Phylum Cnidaria – jellyfish, coral and gorgonian polyps, sea anemones, fire corals and hydroids.

If you happen to touch a sea anemone, a sea plume or coral tentacles (which you shouldn’t do), you’re likely to not feel their sting. Actually, you’ve probably been stung, ineffectively. Those animals’ stingers are calibrated to capture and kill microscopic prey like zooplankton. It’s low-intensity stinging, in scientific parlance.


If you should come in contact with the tentacles of a jellyfish like a sea nettle, a Portuguese man-of-war, a fire coral or a hydroid (don’t do that, either!), you’ll definitely know how nematocysts sting.

And if you try to alleviate it by rubbing it, you’ll feel it even more (so add that to the list of things not to do).


The 10,000-plus species of cnidarians (nahy-dair’-ee-uh n, from the Greek word for “nettle”) have two major things in common. The first is radial symmetry, with circular, concave bodies arranged around central sacs. The second is a capacity for stinging other animals with tentacles armed with nematocysts.

nematocysts sting
This innocent-looking sea rod, embedded with fire coral, got me in the Bahamas.

Cnidarians come in two types: tiny, sac-like polyps that attach to surfaces (like corals and gorgonians) and bell-shaped medusa (that is, jellyfish), upside-down polyps adapted for swimming in the water column.

Other than anemones, polyps are generally tiny and live interconnected lives in colonies of identical brethren whose tentacles are measured in millimeters. Jelly bodies can be as small as an inch across and as large as six inches, with tentacles that can extend as more than 100 feet.


The body tissues of both types are gelatinous (hence, jellyfish), although many of the little polyp versions erect hard surfaces for protection – corals of calcium carbonate, gorgonians of fingernail-like keratin. The jellies just let their often-lengthy tentacles hang out. They have limited swimming abilities and are mostly subject to drifting with the currents.

nematocysts sting
This NOAA schmatic depicts the nematocyst action.


Nematocysts are bulb-shaped capsules embedded by the thousands in tentacle surface tissues, with solid elastic walls and lid structures that keep the capsule closed. Inside are strong, dart-like protein tubules with spiked barbs at the end.

They’re loaded with neurotoxins to kill or stun prey (or, as we know, sting divers). Threads wrap around them, keeping them tethered to their capsules when launched.

When contact with a prey triggers them, the lid pops open and the barbed darts are rapidly expelled. As they do so, the coiled threads unravel with them.

Like arrowheads, they stick into the prey, injecting toxin to invade and paralyze their victim’s tissues.  Strong and stored under significant pressure, they can penetrate even thick crustacean cuticles.

While the polyp’s tentacles move the captured prey to its mouth, the nematocysts are drawn back into their capsules.


Contact with a prey, or a diver, triggers a lightning-fast build-up of seawater inside the capsule, raising hydrostatic pressure to an immense level. The resulting spring action launches the dart with tremendous effect.

It happens within a millionth of a second. It’s considered the fastest process known in animal systems in terms of speed. And it’s close to the force of a bullet in terms of power. Which, of course, explains how a thick (in single-digit millimeters), solid crustacean keratin shell can be pierced by something of much lower mass.

nematocysts sting
Also stingers, albeit less lethal than a lion’s mane: tubularian hydroids (Tubularia crocea), photographed in Boston’s Outer Harbor.


Being stung, of course, is relative. The sting of an anemone or coral can be non-existent so far as we humans are concerned. The sting of a siphonophore likes a Portuguese man-of-war can be very painful, lasting for hours. Even pieces of tentacle, separated from the animal, can be dangerous.

And then, there are the box jellies like Chironex fleckeri, a small deadly species found in the Indo-Pacific basin. Its venom can cause immediate, extreme pain. Sometimes heart failure follows within minutes. Fortunately, an anti-venom has been developed.


One word: vinegar. There are many theories about what to do when you feel nematocysts sting. Most of them are wrong and can even do harm. Vinegar is your best bet.

Treatments are discussed in more detail in the sidebar story “How to Treat Stings: Jellyfish, Fire Coral and Other Cnidarians.”

PRINCIPAL SOURCES:   Marine Biology, Fourth Edition, Peter Castro, Michael Huber;  Reef Creature Identification, Florida, Caribbean, Bahamas, Reef Creature Identification, Tropical Pacific, Paul Humann, Ned DeLoach; Cnidaria,” Animal Diversity Web; “Nematocyst stingers accelerate fast,”; “Nematocycsts,” NOAA Ocean Service Education; “Cinidocyte,”; “How does a jellyfish sting?” A Ted Talk Video.