Shark-bite science: turn the page to learn about the forces behind shark bites
When surfer Bethany Hamilton hit the waves off Hawaii last October, a shark lunged out of the water, bit the 13-year-old, and tore away her left arm and a chunk of her surfboard. Luckily, Bethany’s surfing buddies towed her ashore to seek help. She survived and has since returned to the waters to surf–with one arm.
No one saw Bethany’s attacker, but scientists know which of the world’s roughly 375 shark species is to blame: the tiger shark. The telltale clue: the fish’s feeding method. Turns out, many shark species have very different sets of jaws and teeth, which make for a unique bite. “There’s a remarkable variety of shark teeth,” says marine biologist Dan Huber of the University of Southern Florida. Some sharks have spiky teeth, others have sawlike teeth, and many have rows of teeth. To make sure it doesn’t go hungry, each shark approaches prey with a feeding method that matches its teeth and lifestyle.
To understand how sharks feed, Huber studies dead sharks, carefully measuring teeth, muscle, and cartilage (bendy connective tissue) in the head. He’s learned that a shark’s feeding method can be broken down into one of three basic approaches.
APPROACH #1: RAM AND BITE
Like many large, meat-eating sharks, the tiger shark rams its prey. The 5.5 meter (18 foot)-long fish swims in tropical waters, such as those around Hawaii. And it isn’t picky about its prey. Scientists who have dissected tiger-shark stomachs have found other sharks, seabirds, and even tin cans inside the gut.
To nab food, “the tiger shark surprises its prey by charging with maximum momentum [object’s mass multiplied by its velocity],” says Huber. The collision throws the unsuspecting victim off balance. Then, the big fish secures its prey with sharp, pointy teeth; inside a tiger shark’s jaw are uniquely shaped, serrated (saw-edged) teeth (see photo, p. 13). They look like a row of steak knives, says Huber.
Since the tiger shark lacks hands to grab and tear its catch into bite-size morsels, it digs into its prey with force (push or pull). To do so, the shark shakes its body back and forth, moving like a lever (rigid object that pivots about a fixed point, or fulcrum). The motion’s input force (force exerted) pushes the teeth deeply into its prey. At the same time, the teeth’s serrated edges saw cleanly through flesh and bone.
APPROACH #2: SUCK AND BITE
The so-called blind shark lingers over a meal. Last February, snorkeler Luke Tresoglavic swam ashore, walked across a beach in Sydney, Australia, and drove his car to the lifeguard station–all with a 61 centimeter (two foot)-long blind shark sucking at his leg (see photos, p. 14).
Generally docile unless provoked, blind sharks dwell on the seafloor near Australia’s coastal coral reefs. When it spies chow, such as an octopus, this shark sucks in prey like a vacuum cleaner. “A blind shark keeps pumping water into its mouth and out its gills [respiratory organ used to extract oxygen dissolved in water],” explains Huber. This creates suction, or a force produced by a difference in pressure (force applied over an area) between two spaces.
It’s similar to what happens when you drink from a straw: When you suck, the air pressure inside your mouth becomes lower than the pressure outside. So the liquid in your glass inches up the straw. Similarly, prey gets drawn toward the blind shark’s numerous small, needlelike teeth. But these teeth aren’t designed for chewing; they spear the prey as the shark readies to suck it down whole.
It seems Tresoglavic was too large to gulp. But heavy-duty suctioning sealed the fish’s mouth to his leg. Eventually, a dousing of fresh water on the shark’s gills shocked the saltwater fish into opening its jaws.
APPROACH #3: BITE AND CHEW
Some sharks eat like humans do. They bite and chew their food. These fish tend to be fairly stationary. So their teeth are designed to nab prey that don’t normally make quick getaways, like sea urchins, sea snails, and other slow-moving organisms with hard shells.
The horn shark–which hides among rocks and seaweed in the Pacific–“has knifelike front teeth for grabbing and tearing,” says Huber. “Its back teeth are flatter, like ours, and are used for crushing food.”
How much force does this shark use to crack open shelled prey? To feud out, Huber made a force-measuring device. He wrapped the gadget with squid to lure the horn sharks in his lab to bite. He found that getting bitten by a 2.5-footer, “is like getting a 50-pound weight covered in thumbtacks dropped on your hand.” Ouch!
HUMANS BITE ATTACK
While shark attacks on humans grab headlines, “no shark is really set up to eat us,” says George Burgess, director of the Florida Program for Shark Research at the Florida Museum of Natural History. “We aren’t a part of its natural food source.” He explains: Sharks can mistake splashing humans for fishes, sea turtles, or pinnipeds (seals, sea lions). But most sharks give up after just one bite: You’re not the tasty meal it expected.
Last year, sharks attacked 55 people worldwide, killing four. On the flip side, about 60 million sharks were caught and killed by humans. And many species, like the great white, are endangered. Says Burgess: “Maybe the story isn’t shark bites human, but human bites shark.”
Did You Know?
* The blind shark (Brachaelurus waddi) is not blind. It has this name because it closes its eyelids when it is taken out of the water.
* Record holders: The largest shark–also the largest fish in the ocean–is the whale shark. It can reach lengths over 20 m (60 ft). The smallest shark is the deepwater dogfish shark. This Caribbean Sea dweller is under 20 cm (8 in.) long. The fastest shark is the shortfin mako. It’s been clocked swimming at speeds up to 32 km (20 mi) per hour.
* Sharks do not have real bones. Instead, they belong in the class Chondrichthyes, where fish have skeletons made of cartilage (tough, flexible tissue). Other close relatives in this class: rays and skates.
History: Sharks have been on Earth for a long time–they are even older than the dinosaurs. Research to find out when they first swam in the ancient oceans. Then create a time line highlighting the earliest records of 10 organisms still living on Earth, including the shark.
Review the aquatic food chain and where sharks fit in. Have students imagine what would happen if all shark species were to become extinct. How would the food chain be disrupted? Also, how would that affect humans?
To learn more about sharks, visit the Web site of the shark research program at the University of Florida’s Florida Museum of Natural History: www.flmnh.ufl.edu/fish/Sharks/sharks.htm
You can learn more about marine biologist Dan Huber’s work by visiting his home page at: http://luna.cas.usf.edu/~motta/Dan.html
Dan Huber appears in the new Discovery Channel series Animal Face-Off. For air dates and more information, check out: http://dsc.discovery.com/convergence/animalfaceoff/animalfaceoff.html
The companion Web site to the NOVA program Shark Attack has a section on sharks’ six senses. Site includes a teaching guide visit: www.pbs.org/wgbh/nova/sharkattack/
CHECK FOR UNDERSTANDING
Directions: Fill in the blanks to complete the sentences.
1. Marine biologist Dan Huber categorizes shark-feeding methods into three different approaches: –, –, and –.
2. To prevent from going hungry, a shark’s feeding method must match its — and –.
3. To sever its prey into bite-size morsels, the tiger shark shakes its body back and forth, moving like a –, pivoting around a fixed point called a –.
4. The so-called blind shark pumps water into its mouth and out of this respiratory organ: –.
5. When you drink from a straw, the liquid inches up the straw because of a force called –.
1. ram and bite, suck and bite, bite and chew
2. teeth, lifestyle
3. lever, fulcrum
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