Volume 1, Issue 6
The Toughest Shrimp Around
When a tiny mantis shrimp is hungry, it lines up a snail and delivers a lightning fast and powerful whack to the shell, cracking it open for easy feasting. The amazing thing is that the shrimp's smashing move delivers over 200 pounds of force, hundreds of times more than its own body weight. According to UC Berkeley integrative biologist Sheila Patek, that impressive impact comes from the speed of the motion, decidedly the swiftest kick in all of the animal kingdom.
Sheila Patek, pictured here with a Mantis Shrimp, also studies the biomechanics and physiology of how lobsters communicate. (photo copyright John B. Carnett/POPULAR SCIENCE MAGAZINE)
"The speed of the limb far exceeds most biological systems," says Patek, who last month was named one of Popular Science magazine's "Brilliant 10" for 2004.
To calculate the power of the shrimp's kick, Patek outfitted her shrimp tank with load cells, piezoelectric sensors that measure force. She then coated the devices with shrimp paste, a treat that attracts the predators to the snail stand-ins. Each kick to the load cell squeezes the sensor, generating a measurable electrical signal. It's not unlike the old carnival game where a player smashes a target with a sledgehammer to ring the bell and prove his strength.
Patek's research on the particular crustacean's predation began with a short film starring a peacock mantis shrimp (Odontodactylus scyllarus). The pummeling motion of the shrimp's clublike heel is far faster than the eye, or traditional videotape can follow, Patek explains. Last spring though, Patek, professor Roy Caldwell, and graduate student Wyatt Korff collaborated with the BBC to capture the shrimp's strikes using a high-tech camera shooting at 5,000 frames per second.
For the first time, the scientists could watch the attack in slow motion and really see the biomechanical forces at work.
"The first time I saw the video, I was impressed by how incredibly extreme, energetic, and fast this motion is," Patek says.
Patek's measurements revealed that the shrimp can release its front leg at speeds of 23 meters per second. Previously, scientists thought that the shrimp stored the energy for such speeds by contracting its muscles and latching the leg, essentially cocking it until the energy is released during an attack. However, Patek doubted that the latch-based mechanism was enough to generate such speeds. A spring would do the trick, but if the shrimp was equipped with one, where was it?
"I went to the National Museum of Natural History where they have the world's largest collection of these critters," she says. "I noticed that on the front limbs of every single stomatopod species, there's a beautiful and unique saddle-shaped structure."
The stiff saddle is what's known as a hyperbolic paraboloid. Similar in appearance to a Pringles potato chip, the shape is used by some avant-garde architects to design cement roofs resistant to buckling. This saddle in the shrimp's exoskeleton proved to be the missing spring, bending enough as the muscles contract to store the energy needed to propel the leg with the extreme speed and force required to shatter a tough snail's shell.
"The shrimps wear down their own heel with each attack," Patek says. "Luckily for them, they molt every three months."
A peacock mantis shrimp takes a whack at a Tegula snail with its front leg, which can reach speeds of 75 feet per second. (Sheila Patek, Wyatt Korff/UC Berkeley)
The video also revealed a secondary physical phenomenon of the speedy strike. Negative pressure near the point of impact results in cavitation, erosion caused by the implosive collapse of vapor bubbles. A half-millisecond after the shrimp's heel hits its target, the cavitation bubbles finish the job.
Now that the magnificence of the mantis shrimp's eating habits have been caught on film, Patek is eager to unravel the evolutionary mysteries behind their impressively brutal weapon. For example, she'd like to understand what factors led to the diversification of the limb and how the underlying model of the saddle has evolved in different kinds of mantis shrimp.
"The best part of biology is teasing apart the evolutionary history that goes into something as incredibly powerful and biomechanically complex as this single limb," she says.