Sounds like the kind of teeth you’d like to avoid, doesn’t it, ? Triangular, razor-sharp, with serrated edges—teeth like that should be quite effective.
Fact is, these animals have been known to attack people, or any moving object, for that matter. In a sense, our mystery animal has excellent vision. Its sensitive eyes can see very well in dim light. At the same time, the animal has poor color vision, and it can’t detect the details of an object. This may explain why these animals have been known to attack canoes. For a creature unable to see color and details, a canoe might look like something good to eat.
Good for you, . You’ve already guessed that the answer to our riddle is a shark. Of course, you realize there are many different kinds of sharks—more than we know of. I’m told that scientists are still discovering and naming new varieties.
This particular shark goes by a number of different names. It almost seems that each country where it’s found has a different name for it. In Central America it’s called the Lake Nicaragua shark. In South Africa it goes by the name of Zambezi shark, or shovelnose shark. Whereas in the United States people call it either a bull, a cub, or a ground shark.
Notice something about the names used in Central America and South Africa, . “Zambezi” and “Lake Nicaragua” refer to bodies of fresh water. Does this mean that a shark can live in fresh water? Yes, the ground shark does—and that’s
highly unusual. Sharks mostly dwell in salt water.
I’m not saying that ground sharks don’t go into salt water at all. I’m only saying they’re unusual in that they do at times enter fresh water. Swimmers and boaters in the Mississippi River have been terrorized by ground sharks.
Why is this called the ground shark, you wonder? Because people used to think it lies on the bottom. But further investigation has shown that this is not true. A shark will only lie on the bottom if it is injured or diseased. In fact, a shark that lies on the bottom will die from lack of oxygen. Why so? Well, as you may remember, sharks can’t pump water over their gills the way other fish do. They have to keep swimming around and around in order to force water into their gill openings. You can imagine that makes for a restless life, .
Sharks typically eat other fish—mostly sharks and rays. The ground shark’s mouth, as in most sharks, is on the underside of the head. People used to think that sharks have to turn over on their backs in order to bite with their mouths, but that’s not true either. Sharks can use their teeth to good effect without turning somersaults. And as for those bread-knife teeth, they’re guaranteed to be sharp. They’re constantly being replaced! Some sharks grow new teeth every few weeks.
In Australia, this same shark is called the whaler shark. I’m not sure why. And you must not confuse this name with the whale shark, which is an entirely different fish. Whale sharks are enormous. They grow 40 feet (12 m) long—some say 60—and weigh as much as two elephants. Yes, , these are definitely the world’s largest sharks. In fact, they’re the largest of all fish. However, these big fellows are quite harmless. They swim sluggishly, often near the surface, where they sometimes get hit by ships. Their teeth are tiny. Their food is plankton and small fish. No wonder they’re named after the whales!
By contrast, the world’s smallest sharks are 6-7 inches (16-18 cm) long and fit in your hand.
It seems there is usually an exception to every rule. I’d told you that sharks can’t pump water over their gills. But the one known as nurse shark is an exception. Since they can pump water, they often lie motionless in shallow water. Nurse sharks have been known to attack people—when a swimmer was foolish enough to grab a motionless shark by its tail.
