What Makes Fish Fast?
Sharks and tuna are among the fastest and most efficient swimmers in the animal kingdom. Bigelow professor of ichthyology George Lauder is developing robot replicas to discover the secrets of their speed and usher in a new era of aquatic automatons.
Although aquatic robots are already used for niche applications such as exploring shipwrecks and repairing underwater infrastructure, their broader application has been stymied by the unique challenges of building autonomous systems for marine environments. “Underwater robotics is certainly pretty far behind compared to terrestrial and flying robotics because working in the water is hard,” says Lauder, who is also curator of ichthyology in the Museum of Comparative Zoology. “You have to waterproof everything, communication between robots is hard, vision is hard—it’s just a very hard environment to work in.”
For more than a decade, Lauder and his collaborators have focused on solving one of the most vexing challenges facing underwater robots: how to efficiently move through the relatively dense aquatic environment. Most terrestrial and aerial robots use propulsion systems such as wheels and propellers, but Lauder’s approach to robotics—known as biomimetics—starts with nature’s solutions.
“I’ve had a longstanding interest in the marine world since childhood, so I trained as a biologist,” he explains, “but I’ve also had a strong interest in the mechanical side of how things work.”
Although Lauder has studied dozens of fish, his work has gravitated toward sharks and tuna to understand what endows them with their remarkable swimming abilities. The answer involves a complex interplay of biomechanics, shape, and surface characteristics. Although it’s possible to learn a lot about fish propulsion by studying the animals themselves, disentangling the factors that contribute to their swimming speed and efficiency is difficult using this approach. Developing robots that implement a single aspect of fish propulsion enables him to study each of the essential mechanisms underlying speed in isolation.
Sharks present an interesting case study because unlike other fast swimmers such as tuna or swordfish, which have smooth skin surfaces, shark skin is rough—covered in teeth-like structures called denticles. Although ichthyologists have known for decades that denticles likely hold the key to a shark’s ability to quickly and efficiently move through the water, how they contribute to speed remained a mystery.
Lauder’s laboratory ran a series of experiments last summer that used small pieces of shark skin to explore how they interact with water. Samples were placed in tanks that move water over the skin at a known rate. The researchers added particles to the water so they could see with microscopic imaging systems how it flowed over the denticles. When they analyzed the resulting data to calculate the friction at the interface between skin and water, they found that the water flow created small fluid vortices that reduced drag. Lauder compares the phenomenon to the dimples in a golf ball, which enable golfers to hit the ball about twice as far as a completely smooth ball. “You would think that you would want to be as smooth as possible to be most effective at moving through a dense fluid like water,” he says, “but actually you don’t want to be as smooth as possible. That roughness really matters for efficiency.” Lauder and his colleagues are now using their findings to “print” artificial denticles with properties similar to shark skin that could one day coat the surface of underwater robots to help make them more efficient swimmers.
They are also studying tuna as they search for ways to enhance the propulsion mechanics of aquatic robots. “If I boiled down what makes tuna such fast and efficient swimmers into one word,” Lauder says, “it would be ‘flexibility.’” Tuna are endowed with a highly flexible “lunate” tail—so named because it resembles a crescent moon—which many ichthyologists, including Lauder, believe may be the key to efficient underwater propulsion. “That,” says Lauder, “is their number one advantage over human-made underwater robots, which are all rigid.”
The robot is invaluable for answering questions about how water flows along the tuna’s body.
To better understand their biomechanics, Lauder and his collaborators have built a flexible “robo-tuna.” The robotic fish, which is about a foot long and capable of swimming at speeds of about a meter per second (about 2.3 miles per hour), has an undulating, crescent-shaped tail just like a real tuna. The device has proved invaluable for answering foundational questions about how water flows along the tuna’s body and how its fins generate forces on the fluids. Because the scientists can measure precisely how much power the robo-tuna uses to propel itself, they can easily compare that to the energy cost of alternative propulsion systems.
Moreover, Lauder perceives an opportunity to translate the aquatic-robotics discoveries into other fields. Artificial denticles, for instance, could lead to more efficient wind turbine blades and aircraft, and insights from building the flexible robo-tuna could lead to the development of robots that walk like humans or fly like birds. “Fish have had 400 million years to figure this out,” he says. “There are a lot of clues there that I think will help us bring biology and robotics together.”