There are approximately 365 species of shark today. While sharks are a type of fish, certain characteristics place them into a specific group. One main characteristic is that a shark’s "skeleton" is not made of bone. The skeleton of a shark is actually made of gristle and cartilage, making the shark’s body extremely flexible. Cartilage is the material contained in your outer ear or your nose. It is a tough, rubbery material.
Sharks differ from other fish in their outer body covering also. While most fish have scales, a shark is covered in denticles, tiny sharp projections which make their coverings rough to the touch. Sharks also have gill slits rather than gill coverings and while most fish have a swim bladder, a gas-filled sac which helps with buoyancy, a shark does not.
The Shark’s Body
Since a shark’s body does not contain a swim bladder to help them stay afloat, they have developed other means to prevent them from sinking to the bottom of the sea. One of these developments is an oil filled liver.
A second method by which sharks maintain buoyancy is in the way that they swim. A shark holds its pectoral (or side) fins far out from its body. Then it pushes itself forward by swinging its tail from side to side. This produces a rush of water over the pectoral fins causing "lift," much the same way air flowing over a plane’s wings work. (To learn more about lift check out our explanation Watching "Lift" In Action.)
Sharks eat a wide variety of foods depending on the type of shark. Some sharks seem undeterred by the sting of stingrays, the poison of sea snakes, or the jaws of another shark to grab a meal. Fish, shellfish, shrimp, plankton, jellyfish, turtles, rays, sea birds, lobster, crabs, seals, porpoises, carcasses, other sharks, and sea urchins are just the beginning of the items sharks find edible.
With a mouthful of teeth, shark’s do not seem to need much protection. In fact one shark goes through thousands of teeth in a lifetime. Each row of succeeding teeth is larger than the set before.
But shark’s teeth are built for eating. The shape of a shark’s teeth depend on the type of food it eats, and some sharks’ teeth change shape as they grow older and their diet changes to suit the needs of their growing bodies.
Sharks also have other types of protection. A protective lid, called the nictitating lid, helps keep its eyes safe from harm when it attacks prey or nears an unfamiliar object. Some sharks will also roll their eyes back into the sockets showing only the whites while attacking prey to protect the more important seeing part of the eye.
Bottom feeding sharks, such as the angel shark or wobbegong, have coloration matching their environment. Others will bury themselves in the sandy ocean bottom.
However, sharks have little or no protection from humans who hunt them for almost every part of their body to make leather, jewelry, soup, cosmetics, and other items. Sharks are also hunted for sport and many are inadvertently caught in fishing nets or in nets placed to protect humans.
Helping To Protect Sharks
Since sharks reproduce at a much slower rate and mature more slowly than bony fish, it is important that people are careful not to deplete the shark population to a point where it can not be recovered. In some cases the environments in which the sharks live are being destroyed.
While sharks have a bad reputation, they rarely attack people unless they are provoked or mistake a human for their normal prey. More people drown in the ocean each year than are attacked by sharks. Only a few of the 365 types of sharks, the bull, the great white, the oceanic whitetip, and the tiger shark are aggressive toward humans.
Encouraging the use of electric barriers instead of nets to keep sharks out of an area is one way to help. Also discourage the hunting of sharks for sport or the production of unnecessary articles such as jewelry and souvenirs. In addition learn more about sharks and inform others of ways to live peaceably with these fascinating animals.
A Shark’s Sixth Sense
In order to live and hunt in the ocean waters, sharks have the same 5 senses as people do. They have smell, taste, touch, hearing, and sight. These senses vary in their strength. Many sharks have sharp eyesight, some of which seem particularly sensitive to movement. Most sharks also have a well developed sense of smell.
However, sharks posses a sixth sense which people do not have. Sharks are able to detect weak electrical signals generated by their prey. This sense is detected through small sensory pores called ampullae.
- The smallest shark is the dwarf dogshark which is 6 ˝ inches (16 cm) in length.
- The largest shark, ranging up to 40 ft. (12m) in length is the whale shark.
- Whale sharks are large non aggressive creatures which will sometimes allow divers to hitch a ride by holding on to a pectoral fin.
- Most sharks give birth to live young rather than lay eggs.
- The shark with the longest tail is the thresher shark. Its tail can be 5-8 ft in length, or as long as its body.
- The hammerhead shark swings its head from side to side as it swims, allowing it to use its well developed sense of smell and many ampullae to search for prey in a wide variety of directions in a small amount of time.
- The fastest shark is the mako, which can swim at 20 mph (32 kph) and leap completely clear of the ocean surface when it is excited.
- Angel sharks have extra large pectoral fins resembling angel wings. These sharks spend their lives mostly of the ocean bed resting in the sand waiting for prey to come to them.
- Some sharks migrate hundreds of miles using what scientists believe to be a sense of the earth’s magnetic field to guide them.
- One shark makes oval shape bites in its victim, by creating a suction with its lips and swiveling around for the bite. It is appropriately named the cookie cutter shark.
- One of the smallest sharks, the lantern shark, grows to only 8 in (20 cm) in length and glows in the dark.
- The most recent discovery of a shark was not made until 1976. This was the large megamouth shark, of which only five more have ever been found. This unusual shark which feeds on krill has luminous organs around its mouth.
Books for Children
- Eyewitness: Shark by Miranda MacQuitty - An excellent source of information about sharks with many fascinating photographs to accompany the text. Information includes facts about breeding, anatomy, behavior, species, and lots more. Great for children 7 and older. (amazon.com has it)
- Hungry, Hungry Sharks (Step into Reading Series/ Step 2, Grades 1-3) by Joanna Cole - An interesting book with clear text and nice illustrations. This book has lots of information for the younger reader. Appropriate for ages 4-9. (amazon.com has it)
- A Look Inside Sharks and Rays by Keith Banister - Using acetate pages like windows into these creatures this book is engrossing and enlightening. An intriguing study of shark anatomy and facts. Appropriate for ages 9 and older. (amazon.com has it)
- Sharks by Gail Gibbons - Simple text and interesting illustrations involves even the youngest readers in learning about sharks. Nice for children ages 4-9. (amazon.com has it)
- Draw 50 Sharks, Whales, and Other Sea Creatures by Lee J. Ames - Step by step instructions and illustrations can assist children in noticing and producing drawings of sharks with nice accuracy and detail. Appropriate for children 7 and older. (amazon.com has it)
- Shark Lady: True Adventures of Eugenie Clark by Ann McGovern - The biography of a young girl as she grows up, begins a career working with sharks, and becomes well known for her research and findings. Interesting for children ages 7-12. (amazon.com has it)
- Shark Friends - This is a wonderful site containing information about sharks and other ocean animals. It includes ideas for teachers, information about careers in marine biology and oceanography, a conservation section, postcards, games, puzzles, and more.
- Shark Sufari! by nationalgeographic.com - has a shark quiz and links to shark puzzles and information.
- Sharks by EnchantedLearning.com has information, illustrations, printouts, a pop-up card, classroom activities and more about these sea creatures.
- Fiona's Shark Mania by Fiona Webster - A great site including information about different sharks, art work, literature, links, and more.
- Watson and the Shark by the National Gallery of Art shows the painting (which can be enlarged) by John Singleton Copley, as well as information about the artist and the story behind the painting.