The following is an article from the book Uncle John's Bathroom Reader Plunges Into the Universe.
(Image credit: Flickr user Ken Bondy)
Built-in lights, field detectors, flotation devices: Sharks have mastered the evolutionary tricks of a killing trade.
Hit men from the mob have nothing on sharks. Over the past 400 million years, sharks have evolved into nature's perfect predators. They can attack silently and with great bursts of speed. Their jaws can snap a sea lion in half, and their enormous gullet can swallow both pieces whole.
(Image credit: Alfonso González)
If limb-crunching, man-eating sharks haunt your nightmares, you won't enjoy knowing that there are nearly 400 species, ranging from about six inches to 50 feet long (15cm to 15m), inhabiting every oceanic corner of the globe. The good news is that most of them don't have the least interest in you. Some, like the polka-dotted whale shark (weighing in at 13 tons), are perfectly happy with plankton and schools of small fish. The most common shark, the dogfish, lives mainly on fish, crabs, octopus, and squid. In fact, instead of eating people, the reverse is true. These sharks, among others, actually appear on our dinner tables.
ARMORED AND DANGEROUS
(Image credit: Flickr user michael fontenot)
All the same, sharks must be respected as dangerous predators with evolution's mean streak on their side. The shark's torpedo-shaped body can shoot through the water at speeds of over 30 miles per hour (50 kph). Their skeletons are made of lightweight cartilage rather than bone, which gives them great flexibility in battle. Even the shark's liver helps it catch its prey. Rich in oils and lighter than water, the liver works like a flotation device to keep a shark buoyant, cruising along easily while it hunts for food.
(Image credit: Flickr user Willy Volk)
Then there are those "jaws." The biting strength of some sharks is a crushing 6.5 tons per square inch (Most sharks have at least four parallel rows of teeth -if one tooth breaks or is knocked out, another tooth moves forward to replace it). And a shark's teeth aren't all in its mouth. Its skin is completely covered with thousands of tiny teeth called placoid scales. These sharp, pointy scales are covered with enamel, just like the teeth, and makes shark skin so abrasive that it becomes a weapon. A shark can tear at its prey just by brushing up against it. Scared yet? There's more.
SENSING THEIR SUPPER
(Image credit: Flickr user Willy Volk)
You can swim from hungry sharks, but you can't hide. Their eyes have what's called a "tapetum," a reflecting layer behind the retina that enhances vision. Shark's eyes are so sensitive to individual flickers of light that they can pick up the slightest movement. Lantern sharks, which live in dark water 6,000 feet (1,800m) deep, have phosphorescent lights embedded on their bellies to help them see in the dark. But if a shark decides to kill, it doesn't have to rely on sight alone. Sound travels nearly five times faster in water than in air, and a shark can pick up the sounds of prey from a distance of 3,000 feet (900m) away. They're primed to hear low-frequency sounds, like the sound of contracting muscle tissue in an injured, struggling fish -or human.
A shark's sense of touch is enhanced by a strip of sensory cells along each side of its body that can feel the vibrations of prey moving throughout the water. And if by some chance a shark doesn't see or hear or feel its prey, the hunt isn't over. Its sense of smell is so acute that some sharks can smell one drop of blood diluted in one million drops of seawater! Yikes!
THE SIXTH SENSE
If five powerful senses aren't enough, sharks have an astounding sixth sense. All animals, including human beings, emit electrical signals, which a shark picks up with a special system of gel-filled pores around the head and mouth. In dark, deep water or sandy shallows, electrical detectors allow a hunting shark to position its head and mouth, then close in on a victim it can't even see.
LIGHTNING STRIKES VS. SHARK ATTACKS
(Image credit: Flickr user Ed Garcia)
With all that deadly equipment, it's not surprising that sharks sometimes kill humans. But shark attacks on humans are rare. Why? Probably because we're bony and taste bad. Sharks swim near bathers every day without bothering them. And sometimes people survive because a shark bites them, then abandons them -or even spits them out!
SWIMMING WITH SHARKS
(Image credit: Brocken Inaglory)
In the 1970s, the movie Jaws scared the world with its story of a great white shark devouring swimmers and boaters on the Atlantic coast. But the great white and other large sharks aren't mindless demons with an overwhelming desire for human flesh -they just happen to feed in the shallows where humans like to swim. Researchers now believe that attacks occur when a shark, in murky water, mistakes a swimmer for a school of fish or a threatening enemy. A diver in a wetsuit can resemble the great white's favorite meal: a tasty seal or sea lion.
SHARK KILLS GO OVERBOARD
(Image credit: Flickr user Casey Hussein Bisson)
Peter Benchley, author of Jaws, says that if he rewrote the story today, the great white shark would be a victim, not a villain. Sharks existed before the first dinosaurs appeared, but in the past two decades -partly because they are so terrifying- humans have slaughtered millions of sharks. Now, some species (including the great white) are in danger of extinction. When sharks disappear, other sea life does, too. Lobsters, for example, become endangered when there aren't enough sharks to control the lobster-eating octopus population. Benchley and other naturalists are fighting to save these magnificent underwater killers and warning that the world will suffer if the shark can't do its lethal job.
The article above was reprinted with permission from Uncle John's Bathroom Reader Plunges Into the Universe.
Since 1988, the Bathroom Reader Institute had published a series of popular books containing irresistible bits of trivia and obscure yet fascinating facts.
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