Sharks have been celebrated as powerful gods
by some native cultures.
For example, Fijians believe the shark god Dakuwaqa,
could protect fisherman from the dangers at sea.
And today, sharks are recognized as apex predators of the world's ocean
and include some of the Earth's longest living vertebrates.
What is it that makes these fish worthy of our ancient legends
and so successful in the seas?
Much of their hunting prowess stems from a unique set of biological traits
honed for more than 400 million years.
Their cartilaginous skeletons are less dense than bony ones
and require less energy to move.
Large oily livers lend buoyancy to their streamlined bodies,
and while trunk muscles of bony fishes attach directly to their skeletons,
those of sharks also join to their skin.
This special design transforms them into pressurized tubes
whose springy skin can efficiently transmit muscular forces to the tail.
Shark skin has additional remarkable features.
Despite its smooth external appearance,
at the micro level, it has a coarse texture
thanks to thousands of tiny teeth-like scales
called dermal denticles.
Each denticle is coated in a substance called enameloid,
which turns the skin into a tough shield.
Plus the structure of denticles varies across the body
in such a way as to reduce noise and drag
when the shark moves through water.
As for the teeth in their mouths,
sharks can produce up to 50,000 in a lifetime.
On average, they can lose one tooth a week,
and each time that happens,
it's rapidly replaced.
Thanks to a layer of fluoride coating their teeth,
sharks also avoid cavities.
But teeth aren't the same in all sharks.
They can vary across species and by diet.
Some are dense and flattened,
useful for crushing mollusks.
Others are needle-like for gripping fish.
The mouths of Great Whites contain pointy lower teeth for holding prey
and triangular serrated upper teeth for slicing.
This variety enables sharks to target prey in a diversity of ocean environments.
Many species also have another peculiar trait -
the ability to launch their jaws out of their mouths,
open them extra wide,
and grab prey by surprise.
Over the course of evolution,
shark brains have expanded,
coupled with the growth of their sensory organs.
Modern-day sharks can smell a few drops of blood
and hear sounds underwater from 800 meters away.
They're particularly well-tuned to low frequencies,
including those emitted by dying fish.
And like cats, they have reflective membranes called tapeta lucida
at the backs of their eyes
that dramatically improve their vision in low light.
As if these heightened abilities weren't enough,
sharks have even honed a sixth sense.
They're able to hunt using a network of electrosensory cells
called ampullae of Lorenzini.
These cells are filled with hypersensitive jelly
which allows them to detect electrical signals from prey,
including the slightest twitch of a muscle.
Some of the most iconic shark species,
like Great Whites, Makos, Porbeagles, and Salmon Sharks
owe their success to another surprising trait:
warm blood inside a cold-blooded creature.
Inside their bodies,
they have bundles of arteries and veins
called rete mirabile.
Here, venous blood warmed up by the shark's working muscles
passes right next to arteries carrying cold, oxygen-rich blood from the gills.
This arrangement transfers heat to the blood
that gets cycled back to the body's vital organs.
Warmer muscles enable faster, more powerful swimming,
while warmer bellies aid digestion,
and the more rapid development of young in utero.
And warmer eyes and brains keep the sharks alert in cold waters.
With these amazing adaptations, there's more to revere than fear
from the 500 shark species roaming our oceans.
Unfortunately, one-third of these species are threatened
due to overfishing.
After millions of years in the making,
these apex predators may be meeting their greatest challenge yet.