How Does a Pistol Shrimp Snap?
The short answer
Its big claw slams shut and shoots a jet of water at about 25 m/s (56 mph), as fast as a highway car, for a split second. The jet rips the water open into a bubble. The bang comes when that bubble collapses, not from the claw. The shock can stun small prey close by.
For an instant the collapsing bubble also flashes. Inside, it reaches at least 5,000 kelvin. The Sun's surface is about 5,800 kelvin.
The bang is a bubble
Until 2000, scientists thought the clicking claw made the noise. Then high speed cameras showed what really happens. The team's paper put it plainly: "the sound is emitted at the cavitation bubble collapse and not on claw closure." A cavitation bubble is a bubble that the fast jet rips open in the water itself.
The jet moves at about 25 m/s, and calculations suggest up to 30 m/s. That is 56 to 67 mph, highway speed, but only for a split second near the claw. When the bubble caves in, it sends out a shock wave that "is sufficient to stun or kill nearby small prey," as Physics World reported in 2000.
A flash too dim to see
In 2001, three of the same researchers reported that the collapsing bubble also gives off a brief flash of light. They found "temperatures of at least 5,000 K" inside it. The Sun's surface is about 5,800 K. The flash makes too little light for human eyes, and probably for any eyes. The team caught it with a sensitive light detector called a photomultiplier tube, and wrote: "We have dubbed this phenomenon 'shrimpoluminescence'."
Blurry eyes, a lookout and a spare claw
A pistol shrimp's eyes are completely covered by its shell, which forms a hood over them. The hood is 80 to 90% as transparent as seawater, so the shrimp does see, but coarsely: its eyes pick out detail only about 8 degrees across.
Some pistol shrimp share a burrow with a goby fish. The shrimp digs and keeps an antenna on the goby. The goby keeps watch and flicks its tail when danger comes, and both retreat into the burrow.
If the shrimp loses its snapper, its small pincer turns into the new one. The new claw can snap after the next molt, though it is smaller at first.
Key numbers
| What | Number | Source |
|---|---|---|
| Speed of the water jet from the snapping claw | about 25 m/s (about 56 mph) | Schmitz et al. 2000, J. Acoust. Soc. Am. |
| Top estimate for the jet's speed, from calculations | up to 30 m/s (about 67 mph) | Science News 2000 |
| Temperature inside the collapsing bubble | at least 5,000 K | Lohse, Schmitz and Versluis 2001, Nature |
| Temperature of the Sun's surface | about 5,800 K | Our arithmetic from NASA, Sun facts |
| Year high speed cameras showed the bang comes from the bubble, not the claw | 2000 | Versluis et al. 2000, Science |
| Year the flash was named shrimpoluminescence | 2001 | Lohse, Schmitz and Versluis 2001, Nature |
| How clear the hood over the eyes is | 80 to 90% as transparent as seawater | Kingston et al. 2019, J. Exp. Biol. |
| Finest detail the shrimp's eyes resolve | about 8 degrees | Kingston et al. 2019, J. Exp. Biol. |
| When a new snapper can snap after the old one is lost | after the first molt, smaller at first | Cooney et al., SICB abstract |
What the film simplifies
- The real flash gives off too little light for human eyes, and probably for any eyes. The film shows it only in the slowed camera footage, as a bright white flare.
- "Nearly as hot as the surface of the Sun" rests on a lower limit: the paper says at least 5,000 K inside the bubble, and the Sun's surface is about 5,800 K.
- The sources say the shock can stun or kill small prey close by, but none gives a distance. The film keeps its crab about a bubble's width from the claw tip, and the knockout with spiral eyes and circling stars is cartoon.
- The yellow goby and the red and white shrimp are drawn as a pair the way aquariums keep them. The film names no species.
- The crab snipping off the claw and waving it like a trophy is a cartoon touch. The sources here cover what happens after a shrimp loses its snapper, not how it loses it.
- The jet racing a car shows its speed, but the real jet keeps that speed only for a split second near the claw. The reef town, the offended Sun, the marquee and the grouper with a rock for a head are cartoon.
At a glance
Runtime 1 minute 3 seconds · Topics: Ocean life, Sound and bubbles, Animal partnerships, Animal senses · Captions on YouTube · Full script below
Test yourself
The claw never touches the crab. So what makes the bang and knocks it out?
Answer
The claw shoots a jet of water so fast it rips open a bubble. When the bubble collapses, it makes the bang and a shock wave that can stun small prey close by.
The collapsing bubble reaches at least 5,000 K and the Sun's surface is about 5,800 K. Roughly what fraction of the Sun's surface temperature is that?
Answer
About 86%, a bit less than nine tenths. That is why the film says "nearly" as hot.
Why does a pistol shrimp keep an antenna on a goby fish?
Answer
Its eyes sit under a hood in its shell and its sight is coarse. The goby keeps watch and flicks its tail when danger comes, and the shrimp's antenna is resting on it, so both can retreat into the burrow.
The script
Host: This pistol shrimp can knock out a crab without touching it.
Host: So how? Slow it way down.
Host: His claw slams shut and shoots a jet of water as fast as a car on a highway.
Host: So fast, it rips the water open into a bubble.
Host: Then the bubble collapses.
Host: That's the bang. Not the claw. The bubble. And it hits like a punch.
Host: For an instant it flashes, nearly as hot as the surface of the Sun.
The Sun: Excuse me?
Host: Scientists named it shrimpo luminescence.
The Sun: Oh, come on.
Host: And he did it with blurry eyes.
Host: His eyes peek out through a see through hood in his shell.
Goby: Good thing he's got me. I'm the eyes.
Host: It's true. The shrimp rests an antenna on a goby fish, and the goby flicks its tail at danger.
Goby: Personal space, partner.
Host: But the crab's not done.
Crab: Who's the big shot now?
Goby: Boss! He took your gun!
Host: So he hides out till his next molt, and his little claw comes out as a new snapper.
Host: He switches hands.
Goby: Grouper! Antenna on, partner!
Sources
- The snap's sound comes from the cavitation bubble's collapse, not from the claw closing (high speed imaging, 2000).Versluis et al. 2000, Science
- The claw's water jet moves at about 25 m/s, and the sound comes from the cavitation bubble's collapse.Schmitz et al. 2000, J. Acoust. Soc. Am.
- Calculations suggest the jet moves at up to 30 m/s.Science News 2000
- The bubble's collapse makes a shock wave sufficient to stun or kill nearby small prey, and the sound's peak comes with the collapse, not the claw closing.Physics World 2000
- The collapsing bubble flashes, with temperatures of at least 5,000 K inside, and the authors named it shrimpoluminescence.Lohse, Schmitz and Versluis 2001, Nature
- The Sun's surface temperature, about 5,800 K (our conversion of the Celsius figure on this page).NASA, Sun facts
- The flash is too dim for human eyes or probably any eyes, and it was caught with a photomultiplier tube.Science News 2001
- Pistol shrimp eyes are completely covered by the carapace, which forms orbital hoods.FAO species identification guide
- The orbital hood is 80 to 90% as transparent as seawater, and the shrimp's vision resolves about 8 degrees.Kingston et al. 2019, J. Exp. Biol.
- The shrimp builds the burrow, has poor eyesight and rests an antenna on its goby partner, which warns it of danger.Australian Museum, What is a shrimpgoby?
- If the snapper is lost, the pincer becomes the new snapper.Read and Govind 1997, Biological Bulletin
- The new snapper works after the first molt, despite reduced claw size.Cooney et al., SICB abstract