Quirks & Quarks

The mantis shrimp's violent punches harness brains as well as brawn

Mantis Shrimps have the fastest punch on the planet, and they target them with care and control.
Picture of peacock mantis shrimp (Odontodactylus scyllarus). Taken at Tasik Ria house reef. Manado, Indonesia. (Jens Petersen)

Imagine, if you will, a small lobster with "Rock'em Sock'em Robot" appendages on the front. Add a splash of colour and sophisticated eyes, and you have the mantis shrimp. This crustacean - neither a mantis or a shrimp - is known for the sheer violence with which it attacks its prey, smashing apart crabs, snails, oysters and clams until their squishy and tasty insides come out.

Their weapons are their hinged front appendages which stick out from their thorax and are spring powered like a crossbow, giving them the fastest punch in the animal kingdom. The mantis shrimp strikes out with the same velocity as as a speeding bullet, hitting prey at 30 metres a second, generating 1500 newtons of force - the same as a tiger's bite - all in less than three-thousandths of a second. Their limbs move so quickly, the water around them boils - a process known as supercavitation. These cavitation bubbles produce an underwater shockwave that can kill prey even if the shrimp's bludgeons don't make contact.

In a new paper published by the Journal of Experimental Biology, researchers looked at how much brain is behind this brawn, and showed that these small crustaceans put a lot of "thought" into where they deliver their savage strikes.

A female mantis shrimp (Pseudosquillana richeri). (Credit: Roy L. Caldwell, courtesy of National Science Foundation)

Rachel Crane first noticed the mantis shrimp's odd behaviour when she was a graduate student feeding the resident Mantis Shrimp in the Patek lab at Duke University. Whenever she dropped a crab into the aquarium, the mantis shrimp would turn it over carefully before smashing it to bits. "It's actually this really long process to set up each strike for a shell," she says. "It has these small appendage mouth-parts that it uses to roll the shell around, and then it gently places the shell against the substrate and then it'll tap it with its sensory antennules a couple of times before it strikes." 

The mantis shrimp will take its time setting up its prey. "They'll roll the shell around and then place it and then tap it and then the shell kind of wobble or fall over. So they'll start again and place and tap and then they'll strike." When she began her Ph.D at Stanford University's Hopkins Marine Station, Crane decided to see what those turns were for. Were the strikes random, or a carefully planned assault?

A side view of a mantis shrimp, with its hammer-like appendages tucked in under its head. (Roy L. Caldwell, Courtesy of National Science Foundation.)

The first step was for Crane and her colleagues to collect a number of different mantis shrimp, which, for an animal nicknamed the "Thumbsplitter," proves to be a challenging task. "The dangerous part is when they unfurl their hammer and they use it like a spear. And when they do that they're definitely able to draw blood."  

They collected 77 Caribbean rock mantis shrimp (Neogonodactylus bredini), and separated them into different tanks. Each shrimp would react differently when a shell was put into their tank -- sometimes they would pounce immediately, and other times they would take a more careful approach to their meal. But eventually, each mantis shrimp would approach the shell and turn the shell around, studying it carefully before eventually striking the weakest part of the shell. 

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"They are very definitely targeting specific locations and they're choosing different locations depending on the shape of the shell," she says. "With squat round shells those shells have a really big aperture, which is the hole that the snail comes out of, and the mantis shrimp pretty exclusively target the aperture of those short round shells. "

"However when they're handling the long skinny shells what they do is a lot more complicated. They'll start by hitting that aperture and they'll do that a couple of times but over time they'll switch to striking the skinny pointy apex of the shell."

A female mantis shrimp (Odontodactylus Scyllarus). (Roy L Caldwell, Courtesy: National Science Foundation)

But what surprised Crane is that the mantis shrimp almost never hit the middle portion of the shell, which takes up the most surface area. "If they were hitting at random we would expect most of the strikes to be targeting sort of bulk of the shell and they never strike that area."

The Ninjabot robotic arm (l) poised to strike a shell. (Courtesy of the Patek Lab)

To confirm that the strike points were the weakest part, Crane turned to a robotic mantis shrimp named the Ninjabot, built at Duke University's Patek lab to study the crustacean's brutal blows. And, sure enough, the mantis shrimp's strategy was the most effective way to smash a shell to smithereens. 

"So that middle region they don't ever hit? That's a really ineffective location to be striking the shell, it almost never causes damage. Whereas the aperture and that skinny pointy spire that they focus on, do tend to be associated with more damage."

Ninjabot getting ready to strike the pointy part of a spired shell. (Courtesy of the Patek Lab)

Even with brute force on their side, the mantis shrimp often had to hit the shell a number of times to break it open. "I think there's sort of this impression that it's such a powerful strike. It's easy to imagine just a single impact hitting a shell in the shell just shatters and that's not at all what happens. Our animals took anywhere from 7 to 460 strikes to break the shell enough to start eating."

The next steps for Crane and her team are to use artificial shell shapes with the mantis shrimp to figure out what sensory cues they are looking for, and how they decide which strategy to use.

"I think there's a lot of focus in the literature on mantis shrimp about this powerful striking weapon that they have and it's really easy to be impressed by that, it generates huge forces, and it moves at high speeds and accelerations. But what I get most excited about is this behavioural complexity that they have. That just having this really powerful weapon isn't enough."  

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