All About Sharks

Sharks and their relatives have been swimming in Earth’s oceans for about 450 million years. That's long before there were humans, dinosaurs, or even trees!

Sharks Are Fishes, Not Mammals Like Whales.

Unlike their ancestors the bony fishes, sharks have firm-but-flexible skeletons made of cartilage. So do their close relations the rays, skates, sawfishes , and chimaeras. Cartilage is the same stuff that stiffens your ears and the tip of your nose.

There are more than 500 shark species living today, all different shapes and sizes, from 6-inch dwarf lantern sharks to 60-foot whale sharks.

How Old Are Sharks?

Can you put these organisms in order, from the oldest to the most recent?

Sharks are really, really old!

Sharks survived four mass extinctions. They even survived the one that killed most of the dinosaurs!

Sharks are really, really old!

Sharks survived four mass extinctions. They even survived the one that killed most of the dinosaurs!

timeline showing from left to right shark-like fishes then coniferous tree then dinosaurs thenflowering plants then primates and finally omdern humans

timeline showing from top to bottom shark-like fishes then coniferous tree then dinosaurs thenflowering plants then primates and finally omdern humans

What Makes a Shark a Shark?

Click on a body part to learn more.

How do sharks sense their prey?

NOSTRILS

Shark nostrils are used only for smelling, not breathing.

INNER EAR

Unlike humans, sharks can detect the direction of low-frequency sounds to locate prey.

BLOOD ON THE BRAIN

Shark brains have an extra-large area devoted to scent, helping them smell a drop of blood diluted more than a million times.

SIXTH SENSE

Sensors in the snout called ampullae of Lorenzini help sharks detect electromagnetic fields that reveal prey. Some sharks can even sense Earth’s magnetic field.

photo of shark snout showing dots that are receptors

Why are sharks such good swimmers?

anatomical diagram of shark liver, spine, and fins

FIXED FINS PROVIDE LIFT

Shark pectoral fins don’t flap—they are stiff, like the wings of airplanes or hydrofoils. When sharks swim, these rigid fins provide lift, while the tail provides thrust.

orange graphic of oil and water separating in bottle

OIL-FILLED LIVERS

Unlike most fishes, which use an air bladder to keep from sinking, sharks have a huge, oil-filled liver. Unlike air, oil remains buoyant under high pressure, allowing sharks to swim in a wide range of depths.

Oil is less dense than water, which is why the oil rises to the top when you make salad dressing. Some shark livers contain 30 gallons of oil, which helps keep them from sinking.

transparent graphic comparing swim speeds of shark and human

POWERFUL TAIL

In many sharks, the top of the tail is longer than the bottom. The skeleton extends into this large upper fin, providing more power for swimming.

How can sharks go so long between meals?

illustration highlighting shark intestines

EXPANDABLE, ACID-FILLED STOMACH

A shark’s stomach acid is so strong it can dissolve shells and bones. The stomach expands to store food until it is slowly digested.

three yellow icons; one fish skeleton, one seashell, and one turtle shell

SLOW, EFFICIENT DIGESTION

Long, spiral intestines slow digestion and increase the surface available to absorb nutrients, making meals last a long time.

scan showing the spiral structure of shark intestines

How do sharks reproduce?

illustration of shark highlighting its claspers

EGGS OR LIVE BIRTH?

Some shark species lay eggs—but some give live birth. What makes shark reproduction unique is that all sharks use internal fertilization.

Illustration showing baby shark in egg case

CLASPERS

Unlike most fishes, sharks fertilize their eggs inside their bodies. The male inserts sperm into the female with organs called claspers.

Do sharks have to swim to breathe?

illustration of shark highlighting its spiracle

vector illustration comparing shark gills and fish gills

SWIM OR DIE?

Many sharks must swim constantly to force water through their gills to breathe. That's because sharks lack gill covers, the hard flaps other fishes use to pump water through their gills. But some sharks can breathe while staying still. They breathe through a hole called a spiracle.

BREATHING WITHOUT SWIMMING

Some sharks and rays can draw water in through a hole called a spiracle and pump it over their gills. This allows them to breathe while motionless.

photo of a shark identifying hole in its head

How are shark teeth unique?

illustration of shark that highlights teeth

REPLACEABLE TEETH

Shark teeth are replaced constantly. A single shark may shed 30,000 teeth during its lifetime.

TEETH FOR ARMOR

The hard, toothlike scales covering shark bodies are called dermal denticles, which means “skin-teeth.” They are so hard that sharkskin was once used as sandpaper.

How do sharks reproduce?

EGGS OR LIVE BIRTH?

Some shark species lay eggs—but some give live birth. What makes shark reproduction unique is that all sharks use internal fertilization.

CLASPERS

Unlike most fishes, sharks fertilize their eggs inside their bodies. The male inserts sperm into the female with organs called claspers.

How do sharks sense their prey?

NOSTRILS

Shark nostrils are used only for smelling, not breathing.

INNER EAR

Unlike humans, sharks can detect the direction of low-frequency sounds to locate prey.

BLOOD ON THE BRAIN

Shark brains have an extra-large area devoted to scent, helping them smell a drop of blood diluted more than a million times.

SIXTH SENSE

Sensors in the snout called ampullae of Lorenzini help sharks detect electromagnetic fields that reveal prey. Some sharks can even sense Earth’s magnetic field.

How can sharks go so long between meals?

EXPANDABLE, ACID-FILLED STOMACH

A shark’s stomach acid is so strong it can dissolve shells and bones. The stomach expands to store food until it is slowly digested.

SLOW, EFFICIENT DIGESTION

Long, spiral intestines slow digestion and increase the surface available to absorb nutrients, making meals last a long time.

Why are sharks such good swimmers?

FIXED FINS PROVIDE LIFT

Shark pectoral fins don’t flap—they are stiff, like the wings of airplanes or hydrofoils. When sharks swim, these rigid fins provide lift, while the tail provides thrust.

OIL-FILLED LIVERS

Unlike most fishes, which use an air bladder to keep from sinking, sharks have a huge, oil-filled liver. Unlike air, oil remains buoyant under high pressure, allowing sharks to swim in a wide range of depths.

Oil is less dense than water, which is why the oil rises to the top when you make salad dressing. Some shark livers contain 30 gallons of oil, which helps keep them from sinking.

POWERFUL TAIL

In many sharks, the top of the tail is longer than the bottom. The skeleton extends into this large upper fin, providing more power for swimming.

Do sharks have to swim to breathe?

SWIM OR DIE?

Many sharks must swim constantly to force water through their gills to breathe. That's because sharks lack gill covers, the hard flaps other fishes use to pump water through their gills. But some sharks can breathe while staying still. They breathe through a hole called a spiracle.

BREATHING WITHOUT SWIMMING

Some sharks and rays can draw water in through a hole called a spiracle and pump it over their gills. This allows them to breathe while motionless.

How are shark teeth unique?

REPLACEABLE TEETH

Shark teeth are replaced constantly. A single shark may shed 30,000 teeth during its lifetime.

TEETH FOR ARMOR

The hard, toothlike scales covering shark bodies are called dermal denticles, which means “skin-teeth.” They are so hard that sharkskin was once used as sandpaper.

What Makes Sharks Swim Fast?

Sharks have evolved to swim faster and faster, to keep up with both predators and prey. Watch this video to learn about some of the adaptations that make them so speedy.

SHOW TRANSCRIPT

WHAT MAKES SHARKS SWIM FAST?

[drawing of a shark skeleton and silhouette, then silhouette fades away to show skeleton only]

Voiceover: Shark skeletons are made of flexible cartilage

[magnifying glass appears over skeleton, showing close-up of hexagonal tiles fit close together]

Voiceover: with a hard coating of bone-like tiles that provide extra strength.

[Scalloped hammerhead sharks swim through blue water over a reef. Their heads stretch out squarely from their bodies, with their eyes at the front corners of their heads.]

Voiceover: In addition, vertebrae in the shark’s spinal column

[photograph of mako shark skeleton, magnifying glass appears over spine of shark, with a label that reads ‘vertebrae’]

Voiceover: are made of something called fibrocartilage.

[label changes to read ‘vertebrae made of fibrocartilage’]

Voiceover: And in between the vertebrae are soft disks.

[Silky sharks bend and twist their bodies to attack a ball of sardines.]

Voiceover: So the spine can bend and flex easily.

[A spotted catshark swims out of long, waving fronds of kelp to look at camera, then scoots back under the kelp.]

Voiceover: Together these traits make shark skeletons springy and give them powerful forward thrust.

[Sevengill sharks swim through kelp forest. A soupfin sharks swims through shallow water with kelp.]

Voiceover: Sharks' torpedo-shaped body plan helps them slip through water.

[A basking shark swims through turquoise water with its mouth gaping open. It has no teeth, and its gills look like wide vertical stripes in its mouth.]

Voiceover: Most sharks stay the same temperature as the water around them.

[A pelagic thresher shark with a long tail swims through water. Then it flicks its tail to stun small fish.]

Voiceover: But thresher sharks, makos,

[mako shark swims toward camera, opening mouth, while birds dive into the water around it]

Voiceover: and a few other species

[A great white shark swims toward camera, then opens its wide mouth and bares its teeth, as it breaks through the surface of the water]

Voiceover: exchange heat from their organs with other parts of their bodies.

[A great white comes from deep water and tries to grab a swimming bird, which escapes. The great white swims back down into the deep water.]

Voiceover: This gives them bursts of movement.

Where Do Sharks Live?

You can find sharks in all different parts of the ocean. Some, like bull sharks, live in different places at different stages of their lives. Some sharks always keep to the same region. But others move around.

Sharks move because of the time of year, or even the time of day. They might follow food or seek meeting places to reproduce—like blue sharks, which cross the Atlantic Ocean several times during their lives.

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