By Elizabeth Pennisi, ScienceNOW

Researchers have discovered what makes the shark almost impossible to outswim. By using an engineering imaging technique, researchers have discovered that as a shark’s tail swings from side to side, it creates twice as many jets of water as other fishes’ tails, smoothing out the thrust and likely making swimming more efficient. Sharks do this by stiffening the tail midswing, a strategy that might one day be applied to underwater vehicles to improve their performance.

“The authors have made a persuasive argument that muscles in the fin are modifying the shape and possibly the texture of the fin to modify the [water] flow” throughout the stroke cycle, says Frank Fish, a biomechanist at West Chester University in Pennsylvania.

For fish to move forward, they have to push water backward. And sharks have an added burden: they sink when they stop swimming, so they must be in constant motion. To help generate lift to keep midwater, the top of the tail extends farther back than the bottom, creating a slant along the back edge. Most other fish have tails that are essentially symmetrical from top to bottom.

Curious about how the shark tail works, Harvard University biomechanist Brooke Flammang has been examining its structure and function. In 2005, she discovered a tail muscle that seemed to activate at peculiar times during the tail’s swing back and forth. To understand the muscle’s role, she decided to track in fine detail how the shark pushes water backward.

To do this, researchers typically put a lot of small particles in the water. As the tail swings, the water moves and drags the particles along. The particles reflect light from flashing lasers, so they can be tracked using high-speed cameras. A computer program uses the images to generate pictures of water flow. The jets of water are hard to see, but these jets create rings or vortices of water that resemble smoke rings and can be readily detected.

Typically, this imaging technique employs two cameras to track the particles in the horizontal and vertical directions, and based on that data, researchers estimate how the particles move along the third dimension, depth. But Flammang wanted to see directly how particles moved in three dimensions. So she adapted a more advanced imaging system, one that use three cameras, that until now had only been used to study water flow coming off cylinders with pistons generating the force. “Engineers have employed this technique for years, but its application is new to biology,” Fish notes.

Flammang and her colleagues tested two spiny dogfish and two chain dogfish by putting them in a water tank with a constant water flow so the sharks swam in place. She also looked at the water flow coming off a shark “robot” that had a flexible plastic tail. (For more, see these videos of a spiny dogfish swimming and a robotic fin.) Most fish create a ring of water at the end of each tail flick. The tail pushes the water as it moves to the side, then sends the water twirling away as it stops to change direction. Sharks were thought to produce two rings at that point, one small and one large one because of the shape of the tail, and that’s what happens with the robotic tail.

But in reality, a shark’s tail spins off the second ring right as it reaches the midline of the animal, Flammang and her colleagues report in the 22 December issue of the Proceedings of the Royal Society B. That ring is larger and connects to the ring generated at the end of the tail flick. “That provides a big advantage,” Flammang says. Instead of just getting a push as the tail reaches the extent of its bend, the shark has added thrust midswing. “It may be allowing the animal to produce almost continuous thrust.” Flammang thinks the shark uses the muscle she characterized to stiffen the tail midswing, changing its shape slightly, to throw off the extra vortex.

“The shark has one more degree of sophistication” in generating thrust, says Michael Triantafyllou, an ocean engineer at the Massachusetts Institute of Technology in Cambridge. “Such observations can lead to better designs” for underwater vehicles, he notes, though he cautions that designing shape-shifting components “seems to complicate things.” However, Flammang is undaunted: “I would like to build a fully functioning shark tail model that can [change] the stiffness.”

This story provided by ScienceNOW, the daily online news service of the journal Science.

Image: As it moves, a shark's tail (shown) creates two connected rings of moving water (blue and red represent opposing flows) that help make this fish such a powerful swimmer. (Brooke E. Flammang)