Why can a mantis shrimp's punch crack a shell in one hit?
― It's so fast the water momentarily "boils"
The ocean is a hard place to move fast. Water is about 800 times heavier than air, and it pushes back hard against anything moving through it. Yet the peacock mantis shrimp, barely 10 centimetres long, swings its fist through that water at more than 20 metres per second. It's so fast that the water right behind the fist tears apart and turns to bubbles — at room temperature, no heat required.
In the bath, hold your palm flat and push it through the water as hard as you can. Move slowly and it's easy. Try to move fast, and the water suddenly feels much heavier. Moving quickly underwater is hard work all by itself.
On coral reefs, there's a creature that lives in this same heavy water yet smashes open hard snail shells and crab shells to eat. It's a relative of the shrimp you see at the sushi counter: the peacock mantis shrimp.
Its weapon is a pair of hammer-like legs folded in front of its chest. The strike is so fast that an ordinary camera just sees the leg vanish.
It comes down to just two reasons
Muscles can produce plenty of force, but they can't shorten fast. So the mantis shrimp uses its muscles to slowly compress a "spring," held in place by a latch. The instant the latch releases, all that stored force launches the hammer at once.
Behind anything moving fast through water, the pressure drops. Past about 14 metres per second, the pressure falls so low that water turns to vapour even at room temperature. The shock from that bubble collapsing becomes a "second hit."
The first mechanism produces speed that can overpower the water's weight. The second is an unexpected bonus that speed creates. Let's look at each in turn.
Drawn back by muscle, released by a latch
Muscle has a quirk: the slower it contracts, the more force it can produce, and the faster it contracts, the weaker it gets. Swinging fast while shoving aside heavy water takes more force than muscle alone can deliver.
At the base of the mantis shrimp's leg sits a hard, springy structure shaped like a saddle. The muscle takes its time compressing this part — just like drawing back a bow. While that happens, the leg is held in place by a small latch.
When the latch releases, the stored force is unleashed in just a few thousandths of a second. The tip of the hammer has been recorded reaching roughly 23 metres per second — over 80 kilometres per hour. The acceleration involved is said to be around 10,000 times that of gravity.
Storing energy slowly and releasing it all at once is the same trick a flea uses to jump. Small animals that need big speed turn to springs rather than muscle.
Move fast enough, and the water behind you "tears"
When something moves through water, the water tries to flow in behind it and fill the gap. But if the object moves too fast, the water can't flow in quickly enough. The water behind gets stretched, and its pressure drops.
Look at Figure 1. The horizontal axis is the hammer's speed; the vertical axis is the pressure of the water right behind the hammer. As speed increases, the pressure plunges along a parabola. Double the speed, and the drop is roughly quadrupled.
Once pressure drops low enough, water turns to vapour without any heating at all, because the temperature at which water boils depends on the surrounding pressure. On a high mountain, where pressure is low, water boils below 100°C. Push that further, and even 25°C seawater can turn to vapour on the spot.
This phenomenon is called cavitation. It has long been known to slowly erode the surface of ship propellers through exactly these bubbles. The mantis shrimp triggers the same thing around its own fist.
The instant the bubble collapses becomes the "second hit"
The bubble that forms doesn't last long. As soon as the surrounding water pressure returns, the bubble gets crushed with tremendous force. At the moment of collapse, water rushes in from all sides to where the bubble was and collides, producing a small shock.
Look at Figure 2. From left to right: ① the spring is loaded, ② the hammer strikes the shell (first hit), ③ the bubble in the gap collapses (second hit). From the shell's point of view, almost the same spot gets struck twice within an extremely short span.
When researchers actually measured the forces involved, the hammer's impact reportedly reached up to about 1,500 newtons — close to the weight of a 150-kilogram load, or thousands of times the animal's own body weight. The force from the bubble collapsing was reported to be nearly as large.
A shell that survives one hit can't survive being struck twice in rapid succession, over and over — the cracks slowly spread. Eventually, even a snail's hard shell gives way.
If you hear a sharp "snap, snap" sound at the shore, it might be a pistol shrimp. This shrimp snaps its claw shut in an instant, blasting out a jet of water. The sound turns out to be the collapse of the bubble that jet creates. At the instant of collapse, the bubble has even been observed to give off a faint flash of light.
The hammer's surface is made of a hard mineral in the same family as bone and teeth. Beneath it, layers of fine fibres are stacked with their direction rotating slightly from layer to layer, like a spiral staircase. Even if a crack starts, it's said to be deflected each time it meets a layer running a different way, unable to travel straight through to the core.
Summary
The peacock mantis shrimp releases a spring drawn back by its own muscles, swinging its fist through water at over 20 metres per second. That speed is enough to turn the water behind the fist into vapour at room temperature. The shock from that bubble collapsing follows as a "second hit," adding further damage to the hard shell.
A fist too fast turns the water itself into a weapon.
The mantis shrimp strikes twice with a single punch.
The spring mechanism that produces its speed connects to the flea's jump, and the way bubbles form and make sound connects to the crack of a knuckle. For how the shell on the receiving end holds up, see why the inside of a shell resists cracking.
- In the bath, hold your palm flat and compare how it feels moving it slowly versus about twice as fast. At the higher speed, the push-back should feel far more than just double.
- Draw a little warm water into a needleless syringe (the kind sold at a pharmacy for giving liquid medicine). Seal the tip firmly with a finger, then pull the plunger back hard. Even though you haven't heated it, bubbles may appear from inside the water.
- If you ever see a peacock mantis shrimp at an aquarium, look closely at the hammer-shaped legs folded against its chest. The strike itself is too fast for the eye to follow.
The bubbles that appear from the syringe are mixed with air that was already dissolved in the water. Even so, it lets you feel firsthand that "dropping pressure alone can create a bubble." Moving your hand by itself can't drop the pressure enough to tear the water apart.
Want to go deeper? ― Terms, formulas, and how this links to textbooksWe mark clearly which level each point belongs to, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school physics
- HS+Advanced high-school content, or textbook sidebar material
- Univ.Not covered in high school — university specialist courses (fluid dynamics, biomechanics)
- ResearchNot yet settled even at university level — an active research question
MSTerms: this phenomenon has a name
- Cavitation: when a liquid's pressure drops so low that vapour bubbles form inside it without any rise in temperature.
- Raptorial appendage: the large leg in front of the chest that mantis shrimp use to catch prey. The peacock mantis shrimp uses it as a hammer.
- Boiling point and pressure: the temperature at which a liquid boils drops as the surrounding pressure drops — the same reason water boils while still lukewarm on a mountaintop.
MSHSCheck it with a formula: at what speed does water start to tear?
In a flow, pressure drops in proportion to the increase in speed. Assuming the water right behind the hammer moves at roughly the hammer's own speed, we can estimate the speed at which bubbles start to form. Units: pressure in pascals, speed in metres per second.
| In symbols | p ＝ p0 − ½ × ρ × v² ; bubbles form when p ＜ pv |
| In words | pressure of the water behind ＝ surrounding pressure − ½ × density of water × speed squared. If this drops below "the pressure at which water vaporises," bubbles form |
| Where it comes from | Bernoulli's principle. Along a flow, the sum of "pressure energy" and "kinetic energy" is conserved, so pressure drops wherever the flow moves faster |
| p | pressure of the water right behind the hammer (pascals) |
| p0 | ambient water pressure. About 1 atmosphere in shallow water (pascals) |
| pv | the pressure at which water turns to vapour at that temperature (pascals) |
| ρ | density of seawater (kilograms per cubic metre) |
| v | speed of the hammer (metres per second) |
| Ambient pressure p0 (1 atm) | 101,300 pascals |
| Pressure at which 25°C water vaporises, pv | about 3,200 pascals |
| Density of seawater, ρ | about 1,025 kilograms per cubic metre |
| Maximum reported speed of the mantis shrimp's hammer | about 23 metres per second (reported value) |
| Pressure drop needed to reach vaporisation | 101,300 − 3,200 ＝ 98,100 |
| Work out ½ × ρ first | 1,025 ÷ 2 ＝ 512.5 |
| What v² must reach to get there | 98,100 ÷ 512.5 ≒ 191.4 |
| The square root of that is the speed at which bubbles begin | the square root of 191.4 is about 13.8 metres per second |
| Square the shrimp's maximum speed | 23 × 23 ＝ 529 |
| The pressure drop that speed would call for | 512.5 × 529 ≒ 271,113 |
| How many times 1 atmosphere that is | 271,113 ÷ 101,300 ≒ 2.68 |
At 23 metres per second, the pressure behind the hammer would, on paper, need to drop by 2.7 times the entire atmosphere. But there's only 1 atmosphere of pressure to begin with — there's nothing left to give, so the water tears apart and vaporises instead. That leaves roughly 1.7 times the margin over the threshold speed (about 14 metres per second) where bubbles start. Real flows are more complex, so this is only a rough estimate.
HSHS+Spring energy and water resistance
HSA compressed spring stores elastic potential energy (½ × k × x²). The muscle takes a long time to build up this energy, then converts it to kinetic energy (½ × m × v²) in the instant the latch releases. The larger the ratio between the time spent storing energy and the time spent releasing it, the greater the power (energy per second) that can be delivered.
HS+In fast flows, drag on an object in water is roughly proportional to ½ × ρ × v². It's no coincidence that the same ½ × ρ × v² appears above — this quantity is called dynamic pressure, the flow's momentum expressed in pressure units.
Univ.How the bubble collapses, and how the hammer is built
The way a bubble grows and collapses is described by the Rayleigh–Plesset equation. In the final instant of collapse, the bubble wall moves extremely fast and the gas inside is compressed abruptly. Studies of pistol shrimp estimate that the inside of the bubble can reach thousands of degrees at that moment, and the resulting flash of light is called sonoluminescence. The spring-like part is thought to be a saddle-shaped (hyperbolic paraboloid) section of exoskeleton that stores energy as it bends. The spiralling stack of fibre orientations inside the hammer is called a Bouligand structure, studied as a model material for halting crack propagation.
📖 For the derivation of the formulas and further reading: Cavitation (Wikipedia, Japanese) ／ Bernoulli's principle (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- How much does the bubble actually help? How much the second hit from the bubble contributes to cracking the shell seems to depend on the prey and how the strike lands, and the exact share isn't settled.
- Does the mantis shrimp's bubble also glow? Light from bubble collapse has been observed in pistol shrimp. How much light or heat the mantis shrimp's bubble produces is still being studied in detail.
- Why it survives thousands of strikes The hammer is renewed at each moult, but is said to withstand thousands of strikes in between. Materials researchers are also interested in how far damage can spread before it stops working.
In other words, even this article describes things only "as currently understood." The speed and force figures are reported values, measured from a limited number of individuals using high-speed cameras and force sensors.
Links to textbooks, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: pressure, states of matter and boiling point | Dropping pressure can make a bubble form even at room temperature |
| HS | Physics: elastic energy, kinetic energy | Storing energy in a spring and releasing it all at once |
| HS+ | Physics: fluid pressure, dynamic pressure | Pressure drops where the flow is faster (Bernoulli's principle) |
| Univ. | Fluid dynamics, biomechanics, materials science | How bubbles collapse, the spring's shape, the hammer's layered structure |
| Research | Research on animal movement and materials | The bubble's role, light and heat, resistance to damage |
| ― | Connection to everyday life | Ship propellers and pumps eroded by bubbles |
- Patek, S. N., Korff, W. L., Caldwell, R. L. (2004) Deadly strike mechanism of a mantis shrimp. Nature 428, 819–820.
- Patek, S. N., Caldwell, R. L. (2005) Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp. Journal of Experimental Biology 208, 3655–3664.
- Versluis, M., Schmitz, B., von der Heydt, A., Lohse, D. (2000) How snapping shrimp snap: through cavitating bubbles. Science 289, 2114–2117.
- Weaver, J. C. et al. (2012) The stomatopod dactyl club: a formidable damage-tolerant biological hammer. Science 336, 1275–1280.
- Lohse, D., Schmitz, B., Versluis, M. (2001) Snapping shrimp make flashing bubbles. Nature 413, 477–478.
※This article is a general-interest science explainer. The figures given are approximate, meant to help convey the underlying mechanism. Speed and force values are those reported in research and vary between individuals and conditions. Mantis shrimp can pinch or strike, so please don't handle them with bare hands.