What Makes a Shark a Shark and Not Just Another Fish?

A tiger shark swims with a school of fish.
A tiger shark swims with a school of fish.

A friend and I were discussing sharks the other day and started wondering what exactly makes a shark different from other fish. We considered maybe it is their place on the food chain. Sharks are usually considered apex predators while fish are not. Apex predators are generally animals near the top of the food chain with few if any predators. But it turns out some sharks eat other sharks and some fish eat other fish and the choice of cuisine does not determine shark from fish.

We considered everything from body type to swimming speed and from where they live to political affiliation, but in the end could reach no solid conclusion, so I decided I just had to sit down and do some research. What I learned was that while in some ways we were off, we were actually fairly close in other ways. But the number one difference between the two, I would have never guessed...

1. Bones Vs. Cartilage

The most prominent difference between sharks and other fish is their skeleton. While most fish have bony skeletons, sharks have cartilaginous skeletons made up of cartilage or gristle and no bones. Sharks are not the only fish with this sort of skeletons. Rays and skates also have this sort of skeleton.

Having a skeleton made of gristle instead of bone helps make sharks flexible and relatively light. Sharks have lots of muscle in addition to their lighter skeleton and that gives them a lot of strength and speed. These factors aid them in hunting prey and keeping their spot at the top of the food chain. Sharks have been referred to as "nature's killing machines" and that is true in a way, but they usually only kill for food or to protect their territory.

Some consider shark oil beneficial to health.
Some consider shark oil beneficial to health.

2. Bladders and Livers

Fish generally have something called a swim bladder that is filled with air and makes them buoyant so that they float in the water even if they stop swimming. Sharks do not have these. Instead, sharks have specially developed livers that are less dense than most animals and contain a large amount of oil. This makes it easier for them swim and makes them faster than other fish. If a shark stops swimming, it will slowly sink to the bottom of the ocean.

3. Body Shape

While from the side many sharks appear similar to other fish, if you look at them head on, some differences will become apparent. Most fish have oval-shaped heads when viewed from the front while sharks' heads have a more triangular appearance. Additionally, the difference in the pectoral fins is more obvious from the front with the shark's being well developed to help with swimming while fish tend to have smaller, less functional pectoral fins.

Shark skin up close. Very close!
Shark skin up close. Very close!

4. Texture of the Skin

Neither sharks nor other fish have skin as we think of it. Most people are familiar with the scales that most fish are covered in. Sharks, however, are different and are covered by dermal denticles which are composed of material similar to teeth. The unique composition of these reduce drag and make it easier for sharks to swim quickly and quietly. Again we see that the shark is indeed designed to be a fierce hunter. Nature made shark skin so efficient, in fact, that man has based recent high-tech wetsuits designed for Olympic racing on it.

5. Location of the Gills and Gill Slits

Most fish have gills that they pump water through to take in oxygen and dispose of carbon dioxide. They do this whether in motion or sitting still as they have everything they need to pump water through their system. Sharks are different in that they do not have the ability to pump water on their own and must rely on a process known as ram ventilation to take in and exhale water. Basically this means the forward motion of the shark sends water into the mouth and forces it out through the gill slits.

Here you can clearly see the shark's gill slits.
Here you can clearly see the shark's gill slits.

The location of the gills and gill slits are different as well. A sharks gill slits are usually located behind their head while other fish have their gills behind the operculum, a bony flap that protects the gills. Some sharks who live on the ocean floor and spend a great deal of time with their mouths buried in the sand take in water through an opening on the top of their head called a spiracle.

6. Reproductive Process

The final difference I was able to discover between sharks and other fish is the way in which they reproduce. Fish generally release unfertilized eggs into the water where they are fertilized. Sharks, on the other hand (or should that be "fin"?), fertilize the eggs internally. Some shark species carry the eggs until they are born alive while other species lay the fertilized eggs that later hatch. No matter how the babies are born, they are on their own from birth with neither the mother nor the father caring for the young.

Oddly, I could not find this shark listed anywhere. LOL!
Oddly, I could not find this shark listed anywhere. LOL!

Interesting Facts I Learned While Writing This Hub...

  • The sailfish is the fastest fish in the ocean, capable of reaching speed of 112 kilometers per hour or 70 miles per hour. That's as fast as a cheetah, the fastest mammal on land!
  • Baby sharks are called pups and, as mentioned in the hub, get no help from mom or dad after being born.
  • Sharks have an incredibly well-developed sense of smell and use more than 60 percent of their brain power to process the information they get from odors.
  • Since before dinosaurs roamed the earth, sharks have been swimming in the seas. Sharks are estimated to have been around for more than 400 million years.
  • Sharks have many rows of teeth but only the front teeth are functional. A single shark may lose thousands or even tens of thousands of teeth over its lifetime but already has more teeth ready to replace those from its back-up rows of teeth.

Sharks and Rays

A manta ray flies along. I mean, SWIMS along!
A manta ray flies along. I mean, SWIMS along!

Having learned what differentiated sharks from most other fish, I became curious as to why sharks and rays are considered to be different. There are a few differences, as follows:

  1. A ray has gill slits as do sharks, however, their slits are located under the head rather than on the sides like a shark.
  2. Sharks use their pectoral fins to maneuver but rays must actually use them to swim, almost like wings.
  3. Rays have either no tail fins or greatly reduced tail fins compared to sharks. Rays use their tails for steering while sharks use their tails to swim.
  4. The biggest difference is probably their body types with rays being much flatter than sharks, though angelsharks do have body shapes similar to rays.

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Comments 2 comments

Sneha Sunny profile image

Sneha Sunny 5 years ago from India

wow.... this was a great hub. I really enjoyed reading this. Useful, awesome and voted up!


blah 14 months ago

interesting

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