With the help of a BBC camera crew and the loan of a high-speed video camera, University of California, Berkeley, scientists have recorded the swiftest kick, and perhaps most brutal attack, of any predator. The shrimp flail their club-shaped front leg at peak speeds of 23 meters per second to shatter the hard shells of their prey.
"The speed of this strike exceeds most animal movements by far," said biologist Sheila Patek, a Miller Post-doctoral Fellow at UC Berkeley. "It's insanely fast, but important for generating the forces necessary to crush its preferred food - snails."
When slowed down by a factor of 330, the video shows the mantis shrimp's fist pummeling the shell of a snail like a slow-motion glove smashing into the face of a boxer. Patek is currently conducting experiments which show that the blow yields a tremendous amount of force - well over a hundred times the mantis shrimp's body weight.
In a short note appearing in the April 22 issue of the journal Nature, Patek and her colleagues, graduate student Wyatt Korff and professor of integrative biology Roy Caldwell, report the record-setting strike and the unusual saddle-shaped spring in the hinge of the shrimp's striking appendage that makes it all possible.
This spring is technically a hyperbolic paraboloid, a structure similar to a Pringles potato chip. Very strong, especially when compressed, hyperbolic paraboloids have been used by architects to create structures that don't easily buckle. The nautilus employs this structural element to build a sturdier shell. In mantis shrimp, however, the saddle-shaped structure can also function as a spring, the UC Berkeley researchers found. It stores energy until a quick release propels the shrimp's club in a shell-crushing blow.
"We know of no other biological example where this saddle-shaped structure is used as a spring," Patek said.
Mantis shrimp are distant relatives of the shrimp and lobster, common around the world and major invertebrate predators around coral reefs. Some hide in burrows and dart out to spear fish with their sharp appendages. Others roam the ocean floor in search of other crustaceans - crabs, clams and snails - and smash them open with their club-shaped front appendages. In captivity, these club-equipped stomatopods have been known to break the glass walls of their tank.
Patek, who studies communication in crustaceans and related organisms, has previously looked at the sounds made by the spiny lobster. Three years ago, she discovered that the spiny lobster makes noise in the same stick-and-slip manner of a violin.
Since coming to UC Berkeley, Patek has investigated other animals, including the mantis shrimp, Caldwell's main interest. She and her colleagues attempted to videotape the shrimp's feeding strikes, but they found that the animals moved faster than their video system could capture.
Last spring, however, the BBC rented a new high-speed camera for the team as part of a series on how new technology contributes to biological research. With this camera, the researchers were finally able to visualize the extremely fast strikes. The BBC television crew spent a week in Caldwell's laboratory helping to set up scenes with peacock mantis shrimp (Odontodactylus scyllarus) as the aggressors and local Tegula snails as the victims.
Though the rented $60,000 high-speed camera could shoot at 100,000 frames per second, what Patek valued was its ability to obtain high-resolution video at 5,000 frames per second in hard-to-light situations.
"We got absolutely spectacular images," she said, crediting the nonstop work of the TV crew. The video they obtained was broadcast in the United Kingdom in February as part of the series, Animal Camera. The series is scheduled to air in the United States this spring on Animal Planet.
As the researchers analyzed the video, however, they discovered that the previous estimate of the speed of another mantis shrimp species' strike - 10 meters per second - was much lower than the speeds observed in this particular species. Instead, the peak speeds of the striking appendage were 14 to 23 meters per second, with peak accelerations ranging from 6,300 to 8,000 times that of gravity.
Other animals with fast feeding strikes are the trap-jaw ant, at 17 meters per second, and the much smaller nematocysts of the hydra, which accelerate four times faster but achieve much lower speeds.
The shrimp's speed and acceleration were thought to be created solely by a click mechanism: the shrimp cocks and latches its appendage, the muscles contract, and when the latch is released, the energy stored in the muscles is released in a swift kick. This description, however, could not explain the extreme acceleration of the videotaped strike, so the researchers looked elsewhere.
The answer turned out to be a largely ignored piece of the shrimp's exoskeleton - the flexible, saddle-shaped structure in the striking appendage. They showed that it acted like a spring, storing elastic energy when the appendage is cocked, and releasing it when the shrimp strikes.
"All mantis shrimp have the saddle-shaped structure, though there is a lot of variation," said Patek, who is looking for other creatures that employ hyperbolic paraboloid structures as springs. "Using as few structures as possible with the least amount of energetic investment is a fundamental principle in many biological systems."
The high-speed video revealed other interesting aspects of the strike, including bubbles generated at the point of impact. The researchers suggest that the shrimp is taking advantage of a physical process called cavitation - the destructive effect of exploding bubbles - to break snail shells. The bubbles are created by negative pressure near the point of impact, either during the swift strike or as the heel of the appendage pulls back afterward. The popping bubbles also generate sound, and perhaps even light.
The smashing impacts and cavitation also eat away at the heel of the shrimp's appendage. Some shrimp develop holes completely through the exoskeleton to the flesh below, though periodic molting renews the hard mineralized heel surface.
Stomatopods are unique in many other ways. Caldwell discovered last year that these animals are the only known sea creature to use fluorescence to signal one another. The creatures also have the most sophisticated eyes of any animal on Earth. Some species have more than 10 pigments sensitive to different wavelengths of light, compared to only three pigments in humans. And at least one stomatopod is known to move by curling up and rolling like a wheel - downhill only, of course.
Patek's research was funded by the Miller Institute for Basic Research in Science at UC Berkeley, while Caldwell was supported in part by a campus Committee on Research Faculty Research Grant.