If lions are kings of the jungle, then sharks are masters of the sea. Sharks are a diverse group of ocean predators, different from other fish and different from each other.
Fact or Fiction?
Much of what people believe about sharks has no foundation in fact. Despite all we've learned about sharks over the past few decades, myths about these interesting sea creatures persist even today.
Grouping Sharks
To make the study of sharks--and all animals--easier, scientists put them into different groups, according to their similarities. The 350 or so species of sharks are grouped into 30 families that make up eight different orders.
Well-Known Sharks
We've profiled ten sharks that range from large to small, rare to common, and fast moving to sluggish. Each profile will give you a good idea of how sharks are alike and how each species is different from every other species.
The Great White
Few creatures inspire as much fear and awe as the great white shark. Though it is a fierce predator, the great white rarely lives up to its reputation as a deadly "man-eater."
Whale Shark
Despite the whale shark's enormous size--more than 45 feet in length--it is harmless to humans. To eat, this filter feeder merely swims through the water with its mouth open.
Hammerhead
A wide, flattened, hammer-shaped snout gives these unusual-looking sharks their name. By swinging its elongated snout back and forth, the hammerhead gets a panoramic view of its surroundings.
Shortfin Mako
The shortfin mako is probably the fastest shark in the ocean, reaching speeds in excess of 35 miles per hour. A fierce fighter when hooked, the mako is a highly sought-after game fish.
Bigeye Thresher
Averaging 11 to 13 feet in length, the bigeye thresher has two distinguishing characteristics: a large, upward-looking eye and a tail that measures half of its total body size. This shark uses its tail to whip the water to round up its dinner.
Cookiecutter Shark
This small shark has an interesting name that comes from its curious feeding habits. Although it measures only 6 to 20 inches, the cookiecutter can inflict a nasty wound on its prey.
Tiger Shark
Massive jaws and uniquely curved, saw-like teeth make the tiger shark a formidable predator. Sometimes called the "garbage shark," the tiger shark has been known to eat nails, steel cages, paint cans, and other unusual items.
Angel Shark
Angel sharks look like their relatives, the rays, with flattened bodies and large, flat pectoral fins. Their coloring helps camouflage them from predators as they move along the ocean floor.
Spiny Dogfish
The spiny dogfish is the most common and abundant shark species. This 3-foot shark has two poisonous spines on its dorsal fins and often travels in packs of a hundred or more.
Goblin Shark
Although the goblin shark was first discovered almost a hundred years ago, we still know very little about this elusive species. Only about 20 specimens of this unusual, prehistoric-looking shark have been caught.
Shark Relatives
Sharks have several relatives, including rays and chimaeras. Although these sea creatures also have skeletons made of cartilage, they share few of the sharks' other characteristics.
Shark Evolution
Sharks have inhabited the world's oceans for more than 400 million years. Through the course of time, these animals have continually improved in form and function to accommodate the changes in their environment.
Shark Fossils
Most of the information we have about shark evolution comes from fossils. The most abundant shark fossils are teeth, which can tell researchers about ancient sharks' ages and feeding habits.
Size and Power
Sharks are not the largest or the fastest creatures in the ocean. But the sharks' size, efficiency, and predatory nature make them the most formidable powers of the sea.
Shark Shapes
Although sharks vary in shape, each species is superbly adapted for its particular lifestyle. In both form and function, sharks' body shapes have changed little over the past 400 million years.
Jaws and Teeth
Sharks may lose thousands of teeth in a lifetime and still have plenty to spare. The sharks' jaws are lined with as many as 20 rows of teeth. When one tooth falls out, a tooth from the next row moves forward to replace it.
Eyes
The shape of sharks' eyes depend on their lifestyle, active sharks having larger eyes than sluggish species. Sharks see best in dim light and have a remarkable structure that helps them maintain dim-light conditions even in bright environments.
Snouts
Shark snouts vary in size and shape, depending on the species. Most of the sharks' sensory organs are located in the head and snout region.
Dorsal Fins
Dorsal fins, located on the top of the shark's body, usually come in pairs. These fins prevent the shark from rolling over in the water.
Tail Fins
All sharks have asymmetrical tail fins, with the top lobe larger than the bottom lobe. The tail fin provides the main propulsion sharks need to swim.
Pectoral Fins
The sharks' pectoral fins serve as steering mechanisms, like wings on an airplane. These fins also helps sharks swim level and slow down.
Gills
Sharks have five to seven gills on either side of their bodies, each with its own opening. The gills function as breathing organs and absorb oxygen from the water.
Skin
Although the sharks' skin looks smooth, it is actually covered with tiny scale-like structures that give the skin a rough texture. Called denticles, these structures have the same composition as the sharks' teeth.
Parasites
Sharks frequently travel with companions called parasites that feed on the sharks' skin secretions. Though they may be a nuisance, parasites do not threaten the sharks' health.
Colors and Camouflage
Most sharks are drab in color, but many have ornate patterns and markings that camouflage them from potential predators. Most sharks are also countershaded to help them blend with their surroundings.
Inside a Shark
If you look inside a shark, you'll find many of the same organs that we have. Sharks have systems for breathing, circulating blood, digesting food, and reproducing, just like you do.
Cartilage
All sharks have skeletons made of cartilage rather than bone. Because it is lighter and more elastic than bone, cartilage increases the sharks' flexibility, efficiency, and buoyancy.
Breathing
Every living organism needs oxygen to survive. Fish absorb oxygen from water through their gills, and the sharks' blood carries the oxygen to cells throughout the shark's body.
Male or Female?
The primary difference between male and female sharks is their reproductive organs. Male sharks have testes that produce sperm, and females have ovaries that produce eggs.
Digesting Food
After the shark swallows, food moves into the stomach where digestive enzymes break it down. From there, the food moves to the intestine where it is absorbed as nutrients for the shark's body.
Blood Circulation
The shark's two-chambered heart pumps oxygen-rich blood throughout the shark's body. The blood also transports nutrients from digested food to the body's cells and carries waste products to the kidneys.
Liver
Sharks don't have lungs or swim bladders, the organs that keep other animals and fish afloat. Instead, sharks have large, oil-filled livers that make them buoyant.
Extrasensory Perception
Sharks have evolved an extensive network of sensory mechanisms to cope in their active environment. Sharks have the same five senses as humans, plus an extra, "sixth" sense, called electroreception, that gives them a competitive advantage in their underwater habitat.
Sight
The shark's vision is well adapted to the unique characteristics of the underwater world. Sharks are sensitive to light, movement, and contrast and can learn to distinguish between different-shaped test patterns.
Smell
Some sharks can detect one part tuna extract in 25 million parts of water! It's not surprising that sharks have earned a reputation as "swimming noses."
Taste
Shark experts are divided in their opinions about sharks' ability to taste. But evidence suggests that sharks have a discriminating sense of taste and will reject poisonous prey.
Touch
Sharks have two kinds of touch, physical touch and "distant touch," which allows the shark to feel the vibrations of distant objects. A shark may be able to "feel" an animal move 600 feet away.
Hearing
Sharks have no visible ears, but these animals are highly sensitive to sound and can hear many noises humans can't. This keen sense of hearing helps the shark locate food and determine the shark's orientation in the water.
Electroreception
The shark's amazing "sixth" sense, called electroreception, allows the shark to detect the weak electrical impulses that all animals emit. This remarkable sense helps the shark locate prey it cannot find with its other senses.
Fine Dining
Because sharks are such efficient swimmers, they need little energy and probably eat less than most people believe. Sharks eat a diverse diet of marine life and feed most actively at dusk, dawn, or night.
What's to Eat?
Despite the vast food resources available to the shark, most sharks are pretty choosy about what they eat. The shape and size of a shark's teeth generally determine what appears on its dinner menu.
Detecting Prey
Sharks use all of their six senses to locate their prey. If all senses indicate that an object is suitable prey, dinner is served.
How They Eat
Because all species of sharks have different teeth, they display a variety of feeding techniques. Some sharks carefully investigate their intended prey before biting; others attack without warning.
Filter Feeders
Three sharks--the whale, basking, and megamouth sharks--feed on tiny marine creatures called plankton. Rather than actively hunt their prey, these filter feeders merely swim through the water with their mouths open.
Feeding Frenzy
When sharks become overwhelmed by sensory stimuli in the water, they may display mob-like behavior, known as a feeding frenzy. The sharks may thrash wildly and attack anything in their way--even each other--as they compete for food.
Habitats
Sharks live in all the world's oceans, from the icy polar regions to the sweltering tropics. They also live at different depths of the ocean; some make their home in shallow coastal waters and others inhabit the ocean's depths.
Where Sharks Live
Temperature is the primary factor that determines the area a shark calls home. Some live in cold, arctic waters, while others prefer the warm waters of the tropics; still others inhabit more temperate waters.
Distribution
We've selected four sharks to show you how sharks are distributed throughout the world. The bull, basking, Greenland, and cookiecutter sharks represent cold, temperate, and tropical inhabitants as well as coastal and deep-dwelling species.
Migration
Some sharks travel thousands of miles between continents each year. Sharks undertake these long voyages in response to temperature changes, breeding cycles, and the availability of food.
Life Cycles
Each shark lives a predetermined life cycle that defines its method of birth and reproduction, growth rate, and life span. Each aspect of this life cycle or pattern works together in harmony to ensure that the species continues in perfect balance with nature.
Courtship and Mating
Unlike bony fish, all sharks reproduce by internal fertilization. Few people have witnessed courtship and mating behavior of sharks in the wild, so most of what we know comes from observations of the few sharks that will mate in captivity.
Methods of Birth
Sharks either give birth to live young, or they lay fertilized eggs in the ocean. The gestation period ranges from three or four months to two years.
A Young Shark's Life
After the mother shark gives birth to her pups, she swims away, leaving her youngsters on their own. The pups stay in the nursery area for several years before moving farther out to sea.
Growth and Longevity
Researchers learn about sharks' growth rates through tagging studies and by counting the rings that develop on sharks' vertebrae. These sea animals grow slowly and live about 20 to 30 years.
The Sharks Inspire
Throughout history, people have viewed sharks as creatures over which they had no control. Many early societies saw sharks in symbolic or religious terms and portrayed the sharks' importance in mythology, folklore, art, and literature.
Pacific Island Tales
Perhaps nowhere else in the world does the mysterious shark of ancient times play a more varied role than in the Pacific Islands. In many cultures, sharks were revered as gods.
Capturing Sharks on Film
The underwater environment poses many challenges for photographers. The density and motion of water, as well as the danger of being around sharks, are among the difficulties photographers must overcome to take clear, sharp pictures of sharks.
Sharks and the Movies
"Jaws" was not the first movie about sharks, but it was probably the most terrifying and popular. Twenty years after its debut, "Jaws" is still THE movie about sharks.
The Artful Shark
Sharks have never figured prominently in the history of art and literature. Several cultures, however, have used paintings and books to demonstrate the importance of sharks to their societies.
Shark Attacks
The media often portray sharks as vicious, man-eating beasts. The truth is that only about a hundred people worldwide are attacked by sharks each year, and most of them survive to tell their story.
Why Sharks Attack
Many of the people attacked by sharks are probably the victims of mistaken identity as the shark hunts for food. Sharks may also attack to protect their territory.
How Sharks Attack
Most shark attacks on people involve only a single bite that occurs during a "hit and run" attack. Other attacks--the bump and bite and the sneak attack--are more dangerous and may involve multiple bites.
Keeping Sharks From People
Your chances of ever meeting a shark face to face are slim, but following a few precautions can further reduce your chances of being attacked. You should never swim alone, at night, or in waters where sharks are common.
Protection in the Water
Researchers and sport divers who spend extended periods of time in deep waters have a greater chance of meeting sharks. Several inventions help these divers keep sharks at bay.
The Valuable Shark
Throughout history, sharks have yielded more marketable products than any other single group of fish. At one time or another, people have found a use for almost every part of the shark's body.
Shark Products
The ways that people have used sharks seems to have been limited only by imagination. Weapons, tools, and ornaments are a few of the ways people have used sharks.
Shark Meat/Fins
Most shark meat is edible and is a staple food item in many countries. The most well-known shark dish, shark fin soup, is a Chinese delicacy.
Shark Oil
Shark oil was once the most commercially valuable shark product because of its high concentration of vitamin A. Though no longer used for its vitamin content, shark oil is still used to manufacture cosmetics and medicinal creams.
The Medicinal Shark
Since ancient times, many cultures have used sharks for medicinal purposes. Today, researchers study sharks for clues in treating human diseases.
Research
Despite the sharks' long history on Earth, we still know little about these fascinating sea creatures. Researchers study fossils, dissect dead sharks, and observe live sharks in captivity to better understand the sharks' biology and behavior.
Identification and Tagging
Researchers have spent decades trying to understand where sharks go when they travel. Tagging and recapture studies are helping them find the answer.
Sharks in Captivity
Much of what we've learned about the sharks' behavior comes from observing sharks in captivity. However, most large, fast-swimming sharks, even if they can be caught, don't survive long in captive environments, so our knowledge about shark behavior is limited to just a few species.
Are Sharks Intelligent?
The previously held belief that sharks are mindless eating machines is changing. Scientists now know that sharks are capable of learning a variety of tasks at the same level as pigeons and white rats.
In Danger?
Can an animal that has survived more than 400 years now be in danger? Through overfishing and habitat destruction, humans are slowly jeopardizing the shark's future.
Overfishing
Commercial fishing operations and sport fishermen kill about a hundred million sharks every year. Eventually, we may decrease the stocks of some species that their numbers cannot recover.
Habitat Destruction
In the pursuit of progress and pleasure, humans have left their mark on every corner of the planet, including the sharks' home. Litter, chemicals, and human waste are only a few of the pollutants destroying the sharks' habitat.
Policy
The United States and many other countries now recognize the need to conserve sharks. Governments and organizations are developing polices that limit the number of sharks fishermen can catch and prohibit "finning."
