Common Name:

Sharks

Type:

Fish

Size:

7 inches to 32.8 feet

Weight:

<1 ounce to 20.6 tons

Sharks star in blockbuster movies as blood-seeking villains, but in reality they’re far more fascinating and complicated than they’re often depicted in pop culture.

Based on fossilized teeth and scales, scientists believe that sharks have been around for more than 400 million years—long before the dinosaurs. The ocean’s top predators have evolved into roughly 500 species that come in all different sizes and colors and have varying diets and behavior.

Anatomy

Like rays and skates, sharks fall into a subclass of fish called elasmobranchii. Species in this subclass have skeletons made from cartilage, not bone, and have five to seven gill slits on each side of their heads (most other fish have only one gill slit on each side), which they use to filter oxygen from the water.

Whale sharks, the largest fish species on Earth, can grow to more than 55 feet, while dwarf lantern sharks reach a mere eight inches.

Formidable predators, sharks have mouths lined with multiple rows of individual teeth that fall out and grow back on a routine basis. Their teeth come in all sizes and shapes, from serrated like a razor to triangular like a spear.

Behavior

Sharks are found in deep and shallow waters throughout the world’s oceans, with some migrating vast distances to breed and feed. Some species are solitary, while others hang out in groups to varying degrees. Lemon sharks, for example, have been found to congregate in groups to socialize.

Scientists are still trying to figure out how long sharks live and have only studied the ages of a fraction of shark species. Most notable is the Greenland shark, Earth’s longest-lived vertebrate at 272 years.

Most sharks eat smaller fish and invertebrates, but some of the larger species prey on seals, sea lions, and other marine mammals.

Attacks

People aren’t on a shark’s menu. Even though shark attacks have increased at a steady rate since 1900—a result of better recording of attacks and a rising human population—they are still exceedingly rare: A beachgoer has only a one in 11.5 million chance of being bitten.

Sharks bite people out of curiosity, to defend themselves from a perceived threat, or because they confuse a human with prey.

Threats

Sharks may not be a significant threat to us, but we are to them. Humans are responsible for drastic declines in shark populations.

Overfishing is the biggest threat. An estimated 100 million sharks are killed each year, mostly to supply demand for an expensive Chinese dish called shark fin soup. Some fisheries allow the catch of whole sharks, like any other fish, while others have outright banned shark fishing. Sometimes fishermen cut the fins off live sharks and dump the animals, finless, back into the ocean, where they’ll drown or bleed out. This practice is called shark finning, and it’s done to save space on the boat (the fins are the most valuable part of a shark) and to avoid surpassing fishing quotas.

Rising water temperatures and coastal development are also contributing to shrinking populations by destroying the mangroves and coral reefs that sharks use for breeding, hunting, and protecting young shark pups.

A drop in numbers is bad news for sharks but also for ocean health in general: As top predators of the ocean, sharks are critical for ensuring a balanced food web.

Watch more shark videos from National Geographic here.