Gulp! URI professor studies how sharks eat
KINGSTON, R.I. -- April 17, 2001 -- If you were able to keep your eyes open during the feeding frenzy of Jaws, you have a visual introduction to the work of the University of Rhode Island's Cheryl Wilga, an assistant professor in the Biological Sciences Department.
Wilga, a North Kingstown resident, studies how sharks use their upper jaw when feeding. "It's a mechanism that wasn't known when the movie was made," says the biologist who has always been fascinated by the way things work. In biology lingo, that's called functional morphology.
According to Wilga, a petite Alaskan Indian who came to URI last year following a National Science Foundation postdoctoral fellowship at Harvard University, the shark lifts its head, depresses its lower jaw, its upper jaw comes out, it bites, and then the upper jaw retracts under the head. Unlike humans, the shark's upper jaw isn't fused to its cranium.
To discover exactly how sharks eat, Wilga videotapes small spotted bamboo sharks while they munch on lunch in her lab. The high-speed film produces 500 images every second. Wilga then downloads the video into a computer and slows down the action to see exactly what the shark is doing and how it is doing it.
Wilga takes measurements of the angle of the shark's fins, how wide it opens its mouth, and how fast it captures its prey. She also monitors up to 12 muscles with electrodes as thin as pieces of hair to determine which muscles are used, a process called electromyography.
Wilga also researches how sharks swim. "The old theory was that sharks swim like airplanes fly," says the researcher, who notes that textbooks still state this. The old theory is that as wings of an airplane provide lift, the shark's pectoral fins also provide lift. Using fluid dynamics and lasers -- a technique called digital particle image velocimetry or DPIV, which measures the forces the fish places on the water, Wilga was able to see the water moving around the shark and to see that sharks swim at an angle to the flow. As part of her postdoctoral research, Wilga showed that the shark's body and tail provide the lift not the long suspected fins, which are actually used for steering.
Highly successful predators, sharks have been around for about 450 million years. There are about 800 species of sharks worldwide, including rays and skates that are close relatives and believed to have evolved from the shark.
Rhode Island has its share of sharks -- offshore waters host Blue, Sandtiger, Hammerhead and Thresher sharks while the Spiny Dogfish, Smooth Dogfish, Dusky and Sandbar Sharks and lots of skates such as Little, Winter, Roughtail, Barndoor, and Clearnose swim closer to shore.
Sharks have been clocked up to 60 M.P.H. and perhaps travel as fast as 70 m.p.h. "They have immense power and seem to be expanding little effort," says the URI morphologist, who calls the Jaws antagonist "beautiful, glorious creatures." Wilga says she studies sharks because they have evolved many different mechanisms for feeding as well as swimming and have been understudied, mostly due to the difficulty of working with large predators.
They are unique. Sharks have no bones and are composed of cartilage. They breathe differently too. Their gill structure has five slits on the side while bony fish have one. Their liver is filled with oil and, not surprisingly, their pearly whites are different.
The old adage -- sink or swim -- is true for the shark. Since it's heavier than water and doesn't have a swim bladder like bony fish, the shark must keep moving or bottom out.
Some sharks only eat fish, others only eat crabs, still others only seals. Some sharks only eat each other. Besides his or her own relatives, the shark's biggest predator is man's appetite. Many sharks are endangered because they have been overfished. While they may be speedy feeders, they are slow to reproduce. It generally takes sharks about 10 to 20 years to mature.
Large sharks eat only once every week or two, smaller sharks eat every day, according to Wilga, who notes that most sharks are picky eaters and don't hanker for humans. "Very few would take a bite out of a person," she says, "And if they did, they would usually spit you out."
Most sharks give birth in warm water during the late spring and early summer. If you go swimming in their "nursery," you might get bitten by an overprotective mom, according the researcher who grew up on Kodiak Island.
(digital photos available)
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