How sharks work
By Marshall Brain
http://www.JewishWorldReview.com | (MCT) The summer beach season is prime time for shark attacks in the United States. It seems like sharks are attacking all the time. But that is something of an illusion. Even though the number of attacks is very small (only 23 in Florida in 2006), each attack gets nationwide news coverage. Combined with movies like "Jaws" and sensational TV shows, it all feeds into a natural human fear of predators.
But putting aside the fear factor for a moment, have you ever wondered about the sharks themselves? It turns out that sharks are absolutely fascinating creatures. They have existed on our planet for hundreds of millions of years because the basic shark design is extremely well adapted to life in the ocean.
One key to the shark's success is its diversity. There are over 400 species of shark today. Half of these species are small, at less than a meter in length full grown. But sharks can also get huge. A whale shark can grow as long as 45 feet and it feeds on plankton like many whale species. This wide range of sizes lets sharks fit into many different ecological niches.
Unlike mammals, birds and most fish, a shark does not have a skeleton. Instead it has a cartilage chassis like a ray does. The biggest piece of cartilage that is visible on a human is our ear lobes. By looking at your lobes, you can see that cartilage is strong, light and flexible compared to bone. The most important advantage that cartilage gives a shark is a weight reduction. Unlike most fish, a shark does not need an air bladder to compensate for the weight of calcium-rich bones.
Muscles and fins give the shark its speed and maneuverability in the water. A shark's front fins act like the wings of an airplane and let it "fly" through the water. The tail acts like a high-power propeller. The fastest sharks can swim at more than 20 mph.
But the big thing that gives the shark its edge in the ocean is its sensory package. The package includes the shark's eyes, ears, skin, nose and mouth, as well as electric sensing.
A shark's nose is probably its most important sense. If you were to put a single drop of blood in an Olympic size swimming pool containing more than 600,000 gallons of water, a great white shark could smell that. And most sharks have directional smell, so they can tell the direction that the smell is coming from. If something bleeds, a shark can smell it miles away. Many sharks can also hear sounds of distress from miles away.
Sharks handle their electric sensing using cells located in the head. Whenever something moves using its muscles, a shark can detect the electrical impulses flowing to those muscles. This is a short-range sense, but makes it very easy for a shark to electrically "see" anything that has muscles, even if it is hiding or the water is murky.
Sharks even have vibration sensors in their skin that work a little bit like our ears do. Tubes along the sides of a shark contain small sensitive hairs. When something moves near the shark, the tubes pick the pressure changes and the hairs inside the tubes send signals to the brain. Even if the shark cannot "see" something nearby with its eyes or electrosense, it can "feel" it moving when the shark swims by. This extra sense allows a shark to turn quickly and attack again.
When you put all these different senses together, it makes the shark a nearly ideal hunter. A shark can detect prey from miles away and then use eyes, electrosensing and movement sensing to home in.
Strangely, sharks do not seem to use these senses to home in on people. The very low number of shark attacks tells us that sharks don't hunt human prey on a regular basis. In many cases, if a shark bites a human, the shark will let go and flee. On the other hand, people love to hunt sharks. Millions and millions of sharks die every year, to the point where certain species of sharks may start disappearing. Without protection, extinction is a definite possibility.
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© 2007, How Stuff Works Inc. Distributed by McClatchy-Tribune Information Services.