What Is a Shark?

Characteristics of Sharks

Great White Shark
Jeff Rotman/Oxford Scientific/Getty Images

What is a shark? A shark is a fish -- more specifically, they are cartilaginous fish.  These types of fish have a skeleton made of cartilage, rather than bone.

Sharks, along with skates and rays, are classified in the class Elasmobranchii, which comes from the Greek word elasmos (metal plate) and Latin word branchus (gill). Although their skeletons are made of cartilage, elasmobranchs (and therefore, sharks) are considered to be vertebrates in the phylum Chordata -- the same phylum in which humans are classified.

What Is a Shark? Anatomy 101

Sharks have some key features that can be used to identify species. Starting at the front of their body, sharks have a snout, which has a wide variation in size and shape and may be a way to identify species (think of the difference in the snouts of a white shark and a hammerhead shark, as an example).

On their top (dorsal) side, sharks have a dorsal fin (which may have a spine in front of it) and a second dorsal fin located nearer their tail. Their tail has two lobes, an upper and a lower, and there may be a dramatic difference in size between the upper lobe and lower lobe (thresher sharks have a long, whip-like upper lobe).

Sharks use gills to breathe and their gills are open to the ocean, with five to seven-gill slits on each side. This is unlike the gills in bony fish, which have a bony covering. Behind their gills, they have a pectoral fin on each side. On their ventral (bottom) side, they have a pelvic fin and may have an anal fin closer to their tail.

A shark's body is covered with tough placoid scales, and gender may be distinguished by the presence or absence of claspers near the pelvic fin. Males have claspers that are used in mating, while females don't.

How Many Species of Sharks Exist?

There are over 400 species of sharks, and they have a wide range in size, coloration, and behavior. The largest shark is the massive, relatively passive 60-foot long whale shark and the smallest is the dwarf lanternshark (Etmopterus perryi) which is about 6 to 8 inches long.

Where Do Sharks Live?

Sharks can be found all over the world, in both cold and warm waters. Some, like the blue shark, spend most of their time roaming the open ocean, while others, like the bull shark, inhabit warmer, murky coastal waters.

What Do Sharks Eat?

With a variety of species and sizes, sharks eat a variety of prey. Huge whale sharks eat tiny plankton, while the relatively smaller white shark eats toothed whales, pinnipeds and sea turtles.

Do All Sharks Attack Humans?

Not all sharks attack humans and the , relative to other dangers, is relatively slim. But some species do attack or interact with, humans more than others. The maintains a list of attacking species of shark, along with whether the attacks were provoked or unprovoked, fatal or non-fatal.

What Are the Conservation Issues Facing Sharks?

While shark attacks are a scary prospect, sharks have much more to fear from humans than we do of them in the grand scheme of things. Some estimate that up to 73 million sharks are killed each year just for their fins. Other threats to sharks include intentional harvesting for sport or for their meat or skin, and being caught as bycatch in fishing gear.

Why Should We Care About Sharks?

Sharks are important apex predators in the ocean, which means they play a pivotal role in keeping ecosystems in check. For example, if there was a decrease in white sharks in some areas, seal populations might flourish which could cause a reduction in their prey, which would decrease fish populations.Learn more about why we should protect sharks.