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Sharks

Updated: Jul 27, 2020

Long before dinosaurs roamed the earth, sharks swam in the seas. Sharks have been around for more than 400 million years!


The body of sharks


Cartilage:

Sharks do not have bones in their bodies like humans, and like other bony fish. They are cartilaginous- their bodies are made of cartilage. Feel your ears or nose- that is cartilage. Cartilage allows sharks to be lightweight and flexible, moving quickly through the water without much energy. Being lightweight also helps sharks stay buoyant, or float. Sharks also have a streamlined shape, like a torpedo, which also helps them glide along easily.


Gill Slits:

Sharks have 5-7 gill slits on each side. Most species that live underwater do not need to surface to breathe because they can pull oxygen out of the water. Water contains dissolved oxygen, and to use it, and sharks pull water into their mouths, through their gills, and out their gill slits. During this process, the gills pull oxygen out of the water, and put it into the shark's bloodstream.


7 senses:

Humans have 5 senses: Touch, Smell, Sight, Hearing, and Taste. Sharks have these senses too. The sense of smell is their primary sense and 2/3 of a shark's brain is dedicated to it. Since sharks don't have hands, they use their teeth to learn about objects. Their teeth have many nerves which help them feel objects as they do a test bite. Sharks have an additional two senses that we don't have.

  1. Electroreception. This is the ability to detect electrical currents in salt water. The muscle movements that animals make as they move their bodies create an electrical current. Sharks have tiny dots around their snouts and lower jaws, called ampullae de Lorenzini, which help sharks sense the electrical currents. Once sharks are within 3 feet of their prey, they use electroreception to determine the precise location of the prey, and ensure their jaws are adjusted for an accurate attack.

  2. Water pressure. The lateral line is a sensory organ that runs from the gills to the tail. It opens out into the skin, seen as little holes in the scales making a visual line along the sharks body. The lateral line helps sharks feel the direction and intensity of water movement, which can be caused by fish moving near them.





Size:

Sharks come in a variety of sizes. While the average shark is about the same size as a human, they can range from 6 inches long to 33 feet long!

  • The smallest sharks are about 6 inches long. Dwarf lantern sharks, pale catsharks, cylindrical lantern sharks, and smalleye pigmy sharks all reach a maximum of about 8 inches in length.

  • Whale sharks are the largest sharks. They typically grow to be about 33 feet long, which is about the size of a school bus! They can grow larger though, and the largest one on record was 62 feet long!




Teeth and Diet:

Sharks have a powerful biting force, and lack a hard jaw bone to keep their teeth anchored in place, which means they lose teeth quite often while feeding. To help with this, they have rows of back up teeth that move in to replace the lost teeth. Sharks go though more than 30,000 teeth in their lifetime! There are four basic types of teeth. The type of teeth indicates the kind of food these sharks typically eat.






The most common type of teeth are triangular shaped teeth. They help sharks catch prey and tear off smaller portions which they then eat whole. Sharks with triangular shaped teeth can catch large prey like sea lions, as well as large fish (sometimes including other sharks). This is the kind of tooth Great White sharks have.










There are also needle-like teeth used to feed on small to medium sized fish. They help to give the shark a good grip on the slippery bodies of the fish, which the sharks tend to swallow whole.







Sharks that are bottom feeders have dense flattened teeth that help them crush the crustaceans and bivalves (like crabs and clams) that they pick up on the ocean floor.








Filter feeders such as whale sharks have rows of tiny teeth that are non-functional. Whale sharks actually have 3,000 teeth in their mouth even though they never use them! Though these fish are very large, their prey is very small. They eat plankton, krill, and fish eggs. Whale sharks swim with their mouths wide open to suck in the plankton in the water. They pass the water through their gills, and swallow any small prey that gets filtered by their gills.








Importance in the ecosystem

Most sharks are apex predators, which means they are the top of the food chain. They remove the sick and weak which keeps prey populations healthy. Sharks eat large fish which prevents them from becoming too abundant. If large fish became too abundant they would overeat small herbivorous fish. This would lead to an overgrowth of algae with no herbivores to eat them. Therefore, sharks keep the whole ecosystem in balance. Prey also change where they spend time and feeding habits in order to stay safe from sharks. This indirectly keeps seagrass and coral reefs healthy, and increases species diversity.








Why sharks are vilified


Many predators are portrayed by the media as evil. Shocking and violent stories sell newspapers, and get people to watch the news. While the reporters may choose photos and words for their stories that make sharks sound scary, remember to look for the truth. The truth is that sharks are wild animals, and they are predators. Predators kill their prey in order to survive, but humans are not one of their prey items. VERY occasionally, sharks make a mistake.


When you think about sharks, you probably think about the movie Jaws. The writer for Jaws actually regretted making the shark into a monster after the movie came out, because it created an unrealistic fear of sharks.


Newspapers that report shark attacks sometimes make them sound dramatic. Think about this headline: "Shark horror: 12 foot monster and diver face off in near death battle under water"

Calling sharks 'monsters' or 'killing machines' gives people a negative opinion of them.


 

* The reality is that sharks kill about 5 people a year across the globe.

* Humans kill 100 million sharks a year.


Who should be scared of who?


 

There are only about 4 shark species out of the 440 species that are responsible for attacks on humans. However, people have been able to swim with these 4 shark species without them showing any aggression. But the media highlights every attack which makes it seem like shark attacks are a big problem. No one talks about their peaceful shark experiences. Tourists travel around the world to swim with whale sharks. Beach-goers in San Diego snorkel with a group of leopard sharks that congregate off La Jolla in the summer. Many locations around the world are tourist draws because they offer the chance to swim with various shark species, like hammerheads, thresher sharks, reef sharks, and cow sharks. Swimming with sharks is a fun, exciting, and once-in-a-lifetime opportunity for many people.



Threats to sharks

  • Shark fin soup is the primary threat to shark survival. Shark fin soup is considered a delicacy and seen as a status symbol in Asia. Fishermen remove the sharks fins and waste the rest of their body to make the soup.

  • Sharks are caught as by-catch in fishermen's nets. They have also lost prey due to overfishing of many fish species.

  • 25% of shark species are endangered, threatened, or near threatened. With the high levels of human predation, sharks are slow to recover. Sharks generally live long and take many years to mature. Female Great White sharks aren't ready to reproduce until about 33 years old. Greenland sharks don't reproduce until they are 150 years old! Sharks also have long pregnancies and produce few young.


How to help

  • Join a citizen science project to help with research. Online sites like whaleshark.org let you upload photos of your shark sightings to their database.

  • Learn more about sharks, and share with the people around you. Spread awareness about sharks on social media.

  • Go diving or snorkeling with sharks. Shark tourism supports local conservation efforts.

  • Avoid eating shark. Some nutritional and beauty products also use shark oil and cartilage. By avoiding these products, the demand decreases which will stop the killing.

  • Reduce your seafood consumption. This will increase the prey availability for sharks. It will also reduce the incidents of sharks getting caught in fishing nets.

  • Reduce your use of single-use plastics. Sharks can confuse garbage in the ocean for food, and also eat fish that had consumed plastic. This can make sharks sick or lead to their deaths.



Practice what you learned

Download the activity below, print it, and fill it out:


Shark worksheet
.docx
Download DOCX • 1.08MB


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