10 Facts About Sharks

Sharks Are a Fascinating, Often Feared, Cartilaginous Fish

There are several hundred species of sharks, ranging in size from less than ten inches to over 50 feet. These amazing animals have a fierce reputation and fascinating biology. Here we'll explore 10 characteristics that define sharks.

Sharks Are Cartilaginous Fish

Caribbean Reef Sharks (Carcharhinus perezi)
Stephen Frink/Iconica/Getty Images

cartilaginous fish has a body structure formed of cartilage, instead of bone. Unlike the fins of bony fishes, the fins of cartilaginous fish cannot change shape or fold alongside their body. Even though sharks don't have a bony skeleton like many other fish, they are still categorized with other vertebrates in the Phylum Chordata, Subphylum Vertebrata, and Class Elasmobranchii. This class is made up of about 1,000 species of sharks, skates, and rays.

There Are Over 400 Species of Shark

Whale Shark
Tom Meyer/Getty Images

Sharks come in a wide variety of shapes, sizes and even colors. The largest shark and the largest fish in the world is the whale shark (Rhincodon typus), which is believed to reach a maximum length of 59 feet. The smallest shark is thought to be the dwarf lanternshark (Etmopterus perryi) which is about 6 to 8 inches long.

Sharks Have Rows of Teeth

Closeup of jaw of Bull Shark, Carcharhinus leucas, showing development of rows of teeth.
Jonathan Bird/Photolibrary/Getty Images

Sharks' teeth don’t have roots, so they usually fall out after about a week. However, sharks have replacements arranged in rows and a new one can move in within one day to take the old one’s place. Sharks have five to 15 rows of teeth in each jaw, with most having five rows.

Sharks Do Not Have Scales

Gills of Whitetip Reef Shark (Triaenodon obesus), Cocos Island, Costa Rica - Pacific Ocean
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A shark has tough skin that is covered by dermal denticles, which are small plates covered with enamel, similar to that found on our teeth.

Sharks Can Detect Movement In The Water

Great White Shark Feeding
David Jenkins/Robert Harding World Imagery/Getty Images

Sharks have a lateral line system along their sides which detects water movements. This helps the shark find prey and navigate around other objects at night or when water visibility is poor.

The lateral line system is made up of a network of fluid-filled canals beneath the shark’s skin. Pressure waves in the ocean water around the shark vibrate this liquid. This, in turn, is transmitted to jelly in the system, which transmits to the shark’s nerve endings and the message is relayed to the brain.

Sharks Sleep Differently Than We Do

Zebra shark (leopard shark), Thailand
Fleetham Dave/Perspectives/Getty Images

Sharks need to keep water moving over their gills to receive necessary oxygen. Not all sharks need to move constantly, though. Some sharks have spiracles, a small opening behind their eyes, that force water across the shark’s gills so the shark can be still when it rests.

Other sharks do need to swim constantly to keep water moving over their gills and their bodies, and have active and restful periods rather than undergoing deep sleep like we do. They seem to be “sleep swimming,” with parts of their brain appearing less active while they remain swimming.

Some Sharks Lay Eggs, Others Don't

Shark Egg Case

Sander van der Wel/Flickr

Some shark species are oviparous, meaning they lay eggs. Others are viviparous and give birth to live young. Within these live-bearing species, some have a placenta just like human babies do, and others do not. In those cases, the shark embryos get their nutrition from a yolk sac or unfertilized egg capsules filled with yolk.

In the sand tiger shark, things are pretty competitive. The two largest embryos consume the other embryos of the litter. 

Sharks Live a Long Time

Whale Shark and Divers, Wolfe Island, Galapagos Islands, Ecuador
Michele Westmorland / Getty Images

While nobody seems to know the true answer, it is estimated that the whale shark, the largest shark species, can live up to 150 years, and many of the smaller sharks can live between 20 and 30 years.

Sharks Are Not Vicious Man-Eaters

Bad publicity around a few shark species has doomed sharks in general to the misconception that they are vicious man-eaters. In fact, only 10 out of all the shark species are considered dangerous to humans. All sharks should be treated with respect, though, as they are predators, often with sharp teeth that could inflict wounds (especially if the shark is provoked or feels threatened).

Humans Are a Threat to Sharks

NOAA officer sorting confiscated shark fins.

Humans are a greater threat to sharks than sharks are to us. Many shark species are threatened by fishing or bycatch, which lead to the deaths of millions of sharks each year. Compare that to shark attack statistics - while a shark attack is a horrifying thing, there are about 10 fatalities worldwide each year due to sharks.

Since they are long-lived species and only have a few young at once, sharks are vulnerable to overfishing. One threat is the wasteful practice of shark-finning, a cruel practice in which the shark's fins are cut off while the rest of the shark is thrown back in the sea.