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.
