University of Massachusetts Amherst researchers are using X-ray scans and computational models to learn the secrets of mantis shrimp, crustaceans who fire their appendages with amazing speed and force to ward off enemies and capture prey. On the left is a freeze frame from a high-speed video of an experiment in which a materials-testing machine compresses a mantis shrimp appendage to mimic the way the crustacean would prepare to strike. On the right is a finite element computer model of the appendage under similar loading conditions. Blue, or cold, regions represent areas with low calculated strain energy density. Red, or hot, regions have high calculated strain energy density. The comparisons show the model’s predicted behavior resembles the appendage’s physical behavior. (Images: Michael Rosario, University of Massachusetts Amherst. A video, “An inside look at the mantis shrimp’s punching mechanism,” is available in the Related Links box at right.)
An archery avocation got Michael Rosario thinking seriously about killer shrimp.
As an undergraduate integrative biology student at the University of California, Berkeley, Rosario co-founded an archery club. As he won competitions, he also studied with Sheila Patek, an evolutionary biology, biomechanics and animal behavior researcher. The intersection of the two interests has led Rosario, a Department of Energy Computational Science Graduate Fellowship (DOE CSGF) recipient, to apply computers to crustaceans and to a featured role on a National Geographic television program.
Rosario and Patek focus on the mantis shrimp, a group of around 400 species named for their resemblance to true shrimp and to the praying mantis, the garden insect hunter. These denizens of the ocean floor and coral reefs deliver powerful, lightning-fast blows. Their club-like appendages, which also can open to deploy needle-sharp claws, reach top speeds of around 50 miles per hour in less than 3 milliseconds. That’s so powerful the claws pull water molecules away from each other to form a cavitation bubble that delivers an additional shock, emits sound waves and a spark of light, and creates a temperature spike of up to 7,000 degrees centigrade – about as hot as the sun’s surface.
What’s more, a mantis shrimp appendage delivers a blow measured at up to 300 pounds, thousands of times the creature’s body weight. It’s one of the highest peak forces any animal produces.
“I kept looking at this animal and trying to figure out its relationship to how I knew bows and arrows work,” Rosario says. Shrimp claws and bows both store muscle power gradually and release it in a burst. But while each of a modern bow’s parts has a specific duty – acting as a brace or storing elastic energy – “when you look at a mantis shrimp appendage, it’s just the exoskeleton. It’s one structure that has to deal with these competing demands.”
As an undergraduate, Rosario largely focused on experiments testing the exoskeletons of mantis shrimp appendages. By using wire to replace the muscle that loads the appendage’s spring, he could measure the amount of force required as a function of displacement in the claw over time. Rosario found it took 40 to 50 Newtons to fully compress it – or the force necessary to lift between 9 and 11 pounds. “I couldn’t close the appendage with my hands.”
Rosario felt stifled by the experiment’s limitations. “What I was really interested in was elastic energy” – the potential energy stored in a crumpled or stretched or otherwise deformed bendable object, like a retracted bowstring. “There’s no good way to measure elastic energy in these systems.” He wanted to know how a mantis shrimp appendage, as a single structure, handled demands that usually require specialized structures.
Rosario joined Patek as a graduate student after she moved her lab to the University of Massachusetts Amherst. Computational models were the key, he decided, to discovering the mantis shrimp’s elastic energy secrets. In an intensive summer course he participated in Biomesh, a finite element analysis (FEA) workshop that Elizabeth Dumont, another UMass biology professor, had helped create. FEA, more typically used in engineering applications, decomposes a model into a series of simple digital bricks, Rosario says, then computes the physical properties and changes in each. Starting with computer tomography (CT) scans of mantis shrimp appendages, he created models to calculate what parts of the appendage bear the load when it’s compressed to strike – in essence, how and where it stores elastic energy.The CT scans and other research found high material density in two bar-like structures in an appendage from G. smithii, a mantis shrimp species known for its speedy blows. “That got me thinking,” Rosario says. “Functionally, does that play a role? If we load this model of the appendage, do we see a high density of energy where we also see a high density of mineralization?”
Rosario’s models calculated how much energy is stored in each virtual brick – or region of the appendage – under different loading conditions. In initial studies, he found that when the claw was compressed, elastic energy was highly concentrated in the mineralized bars, as he suspected. Rosario compared that against a model of an appendage from L. maculata, a mantis shrimp species that typically spears its prey while releasing its claws at a slower speed. The calculations showed energy was spread throughout the entire appendage, rather than focused in particular places.
That could mean the more diffuse energy density in L. maculata translates into a slower speed than the concentrated energy approach in G. smithii, Rosario says. But he also speculates “these different structures are viewing energy density in different ways.” Loading elastic energy into one or two areas, like the mineralized bars, puts added strain on those parts, increasing the risk of failure. Spreading the energy throughout the appendage lessens the strain on any one spot. “You end up with somewhat of a more robust structure.”
Patek’s work has gained international attention, most recently from the National Geographic WILD channel on cable television. “Ninja Shrimp,” which has been showing this spring, includes Rosario demonstrating both his claw-compressing experiment and the computer models.
Studying biological structures involved in elastic energy storage could provide ideas to build better structures for similar purposes, Rosario says. Robots designed to explore planetary surfaces, for instance, would benefit from an energy-efficient way to jump over obstacles.
The research also provides insights into the evolutionary process. In broad terms, Patek says, “what we’re hoping to understand is the interface between physics and evolution.” Many organisms have elastic energy mechanisms, and biologists generally understand how each works. Little has been done, however, to compare these systems across species. By examining how they differ “we can probe the evolutionary history of the system,” including why some parts changed and others didn’t to achieve different purposes.
Comparing elastic energy mechanism parts and variations has many computational components, Patek says, making the rigorous training Rosario receives through the DOE CSGF especially useful. Although experience in computation and mathematics isn’t uncommon among biomechanics researchers, “he has a skill set we didn’t have. That means he now can think about asking questions we didn’t think we could ask before. We can do things we didn’t think we could.”
Rosario plans to continue adapting computer codes, which will let him explore deeper mysteries of the mantis shrimp. For instance, the model now calculates energy distributions only at the beginning and end of compression and merely interpolates what happens in between. Rosario wants to base the progression on hard calculations.
The model also assumes an appendage’s composition is pretty much homogeneous throughout. Working with polymer scientists, Rosario hopes to characterize an appendage’s material properties at the nanoscale and address questions connecting them to time scales. “It takes about 2 seconds for these appendages to load, but they strike within milliseconds, so there’s definitely something going on at the polymer level. I’d like to test some of my theories out with these computational models.”
Greater understanding of modeling has opened new horizons, Rosario says. “Without the mathematics and the computation, I would be blind in terms of where this energy is being stored. This computation has given me the ability to look at things that weren’t possible” to see.