(ominous music)
- [Narrator] They glide through the water
with unmistakable grace,
remnants of an ancient past,
they dive and they rise
from the ocean's murky depths
to it's sun-kissed shallows,
rousing fear and awe
like no other creature in the sea.
The world's biggest living fish is a shark.
Of the estimated 34,000 species of fish,
the largest are whale sharks.
These gentle giants usually grow to about 40 feet long
and weigh an estimated 15 tons.
Their mouths alone can span four feet wide.
The gigantic whale shark however,
pales in comparison to the largest fish
that ever existed, the megalodon.
Dating to over 20 million years ago,
it's thought that the prehistoric shark
could of reached 80 feet long,
weighing up to around 70 tons.
Unlike whale sharks, the megalodon was carnivorous,
and consumed any creature that fit
into it's nearly 10 foot wide mouth.
Throughout their lives some species of shark
can shed over 30,000 teeth.
Unlike humans who are born
with a set number of teeth in their jaws,
sharks have a seemingly limitless supply.
They can grow, lose, and replace their teeth as needed.
Furthermore, most sharks
have multiple rows of teeth in their jaws.
The jaws of a great white shark,
the largest predatory fish in the sea,
can contain up to seven rows
that hold up to 300 teeth at any one point.
Most sharks, as they hunt their prey,
end up losing their teeth individually.
However, the cookiecutter shark loses
and replaces the teeth in it's lower jaw all at once.
Sharks are built for speed.
The fastest known shark, the mako shark,
can reach speeds of up to 46 miles per hour.
This speed is largely due
to their body's hydrodynamic design.
Many sharks have torpedo shaped heads
that allow them to cut through the water
with little resistance.
Plus, shark skin is covered with flat, v-shaped scales,
called dermal denticles.
The denticles help water flow smoothly over the skin,
which reduces friction
and helps sharks swim quickly and quietly.
Sharks also have skeletons
made of cartilage instead of bone.
Cartilage is a much lighter material than bone
so sharks have less weight to carry.
Sharks may lay eggs, or bear live young.
Egg laying sharks only lay a few large eggs.
They may come in various forms,
such as sacks called mermaid purses or corkscrews.
These eggs act as external wombs
in which shark embryos complete their development.
However, most sharks give birth to live young.
Called pups, the young of most live bearing species
gestate for around one year.
Some even begin practicing their predation skills
while in the womb.
Before they are born, the sand tiger shark pups compete
with their siblings.
In fact, the strongest pup in each of the two wombs
devours its weaker brothers and sister.
Some sharks are at risk of extinction.
Every year an estimated 100 million sharks
are killed worldwide, in large part
for the shark fin trade.
The sharks are caught and their dorsal fins
are removed and sold at a hefty price,
primarily in Asia.
In traditional Chinese culture,
serving and eating shark fin is a sign
of status and wealth.
Because of the high demand and value of shark fins,
some shark populations have plummeted
by up to 70% causing a ripple effect
in ecosystems and endangering at least 74 shark species.
However, measures are being taken to protect sharks
with a number of countries and jurisdictions cracking down
on unsustainable shark fishing.
In China, shark fin soup is no longer allowed
to be served at government banquets.
A move hailed by shark conservationists.
Through continued international conservation efforts,
the loss of sharks may be curbed,
allowing the creatures in all their power and grace
to survive for many generations to come.