More than 40,000 images per second. That is the speed of the camera that physicists from the universities of Twente and Munich aimed, in 2000, at the claw of Alpheus heterochaelis, a pistol shrimp about six centimeters long. Their question seemed simple: where does this snap that saturates reef hydrophones come from?
The answer, published in the journal Science, overturned the received idea. And it launched a series of spectacular figures that have been circulating since, often distorted.
We review them here one by one, with what the original articles really say.
Where does the snap of the pistol shrimp come from?
The snap is born from the collapse of a cavitation bubble. When the large claw closes, it expels a jet of water so fast that the pressure falls below the vapor pressure of water. The bubble grows, then implodes. It is this implosion that the hydrophones capture, according to Versluis and colleagues (Science, 2000).
The detail that changed everything lies in the timing. According to the Science News report published in September 2000, the sound arrives about 700 microseconds after the claw closes. If the collision of the two fingers produced the noise, the two events would coincide. But they are offset, and the sound peak falls exactly on the bubble’s implosion. The researchers reproduced this delay with a Rayleigh-Plesset type model, the classical equation that describes the life of a bubble in a liquid.
According to the very summary of the 2000 article, this shot serves to stun or kill small prey within claw range, and the AAAS press release specifies that the shrimp targets prey located only a few millimeters away.
In short, the shrimp shoots with water.
Which figures really hold about the jet, the bubble, and the light?
The solid measurements concern the water jet, which reaches about 30 meters per second, the bubble, which expands up to 3 or 4 millimeters, and the temperature at the moment of implosion, estimated at at least 5,000 kelvins by Lohse and colleagues (Nature, 2001). The rest requires caution.
| Circulating figure | What the publications say |
|---|---|
| A shrimp 1.2 to 2 cm long | About 6 cm for Alpheus heterochaelis, the species studied in 2000 (Science News, 2000). The large claw can reach half the body size (AAAS, 2000). |
| A claw snapping at 60 km/h | The published speed concerns the water jet, not the claw: up to about 30 m/s, or around 108 km/h (Science News, 2000). |
| 218 decibels, louder than a siren | About 80 kPa peak-to-peak measured 4 cm from the claw, in water decibels. No direct comparison possible with a siren in air. |
| A bubble hotter than the Sun | At least 5,000 K, deduced from a light flash (Nature, 2001). Same order of magnitude as the surface of the Sun, without direct measurement. |
| A visible flash | A flash too weak for the human eye, according to authors interviewed by Science News in 2001. |
The figure of 60 km/h probably comes from a failed conversion. Some English-speaking media talk about 62 miles per hour for the jet, about 100 km/h, and the value may have lost units along the way. Yet even 100 km/h describes the expelled water. The claw itself closes in a flash, but the cited articles do not attribute a speed in kilometers per hour to it. So keep the jet figure, and forget the claw one.
For the temperature, the nuance matters.
Lohse, Schmitz, and Versluis did not stick any thermometer into the bubble; they observed a brief light emission at the moment it collapses, then deduced that at least 5,000 K inside was needed to produce it. They named the phenomenon “shrimpoluminescence,” by analogy with sonoluminescence, the light emitted by a bubble excited by ultrasound.
218 decibels, is that louder than a gunshot?
No, the comparison does not hold. The 218 dB correspond to about 80 kPa peak-to-peak measured 4 centimeters from the claw, a laboratory value cited by the NGO Ocean Conservation Research. These water decibels use a reference of 1 micropascal, while air decibels use a reference of 20 micropascals.
The calculation fits on one line. Twenty times the logarithm of 80,000 pascals divided by 1 micropascal indeed gives 218 dB. And acousticians generally refer a source level to 1 meter from the animal. With a simple spherical decay, the pressure is divided by 25 between 4 centimeters and 1 meter, which removes about 28 dB and brings the figure around 190 dB, as an indication. Added to this is the difference between water and air, which prevents placing this number next to a plane or an explosion.
So this figure is found copied from site to site without its measurement distance, and this is where the comparison with a gunshot or an ambulance siren loses all meaning for a reader who does not know the convention.
What remains established is already impressive. Lohse and his colleagues wrote in 2001 that this crackling is intense enough to disrupt underwater acoustic communications.
Where does the pistol shrimp live and with whom does it share its burrow?
Pistol shrimps, family Alpheidae, live mainly in tropical and subtropical coastal waters: coral reefs, seagrass beds, oyster banks, sandy or muddy bottoms. They mostly stay hidden in a burrow or a crevice, so they are heard much more often than seen.
This crackling sound you hear underwater on many tropical reefs largely comes from them. Some species of the genus Alpheus share their burrow with a goby. The shrimp, which has poor eyesight, digs and clears the hole. The goby stands guard at the entrance, and the shrimp keeps an antenna resting on it. At the slightest escape movement of the fish, both disappear into the sand.
But no encounter is guaranteed.
If you want to try your luck, the goby and shrimp duo on a sandy bottom remains the most common clue, provided you keep your distance, do not slip your hand towards the burrow entrance, and leave the sand in place, for the reasons detailed in our article on why you should never touch marine life. A collapsed burrow means hours of lost work for the animal.
What do engineers copy from its claw?
In 2019, Xin Tang and David Staack, from Texas A&M University, described in Science Advances a mechanical claw 3D printed, five times larger than a real one and powered by a spring. When closing, it creates a cavitation bubble that produces plasma in water, without electricity.
The university press release mentions possible uses such as water sterilization or drilling. This remains at the laboratory stage. Thus, the shrimp joins the list of extraordinary defense mechanisms in marine animals that inspire laboratories. The previous version of this article went further, with fragmented kidney stones, robotic arms, and reinforced propellers thanks to it. We found no publication establishing this link, and we removed it.
The story actually goes the other way, since the 2000 AAAS press release recalls that cavitation already intrigued engineers in 1916, because of the damage it caused to ships, long before any shrimp was filmed.
The shrimp simply found the recipe before us.
Does warming make reefs noisier?
Yes, according to the work of Aran Mooney and Ashlee Lillis, from the Woods Hole Oceanographic Institution, presented in February 2020. When the water warms, pistol shrimps snap more often and the soundscape becomes more intense. These cold-blooded animals adjust their activity to the water temperature.
The researchers see a risk in this. A noisier reef could blur sound exchanges between fish, and interfere with sonars or fish detectors on boats. However, the study describes an effect of temperature and does not count populations. We found no published data quantifying an increase or decline of pistol shrimps worldwide. Impossible to decide here.
Their fate depends mainly on that of the reefs and seagrass beds where they live, and these habitats are under pressure, as shown by our dossier on coral bleaching, which details marine heat episodes.
Listen carefully next time.
FAQ
Why is it called a pistol shrimp?
Because its large claw snaps shut sharply and produces a popping sound. The noise comes from the implosion of a cavitation bubble created by the water jet, according to Versluis and colleagues (Science, 2000).
Is the bubble really as hot as the Sun?
Lohse and colleagues (Nature, 2001) estimate the temperature to be at least 5,000 kelvins at the moment of implosion. This is the same order of magnitude as the surface of the Sun. This figure is deduced from a light flash, without any thermometer.
Are the 218 decibels comparable to a gunshot?
No. This figure corresponds to about 80 kPa measured at 4 centimeters from the claw, in water decibels. Air decibels are based on a different reference, so the comparison with an airborne noise does not hold.
Is the pistol shrimp dangerous for a diver?
The publications consulted describe a shot aimed at small prey a few millimeters away. They do not mention any human injury. Still, avoid handling it or rummaging through its burrow.
Sources
- How snapping shrimp snap: through cavitating bubbles (Versluis, Schmitz, von der Heydt and Lohse, Science, 2000)
- Snapping shrimp make flashing bubbles (Lohse, Schmitz and Versluis, Nature, 2001)
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