Question

鲨鱼的资料（英文）

Answer 1

How long have sharks existed?

Fossil records indicate that ancestors of modern sharks swam the seas over 400 million years ago, making them older than dinosaurs! They are considered to be the first vertebrate with a complete jaw. Throughout time sharks have changed very little.

What is an "elasmobranch"? Elasmobranchs are a closely related group of fishes, differing from bony fishes by having cartilaginous skeletons and five or more gill slits on each side of the head. In contrast, bony fishes have bony skeletons and a single gill cover. Elasmobranchs include sharks, rays, and skates.

Do sharks have bones?

No, sharks and all other fishes belonging to the class Chondrichthyes lack true bone, but rather have cartilaginous skeletons.

How long do sharks live?

While longevity data are not available for many sharks, maximum ages do vary by species. Some sharks like the smooth dogfish (Mustelus canis) may only live 16 years, while others such as the porbeagle shark, (Lamna nasus) may live as long as 46 years. Whale sharks (Rhincodon typus) the largest fish in the world may live over 100 years.

What does a shark's skin feel like?

Shark skin feels exactly like sandpaper because it is made up of tiny teeth-like structures called placoid scales, also known as dermal denticles. These scales point towards the tail and helps to reduce friction from surrounding water when the shark swims. Because of this, if someone rubbed the skin from the head towards the tail, it would feel very smooth. In the opposite direction it feels very rough like sandpaper. As the shark grows, the placoid scales do not increase in size, but rather the shark grows more scales. The silky shark (Carcharhinus falciformis) has small scales giving it a "silky" feel to the touch.

Do sharks sleep?

It was once believed that all sharks had to swim constantly in order to breathe and could not sleep for more than a few minutes at a time. Oxygen-rich water flows through the gills during movement allowing the shark to breathe. While some species of sharks do need to swim constantly, this is not true for all sharks. Some sharks such as the nurse shark have spiracles that force water across their gills allowing for stationary rest. Sharks do not sleep like humans do, but instead have active and restful periods.

Answer 2

Out of all the species walking, flying, slithering or swimming, there aren't many who have been around as long, survived as well, or come in so many shapes and kinds as the shark. The earliest evidences of sharks are isolated spines, teeth and scales that appeared about 430 million years ago in the Silurian Period, known as the "Age of Fishes". Sharks have a sleek, streamlined design which helps them swim without using up a lot of energy.They certainly need to conserve their energy because they never really sleep and most of them never stop swimming.

Some sharks are fierce predators, and would be happy to eat you if they encountered you. Almost any shark six feet or longer is a potential danger, but three species have been identified repeatedly in attacks: the Great White Shark, the Tiger Shark and the Bull Shark . All three live world wide, reach large sizes and eat large prey such as marine mammals or sea turtles. But most sharks never grow longer than five feet and never even see anyone with legs and arms anyway. People kill thousands more sharks every year than sharks kill people.

Sharks take about as long to mature as we do. Some of them become adults in their teens. A mother shark carries her babies inside her body while they develop, sometimes for more than a year. Even so, some sharks are born inside an egg which they have to crack open. They spend early portions of their lives in nursery grounds. Some of the advantages sharks have over people is that they keep growing new teeth, they don't have breakable bones, and they are not prone to get cancer. Sometimes sharks are referred to as swimming computers because of the six senses which they possess: vision, hearing, vibration, smell, taste and electro-perception.

Answer 3

In some form, sharks have been around for about 400 million years.

Even before dinosaurs roamed the earth, sharks hunted through the oceans! They're such good survivors that they've had little need to evolve in the last 150 million years.

These ancient predators fascinate adults and children alike.
Sharks have the most powerful jaws on the planet. Unlike most animals' jaws, both the sharks' upper and lower jaws move.

A shark bites with it's lower jaw first and then its upper. It tosses its head back and forth to tear loose a piece of meat which it swallows whole.

Each type of shark has a different shaped tooth depending on their diet (the shark in the photo is a great white -- you can tell he's a carnivore just by looking at those sharp, pointy teeth!).
A shark may grow and use over 20,000 teeth in its lifetime!
Normally, sharks eat alone. But sometimes one feeding shark attracts others. They swim up as quickly as possible and all begin to try to get a piece of the prey. They bite wildly at anything that gets in their way -- even each other.

The great white shark rarely partakes in feeding frenzies.
Almost all sharks are "carnivores" or meat eaters. They live on a diet of fish and sea mammals (like dolphins and seals) and even such prey as turtles and seagulls.

Sharks even eat other sharks. For example, a tiger shark might eat a bull shark, a bull shark might eat a blacktip shark and a blacktip shark might eat a dogfish shark!

The teeth of the carnivores are sharp and pointy. Their skeleton is made of cartilage instead of bone, which allows greater flexibility.

Their skin is made of denticles instead of ordinary fish scales. The denticles are constructed like hard, sharp teeth and help to protect the shark from injury.
Not all sharks are fierce carnivores. Some are quite harmless. Oddly enough, the most harmless sharks tend to be the largest! The basking shark, the whale shark and the megamouth sharks all fit this description.

These huge sharks eat plankton, a tiny shrimp-like creature found in the ocean. To do this, they swim forward with their mouths wide open. "Gill rakers" at the back of their throat strain the tiny food from the water.
Think sharks are dangerous? The most dangerous sharks are the Great White shark, the Tiger shark, the Hammerhead shark, the Mako shark and the Bull shark. On average, there are only about 100 shark attacks each year and only 10 of those result in a human death.

You should check it out from their perspective, though! People kill thousands of sharks in a year for sport and for food. Shark skins are used to make products like any other leather would be. Up until the 1950's, shark livers were used as a vitamin A supplement. Shark fin soup and shark steaks are both eaten in many countries (Mako, seen in the top photo, is the most popular in the United States).
Baby sharks are called pups. Just like there are many types of sharks, there are also different ways that sharks come into this world. There are three ways that sharks are born:

eggs are laid (like birds)

eggs hatch inside the mother and then are born

pups sharks grow inside the mother (like humans)

Sharks can have from 1 to 100 babies at a time, depending on the type of shark. The ones with pups that grow inside the mother have fewer babies at a time than sharks that lay eggs outside the body.

Sharks do not care for their babies after they are born, but they do search for a safe place where they can lay their eggs or give birth.

One of the reasons that sharks are such successful predators is that they have such super senses.

Two-thirds of a shark's brain is dedicated to its keenest sense -- smell.

Some sharks have eyes similar to a cat. A mirror-like layer in their eyes allows them to see better in the water. This allows the shark to hunt in clear seas or murky water.

To top it off, sharks have a few unusual senses. For instance, they are able to feel vibrations in the water using a line of canals that go from its head to its tail. Called a "lateral line", these canals are filled with water and contain sensory cells with hairs growing out of them. These hairs move when the water vibrates and alerts the shark to potential prey.

Sharks also have a sensory organ called the "ampullae of Lorenzini" which they use to "feel" the electrical field coming from its prey.

Answer 4

What
is
a
Shark?
Sharks
are
a
type
of
fish.
Sharks
are
amazing
fish
that
have
been
around
since
long
before
the
dinosaurs
existed.
They
live
in
waters
all
over
the
world,
in
every
ocean,
and
even
in
some
rivers
and
lakes.
Unlike
bony
fish,
sharks
have
no
bones;
their
skeleton
is
made
of
cartilage,
which
is
a
tough,
fibrous
substance,
not
nearly
as
hard
as
bone.
Sharks
also
have
no
swim
bladder
(unlike
bony
fish).
________________________________________
MAJOR
DIFFERENCES
BETWEEN
SHARKS
AND
BONY
FISH
ATTRIBUTE
SHARKS
BONY
FISH
Skeleton
Cartilage
only
Bones
and
cartilage
Swimming
Can
only
swim
forward.
Can
swim
forwards
and
backwards
Buoyancy
(floating)
Large
oily
liver
Gas-filled
swim
bladder
Gills
Gill
slits
but
no
gill
cover
Covered
gill
slits
Reproduction
Eggs
fertilized
in
female's
body.
Eggs
usually
fertilized
in
the
water.
Skin
Rough,
sandpaper-like
placoid
scales
Slippery,
overlapping
scales
SIZE
A
scuba
diver
above
a
Whale
shark.
The
Whale
shark
is
the
largest
fish
in
the
world;
the
basking
shark
is
the
second
largest
fish.
There
are
many
different
species
of
sharks
that
range
in
size
from
the
size
of
a
person's
hand
to
bigger
than
a
bus.
Fully-grown
sharks
range
in
size
from
7
inches
(18
cm)
long
(the
Spined
Pygmy
shark),
up
to
50
feet
(15
m)
long
(the
Whale
shark).
Most
sharks
are
intermediate
in
size,
and
are
about
the
same
size
as
people,
5-7
feet
(1.5-2.1
m)
long.
Half
of
the
368
shark
species
are
under
39
inches
(1
m)
long.
________________________________________
BODY
SHAPES
Sharks
have
a
variety
of
body
shapes.
Most
sharks
have
streamlined,
torpedo-shaped
bodies
that
glide
easily
through
the
water.
Some
bottom-dwelling
sharks
(e.g.
the
angelshark)
have
flattened
bodies
that
allow
them
to
hide
in
the
sand
of
the
ocean
bed.
Some
sharks
have
an
elongated
body
shape
(e.g.,
cookiecutter
sharks
and
wobbegongs).
Sawsharks
have
elongated
snouts,
thresher
sharks
have
a
tremendously
elongated
upper
tail
fin
which
they
use
to
stun
prey,
and
hammerheads
have
extraordinarily
wide
heads.
The
goblin
shark
has
a
large,
pointed
protuberance
on
its
head;
its
purpose
is
unknown.
________________________________________
VARIETIES
OF
SHARKS
There
are
about
368
different
species
of
sharks,
which
are
divided
into
30
families.
These
different
families
of
sharks
are
very
different
in
the
way
they
look,
live,
and
eat.
They
have
different
shapes,
sizes,
color,
fins,
teeth,
habitat,
diet,
personality,
method
of
reproduction,
and
other
attributes.
Some
types
of
shark
are
very
rare
(like
the
great
white
shark
and
the
megamouth)
and
some
are
quite
common
(like
the
dogfish
shark
and
bull
shark).
Sharks
belong
to
the
group
of
cartilagenous
fish,
the
Elasmobranchii,
that
includes
the
sharks,
rays,
and
skates.
There
are
over
350
different
species
of
sharks.
The
spiny
dogfish
shark
is
the
most
common
shark.