READING LEVEL – MIDDLE
FACT SHEET FROM THE EDUCATION DEPARTMENT
Sharks
Sharks are some of the most mysterious
and misunderstood animals on Earth today.
They are feared by most, loved by some
and hunted by many people. Many movies
and books have furthered myths about
sharks. However, the more we study
sharks, the more we learn the truth
about these magnificent fish.
SHAPE, SIZE AND COLOR
There are approximately 400 species of
sharks, each with a unique shape and size.
Many sharks have a sleek, streamlined
body shape designed to help them swim
quickly through the water to capture their
prey. The fastest shark is the torpedoshaped shortfin mako, which can swim at
20 mph (32km/h). Sharks range in size
from the 8 inch dwarf lanternfish (also
known as the deepwater dogfish shark)
to the 45 foot whale shark. The color of
a shark’s body is designed to allow the
shark to blend in, or camouflage, with
the ocean and surrounding environment.
The top of a shark is typically a darker
color that blends in with the dark water
below when seen from above. Sharks
have a light color underneath that blends
in with the sunlight above when seen
from below. This type of camouflage is
called countershading. Some sharks may
have stripes or spots, become lighter
over time or remain the same color
their entire lives.
NO BONES ABOUT IT
Sharks and their relatives (including
skates, rays, sawfishes and guitarfishes)
belong to a group of animals called
elasmobranchs, which includes fish with
skeletons made of cartilage, not bone.
Cartilage is a tough, flexible material
(connective tissue) that can be found in
the human nose and ears. Sharks have five
to seven gill slits visible on the outside of
their bodies. In order to breathe, water
passes in through their mouth, across their
gills where oxygen is absorbed, and exits
through their gill slits. Sharks do not
sleep like humans do, but instead they
have active and restful periods. Some
species of sharks need to swim constantly
to allow oxygen-rich water to flow through
their gills. However, some sharks, like
the nurse shark, can rest at the bottom
because they have small holes called
spiracles that force water across their
gills in addition to taking in water
through their mouth. Sharks also have
an oily liver to help them stay afloat.
The skin of a shark is covered in tiny
scales or skin teeth called dermal denticles.
These skin teeth point toward a shark’s
tail, so a shark feels smooth if touched
from head to tail but feels like sandpaper
if felt from tail to head. As a shark grows,
more skin teeth are added to its body,
which helps it to become a faster, more
efficient predator.
KEY TERMS
Ampullae of Lorenzini
Small pores in a shark’s snout that
detect electrical signals.
Bycatch
Marine animals caught accidentally
by fisherman who intended to catch
another species.
Cartilage
Hard, flexible connective tissue that
forms a shark’s skeleton.
Countershading
A type of coloration where an animal’s top side is dark while the bottom
is light. This coloring helps the animal
to blend in with its surroundings.
Dermal Denticles
Small tooth-like scales covering a
shark’s skin.
Elasmobranchs
Subclass of fishes with skeletons made
of cartilage and possessing five or more
gill slits: includes sharks, skates and rays.
SHARK SENSES
Sharks have a number of specialized
senses and adaptations, making them
one of the most efficient predators on
Earth. Depending on how far away their
prey is located, a shark will utilize different senses to detect it. A shark’s most
acute sense is smell. When swimming, the
shark moves its head from side to side
in order to detect and follow the area of
the most concentrated scent. A shark can
smell blood or fish oil up to 1320 ft (400
m) away. Sharks hear underwater using
internal ears on either side of their head.
Sharks specialize in hearing low frequency
sounds, like those made by injured or sick
animals splashing in the water. Scientists
have found that most sharks have ears
that are capable of hearing low frequency
sounds from 295 feet (90 m) away. However, some sharks can hear these sounds
from up to 800 feet (250 m) away. At 100
feet (30 m) away, the shark’s lateral line
becomes effective. The lateral line is used
to detect sound waves, vibrations and
disturbances in the water. It is a complex
network of holes that runs horizontally
along the body. The holes are attached
directly to nerves that stimulate a muscle
response from the brain when vibrations
are detected. Sharks can typically see
objects from about 50 feet (15 m) away.
They see best at dawn and dusk by using
a structure called a tapetum lucidum, which
is a special layer of cells behind the retina
in the shark’s eye. The tapetum lucidum
acts like a mirror within a shark’s eye,
allowing it to see 10 times better in dim
light than humans. Sharks also have sensory organs called Ampullae of Lorenzini,
which are most effective at about 6 feet (2
m) or less. They are fluid-filled pores surrounding the mouth and nose of sharks
and rays.These pores detect weak electrical fields given off by animals as well as
the Earth’s magnetic field. The Ampullae
of Lorenzini are particularly useful in
dark or murky waters when prey animals
are hiding under the sediment.
THE TOOTH OF THE MATTER
Different species of sharks have different
shapes and sizes of teeth that are specialized for the kinds of food they eat.
Notched teeth with serrated edges like
those of the tiger shark are specialized
for cutting and tearing into hard objects.
These teeth make it possible for adult tiger sharks to eat through sea turtle shells.
Thin, long, pointy teeth allow the shortfin
mako shark to grasp and hold onto prey.
Great white sharks have narrow teeth in
their bottom jaw used for piercing and
holding prey, plus sharp, serrated, triangular teeth in their top jaws for cutting. Port
Jackson sharks have hard, flat teeth for
crushing crabs, clams, oysters and other
shellfish. Whale sharks have numerous
tiny teeth—they filter-feed plankton and
fish through screens on their internal gill
slits. Some sharks, like the nurse shark,
use barbels or whiskers to probe the
bottom in search of food hidden in
the sediment.
Sharks lose teeth constantly throughout
their lives. When they eat animals with
hard bones or shells, they lose teeth from
their soft, cartilaginous jaw. Some sharks
can lose up to 30,000 teeth in their lifetime. The inside of a shark’s jaw has five
to fifteen rows of teeth that usually lie
flat until the tooth in front of it falls out.
When a tooth is lost, another rotates forward to replace it within 24 to 48 hours.
The process of tooth replacement in sharks
is very similar to the movement of a conveyor belt or the steps on an escalator.
SHARK PUPS
Sharks reproduce by laying eggs, giving
birth to live young or a combination of
both. Some species of sharks lay eggs that
have hooks to attach to underwater plants.
Other shark species bear live young, which
are fully formed. The pups are not cared
for by their parents and are ready to hunt
for their own food as soon as they are
born. The number of young varies by
species. Sand tiger sharks have 1-2 pups
while whale sharks can have up to 300
pups at one time.
SHARKS AND HUMANS
For years, a great deal of attention has
been focused on the interaction between
sharks and humans. Of the approximately
400 species of sharks, only 32 species have
ever been documented in attacks on
humans. According to the International
Shark Attack File (ISAF), there are an
average of 50-75 unprovoked shark attacks
each year worldwide, and only about 5
result in a fatality. Scientists theorize that
human attacks may happen as a result of
territorial responses, inquisitive testing or
mistaken identity.
KEY TERMS
Despite their reputation, most sharks
are shy and harmless, avoiding people
and other large animals whenever possible. Only a dozen species of sharks are
considered dangerous to humans. On the
other hand, people kill millions of sharks
every year by accident and intentionally.
Many sharks die as accidental bycatch,
caught by fisherman who intended to
catch another species. Sharks are also
killed intentionally for their rough skin
and sharp teeth to be used as sandpaper,
leather and jewelry. Many sharks are also
hunted for their liver oil, meat and fins to
make shark fin soup.
Shark populations are declining and,
without protection, may not be able to
bounce back from the pressures put on
them by humans. Sharks are apex, or top,
predators in the world’s oceans and play
an essential role by feeding on sick and
dying animals as well as larger animals
that have few predators. Scientists who
study sharks are working to find out more
about these amazing animals and how to
better ensure their survival.
FAST FACTS
Lateral Line
Sense organ in sharks, running horizontally along the body, that can detect
movement and vibrations.
• The shortfin mako is the fastest shark
and can swim at 20 mph (32km/h).
Tapetum lucidum
A special tissue layer in the eye that
acts like a mirror bouncing light through
the eye and improving a shark’s vision
at night and in low light.
• Sharks have a skeleton made of
cartilage, not bone.
• Sharks range in size from 8 inches to
45 feet in length.
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• Sharks can replace lost teeth in
as little as 24 hours and may lose
thousands of teeth in their lifetime.
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