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Dave swims with the sharks to learn what makes them unique.


Show Number 1402

How do sharks find their prey?

If a shark sees you, will it bite?
And if it bites, will it kill you?

[Apple Image] Getting Started

Do vibrations travel differently through different substances? Begin with a large, empty, plastic zip-lock bag. First, blow air into the bag and zip it shut. Place your ear on one side of the bag and have a helper make a noise or say something on the other side. Now fill the bag with water and try again. Does the sound travel differently in water than in air? How does this affect how ocean animals hear?

What animals do you think are dangerous to humans? Why? What are these animals' primary diets? Are humans included? Why would an animal attack a human? How are these animals important to the world environment?

How many different types of sharks can you name? What do they eat? How do they find their food? What do they all have in common? What makes them different from other ocean animals?

[Apple Image] Overview

Unlike the ferocious star of Jaws, most sharks pose little danger to humans. The world's 370-plus species of sharks eat a variety of foods. Whale sharks gulp in water and filter it out through their gills, trapping tiny plankton for food. Cookie cutter sharks fasten onto prey, then twist and pull away flesh, leaving a hole like that made by a cookie cutter. Nurse sharks use thick lips to suck food from rocky crevices.

A keen sense of smell helps sharks find food. Through touch, hearing, and sight, they detect movement in the water. A special organ on the snouts of sharks, called the ampullae of Lorenzini, allows them to detect weak electrical voltages from living creatures or ocean currents.

Humans pose a greater danger to sharks than sharks do to humans. People kill about 100 million sharks each year, eating shark fin soup and shark meat, using shark oil cosmetics, wearing sharkskin boots and shark tooth jewelry, and using parts of the shark to make medicines.

Despite many differences, sharks have one thing in common -- no bones! Most fish have bony skeletons, but sharks have skeletons made of cartilage. Also, throughout their lives, all sharks grow new teeth, with one set of teeth constantly growing in behind another. And instead of scales, sharks have a rough surface covered with denticles or skin-teeth.

Sharks come in many sizes. Whale sharks weigh up to about 13 metric tons (14 U.S. tons) and grow up to 18 meters (60 feet) long. The dwarf dog shark measures less than 20 centimeters (8 inches) long.

Some sharks lay eggs from which young sharks, called pups, hatch. Other species give birth to live pups. Pups fend for themselves from the first day, growing slowly to adult size over as long as 15 years.

Fossil records show that the earliest sharks lived nearly 400 million years ago. Despite their long history on earth, many mysteries about them remain.

[Apple Image] Connections

  1. The gray reef shark makes a "threat display" with specific posture and movements. How do other animals (including humans) use physical posture or gestures to show that they are threatened or about to attack?
  2. What steps can be taken to minimize danger to sharks from humans?

[Apple Image] Resources

Bullock, D.K. (1991) The underwater naturalist. New York: Lynn & Burford.

Conniff, R. (1993, May) From jaws to laws. Smithsonian, pp. 32-43.

Curtsinger, B. (1995, Jan) Close encounters with the gray reef shark.
National Geographic, pp. 44 - 67.

Michael, S.W. (1993) Reef sharks and rays of the world.
Monterey, CA: Sea Challengers.

The ocean planet. (1995, Mar) Popular Science, pp. 29-49.

Perrine, D. (1995) Sharks. Stillwater, MN: Voyageur Press.

Springer, V.G. & Gold, J.P. (1989) Sharks in question. Washington, DC: Smithsonian Institution.

Taylor, L. (1993) Sharks of Hawaii: Their biology and cultural significance.
Honolulu: University of Hawaii Press.

Ben S. Roesch's ultimate shark page:

Student Activity:
Taxonomy Computer Image


Try your skill at setting up a classification system for items in your room.

[Apple Image] Main Activity

Have you ever wondered how scientists classify animals? Why, for example, are some fish that lay eggs and some fish that give birth to live pups both classified as sharks? What is the common link between a 6" shark and a 60' shark?

The classification of living organisms into a system is called taxonomy. In this activity, you will try to establish a classification system for the world of your classroom. There are many ways to classify things. For example, sharks might be put in separate classes according to size, according to whether they are meat eaters and/or plant eaters, according to how they bear young, or according to where they live.

1. Working in groups of three or four, devise a system for classifying every object in your classroom. First, list all the objects that you will classify. Then decide on general categories and subcategories that make sense to your group.

2. Draw a chart showing the categories and subcategories that your group has chosen. List objects in the classroom in each category.

3. Each group should explain its classification system to the class. Display and compare the charts. Are there ways in which they are similar? How are they different?


  1. How many different ways of classifying things did your class list?
  2. What kinds of things were easier to classify?
  3. Were there some things that were classified by one group but ignored by another?
  4. What are the advantages and disadvantages of each system of classification?

[Try This]
How fast and how far do smells travel? Slice an onion at the front of the classroom. How far away can the onion be smelled? Wait 30 minutes. Now how far away can the onion be smelled? What has happened?

[Try This]
Make a model of a coral reef. Paint the inside of a large box blue and then cut and color cardboard, Styrofoam, or clay to resemble coral. Research accurate colors and shapes of fish and make them. Two-dimensional fish can be painted or three-dimensional models can be made from fabric or clay.

[Try This]
Fill one container with a liter (1 qt) of water and another container with a mixture of one liter of water and 30 grams (1 oz) of salt. Do objects float differently in the two containers?

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Educational materials developed with the National Science Teachers Association.
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