Study: Sharks Bite With Upper Jaws

Bamboo sharks were photographed in the lab with a high-speed camera that produces 500 images every second. Tiny electrodes, about the size of a human hair, were inserted into the muscles of the shark's jaw to record every movement of the muscle. The two sets of data were later combined to see which muscles moved while the shark was gobbling down its lunch.

The evidence revealed that when a shark eats, its upper jaw extends out from the head, then the fish bites, then the upper jaw retracts under the head. Think of it as a conveyor belt pulling flesh into the mouth.

Once that question had been answered, Wilga turned to another area that is poorly understood: How the shark, which is not naturally buoyant, manages to swim.

The old theory was that "sharks swim like airplanes fly," using their pectoral fins to provide lift, just as an airplane uses its wings for the same purpose.

That, however, turns out to be dead wrong.

It is the shark's body, not its fins, that provide the lift, the research shows, but how Wilga found that out is as interesting as the conclusion itself.

Even if you study it in a glass tank, you can't really see how the fish gets its lift, because water is clear and such things as stream lines and vortexes are invisible.

So Wilga borrowed from the field of engineering.

"Its called digital particle image velocimetry," she says, and it's a system that allows her to determine just what the shark is doing to maintain its buoyancy by revealing the effect the fish is having on the water. In other words, she can actually see the way the water moves as the fish swims through it.

"Its really cool," she says.

Swimming in Beaded Water

Tiny glass beads, so small they can pass through the shark's gills without bothering the fish, were dumped into the tank. Each bead is coated with silver.

Then, a low powered laser that puts out a flat beam like a sheet of paper shines up through the tank. As the fish swims, the beads flow through the water, depending on how the fish maneuvers, and the beads reflect the movement of the water.

That data is recorded on a high-speed video camera, providing a complete record of the shark-water interaction. What it revealed, she says, is that the shark uses its tail fins to push it through the water, but it uses the shape of its body — not its fins — to provide the lift.

This breakthrough in shark aerodynamics is so new it's not even in the textbooks yet. None of this data is likely to ease the fear of sharks. But if you get hit, at least now you'll have a better idea of what's going on.

Enjoy the summer.

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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