HARVARD GAZETTE ARCHIVES

George Lauder uses his hands to tell the tale of a shark's tail and how it is different from most fish tails. (Staff photo Stephanie Mitchell/Harvard News Office)

Sharks like this dogfish boast asymmetrical tails that may make them more maneuverable then fish with symmetrical tails. (Courtesy of George V. Lauder)

Sharks' tails have always mystified biologists.

Their relatives, hundreds of different species of fish, happily push themselves through the water with symmetrical tails that move from side to side. But most sharks are asymmetrical; the top part of their tails is larger than the bottom part, sometimes grossly so.

George Lauder and Cheryl Wilga decided to look into this uneven enigma. Lauder is a professor of biology at Harvard University who has a life-long interest in the design of animals that live in water. He often works with Wilga, a biologist at the University of Rhode Island. The Navy helps support their research because the things they learn could lead to robotic submarines that move more like fish and less like robots.

Lauder maintains a large aquarium at Harvard's Museum of Comparative Zoology in which swim, among other things, four dogfish, small sharks that live near shore.

See video of shark's tail in motion: Real / Quicktime

Using a complicated setup of laser light sheets that the dogfish swim through, high-speed video cameras, and sophisticated computer software, the researchers studied the flow of water over their asymmetric tails. Wilga and Lauder describe this setup and their results in the Aug. 19 edition of the journal Nature.

While a symmetrical fish tail leaves a one-part wake behind, the shark experiments clearly show a two-part wake. The larger upper lobe of a shark's tail cuts the oncoming water slightly before the smaller lower lobe. This creates a wake within a wake, giving the shark both thrust and lift, both forward and upward motion.

Lauder believes this increases maneuverability, although that idea has yet to be tested. An uneven tail may not be better or worse than an even one. Sharks lack bones; their skeletons are made of cartilage. They followed a different evolutionary path than fish that boast bony bodies. Both the cartilage and asymmetrical tails represent different designs for living in the water. "One design is not necessarily superior to the other," notes Lauder.

Most sharks and fish must swim or sink. If they stay motionless they will fall to the bottom. Fish have an advantage in this battle against negative buoyancy because many of them have gas-filled swim bladders. A trout or herring, for example, can raise itself by removing gas dissolved in its blood steam and using it to inflate its bladder. Submarines operate on the same principle, adding water to large tanks to dive, blowing it out with compressed air to rise.

Fishing for submarine designs

Would submarines be more maneuverable if they had tails or fins like fish? That's the kind of question Lauder wants to answer. "We see fish fins, including tails, as models of systems that could be used for increasing the maneuverability of underwater vehicles," he says.

Lauder's not talking about large, manned submarines. Rather, he aims for small, unmanned subs, turning quickly and moving quietly in near-shore waters. Such robots could be used for tasks, including search and rescue, scientific, commercial, and security purposes.

"Our goal is to provide new designs for flexible, finlike propulsion and steering on autonomous underwater vehicles," Lauder notes. Such vessels roam free of cables attached to surface ships. "Propellers make lots of noise; fish move very quietly," he adds. "Also, it's difficult for even small underwater vehicles to turn sharply, spin, and move laterally."

Such capabilities would come in handy for maneuvering among coral reefs, pier pilings and other near-shore structures. Equipped with cameras and other types of detectors, swimming robots could spot explosives, bodies, and interesting aquatic life.

As a teenager, Lauder did a lot of SCUBA diving. As he pursued a bachelor's degree (1976) then a Ph.D. (1979) at Harvard, he thought about how animals adapt to the water world, and how to build machines that do what animals can do. Those kinds of machines will come, but in the meantime he has more puzzles to solve, such as would a robot be better off with a fish or a shark tail.