Shark
The Shark is one of the most feared sea animals. Scientists classify more than 400
species of fish as sharks. These fish live in oceans throughout the world. But sharks are
most common in warm seas.
Sharks vary greatly in size and habits. Whale sharks are the largest kind of shark and
the largest of all fish. They may grow up to 40 feet (12 meters) long. Whale sharks may
weigh over 15 tons (14 metric tons), more than twice as much as an average African
elephant. The smallest sharks may measure about 6 inches (16 centimeters) long and
weigh about 1 ounce (28 grams).
Some kinds of sharks live in the depths of the ocean. Others are found near the surface.
Some species of sharks live in coastal waters. Others dwell far out at sea. A few
species enter rivers and lakes that have outlets to the sea. Scientists believe that one
species, the Ganges River shark of India and Pakistan, lives only in rivers.
All sharks are carnivores (meat-eaters). Most of them eat live fish, including other
sharks. In fact, a shark's most common natural enemy is a larger shark. Most sharks eat
their prey whole. They may also tear off large chunks of flesh. Some sharks crush their
prey. Others scoop out small pieces of flesh from large fish. Sharks also feed on dead
or dying animals. Sharks have the reputation of attacking human beings. But fewer
than 80 shark attacks a year are reported throughout the world.
The body of a shark
Sharks differ from most other kinds of fish in a number of ways. For example, sharks
have a boneless skeleton. The skeleton is made of a tough, elastic substance called
cartilage. Most species of sharks have a rounded body, shaped somewhat like a
torpedo. This streamlined shape helps sharks to swim. Angel sharks have a flat body
similar to that of skates and rays. Angel sharks live near the ocean bottom.
Sharks have fewer young at a time than most fish do. Some species give birth to 60 or
more pups in a litter. Most sharks, however, have far fewer. The parents do not take
care of the young. The mother may even eat the babies.
Shark eggs, unlike those of most fish, are fertilized inside the female's body. The male
shark has two organs called claspers. The claspers release sperm into the female, where
it fertilizes the eggs. Among most species of sharks, the eggs hatch inside the female.
The pups are born alive. At least 40 species lay their eggs outside their bodies.
Scientists have observed one instance of a female shark giving birth without mating
with a male. Such reproduction without mating is called parthenogenesis.
Parthenogenesis also occurs in a number of insect and reptile species.
Tail and fins. Sharks can travel with great bursts of speed when excited. The fastestswimming sharks have a crescent-shaped tail that provides power for swimming. The
upper part of the tail usually is longer than the lower part. Stiff pectoral fins (side fins)
help lift and balance the front of the body.
Most fish have a swim bladder. The swim bladder is a gas-filled organ that helps them
remain at a certain depth without sinking. Sharks lack this organ. Instead, they have a
large liver filled with oil. This oil is lighter than water. As a result, it helps the shark
keep from sinking. Even so, most sharks must swim constantly or they will sink.
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Teeth and scales. A shark's mouth is on the underside of the head among most species.
The exceptions are the angel, megamouth, whale, and wobbegong sharks. The mouth
of these sharks is at the front of the head. A shark has several rows of teeth. New teeth
replace the rows of older ones regularly. New teeth emerge as often as every week in
some species. Some sharks have molarlike grinding teeth. Others have razorlike cutting
teeth. Still others have pointed teeth. Some people believe that sharks must turn over
on their back to bite, but this is not true.
Small, toothlike scales cover a shark's body. These placoid scales make the skin of a
shark rough. Dried sharkskin, called shagreen, was once used as sandpaper.
Gills. Sharks get oxygen from the water through gills, as do other fish. But sharks have
no gill cover. A gill cover is a bony plate that shields the gills of most fish. Instead,
sharks have from five to seven slits in the skin on each side of the head. Water passes
out of these slits after the shark's gills remove the oxygen. Many sharks cannot pump
water over their gills, as do the majority of fish. These sharks rely on their constant
swimming to force water through their mouth and over their gills. This process of
forcing water is known as ramjet ventilation.
Senses. Sharks have keen senses that enable them to compete successfully for prey.
They have excellent hearing, though it is limited to low-pitched sounds. Sharks can
home in on prey by its sounds. Some scientists believe that the shark's lateral line
detects the lowest-pitched sounds. The lateral line is a sensory system of fluid-filled
canals that runs down both sides of the shark's body, from its head to its tail. The lateral
line, sometimes called distant touch, also detects water movement.
Sharks have sensitive eyes that can see well in dim light. Sharks even possess a crude
form of color vision. However, they may not be able to see details clearly. Sharks have
been called "swimming noses." It was once widely believed that sharks relied mainly
on their sense of smell to hunt prey. However, little evidence exists of a special
sensitivity for smells in sharks.
Sharks can detect electrical fields. The head of a shark has small pores that lead to a
system of sensory tubes. These tubes, called ampullae of Lorenzini, are sensitive to
electrical fields. Sharks can locate and capture a fish by sensing the small electrical
field produced by its gills. Sharks also seem to use their electrical sense for navigation
and migration.
Kinds of sharks
Sharks belong to a group of fishes known as elasmobranchs. Rays and skates also
belong to this group. Scientists do not know exactly how many kinds of sharks there
are. They occasionally identify new species. But sometimes they find that two sharks
that had been considered different species are actually the same. This section describes
some of the best-known sharks.
Basking sharks may measure over 30 feet (9.1 meters) long. They live in tropical and
temperate waters. They eat only small water organisms called plankton. These sharks
have a habit of slowly swimming on the surface. They seem to be basking (warming
themselves). But they are actually filtering plankton from the surface.
Bull sharks can live in fresh water. They often enter rivers that empty into the sea. For
example, bull sharks have been captured in the Amazon River in Brazil, in the Ganges
River in India, and in the Mississippi River in the United States. They are also found in
Lake Jamor in Indonesia and in Lake Nicaragua in Nicaragua. Bull sharks often feed
on other sharks. They have attacked swimmers.
Great white sharks rank among the most dangerous sharks. They measure up to 21
feet (6.4 meters) long. They live in cool temperate to tropical waters. Great white
sharks swim powerfully. They prey on such large animals as sea lions, tuna, and other
sharks. They have attacked human beings and even fishing boats.
Hammerhead sharks rank among the strangest-looking sharks. They have a flattened
head. The head of most hammerhead species resembles the head of a hammer. Their
eyes and nostrils are at the ends of the "hammer." Hammerhead sharks include a
number of similar species. The largest hammerhead, the great hammerhead, may grow
as much as 20 feet (6.1 meters) long. Hammerheads live in shallow tropical and warm
temperate waters. They have attacked people.
Mako sharks are swift, powerful creatures. Shortfin makos are considered one of the
best species of game fish among all the sharks. When hooked, a shortfin mako fights
by leaping high into the air. Most shortfin makos live in tropical and warm temperate
waters. They may reach a length of 13 feet (4 meters). Makos feed on such fish as
bluefish, sharks, squid, swordfish, and tuna. Mako sharks can be dangerous to people,
but attacks are rare.
Nurse sharks are slow-moving fish. They live among reefs in shallow tropical and
subtropical waters. Unlike most species of sharks, the nurse shark can pump water over
its gills. Therefore, it does not have to swim constantly and often lies motionless on the
sea bottom. Nurse sharks may grow to 14 feet (4.3 meters) long. They eat bottomdwelling fish, crabs, lobsters, and shrimp. They have attacked people. But most of
these attacks were caused by the victim. For example, some people foolishly grab a
motionless nurse shark by the tail.
Thresher sharks are long-tailed subtropical fish that swim along the surface. They
grow to a length of about 20 feet (6.1 meters). Half of that length is the tail. The
thresher uses its long tail to gather into "herds" the fish it feeds on and to stun fish
before eating them. No thresher shark has been known to attack a person.
Whale sharks, in spite of their enormous size, eat only plankton and small fish.
Therefore, they are harmless to people. Whale sharks live in tropical waters.
Sharks and people
Shark attacks. About 50 species of sharks are considered potentially dangerous to
human beings. Most shark attacks do not result in death or serious injury. However,
swimmers should take care in areas known to have sharks.
People once thought that sharks ate constantly and would attack any human being
immediately. But scientists have learned that some species of sharks can go for days or
weeks without feeding. During this time, the oil stored in their liver probably nourishes
them.
Scientists do not know why sharks sometimes attack people and at other times leave
them alone. The American Elasmobranch Society studies shark attacks throughout the
world. The society supports safety rules for people who swim in water where sharks
may be nearby. These rules include the following:
Never swim or dive alone.
Never swim or dive with an open wound. Blood attracts sharks.
Never swim or dive at night or in dirty water, where there is less chance of spotting a
shark.
Leave the water immediately if a shark is sighted. Swim as smoothly as possible
because thrashing movements might attract the shark.
Never grab or injure any shark, even a small and apparently harmless one.
How people use sharks.
Commercial fishing crews catch sharks mainly for their hides, fins, and flesh. Shark
hide makes a luxurious leather after the scales have been removed. Chinese people
often use dried shark fins to make a popular and expensive soup.
People in various parts of the world eat the flesh of many species of sharks. In
England, for example, dogfish sharks are often used in preparing fish and chips, which
consists of fried fish and French fried potatoes.
The liver oil of certain species of sharks contains large amounts of vitamin A. Until the
late 1940's, shark liver oil ranked as a main source of this vitamin. Then scientists
discovered an inexpensive way to manufacture vitamin A.
Since the late 1950's, sharks have been used increasingly in scientific research. Medical
researchers are interested in the fact that few sharks ever have a cancerous tumor.
Scientists hope to discover what protects sharks from disease.
Commercial shark fishing increased during the 1980's. One reason for the increase was
that shark fins brought high prices in Asian markets. Fishing crews caught so many
sharks that populations dwindled worldwide. Conservationists began to fear that many
shark species would become extinct.
During the 1990's, some countries began to protect sharks. South Africa, for example,
declared great white sharks a protected species. South Africa banned fishing for these
sharks or selling their jaws or other body parts. The United States issued rules in 1993
to protect a number of shark species in U.S. waters. The rules included commercial
fishing quotas and bag limits for sport fishing. The United States also banned catching
several species, including the great white shark and whale shark.
Citation:
Gruber, Samuel H. “Shark.” World Book Advanced. World
Book, 2013. Web. 30 September 2014.