19 Apr 2016

Is a Shark a Fish?

is a shark a fish or animal?

Are Sharks Mammals ?


Sharks are big and dangerous. That makes people wonder is a shark fish or anything else. Some people believe that they are mammals like whales. But it is not true.


A shark is actually a type of fish. They are not mammals like a dolphin or reptile like a crocodile. They are closure to a Tuna or Mackerel. 

Sharks are different from the marine mammals like whales and dolphins. They are fish because they use gills to take oxygen from the water, have fins with no digits, and can lay eggs. There blood is also not as warm as whales or seals. Most importantly, sharks have no mammary gland to produce milks for their cubs.

A shark is a fish but it is not like most other common or bonny fishes. They have many differences. These are discussed below.

Difference between Sharks and Bonny Fishes


  • We know that many sharks are very large. On average a shark is larger than a common bonny fish. In fact, the largest fish is the Whale Shark. It can be as big as 40 ft with a body weight of 21 ton. The biggest bonny fish is Ocean Sunfish or Mola mola. It cannot be much bigger than 8 ft.
  • Sharks lives in marine environments but bonny fishes are abundant in both oceans and rivers. Some sharks like the Bull Sharks can also live in freshwaters but it is not their natural habitat.
  • They belong to the class “Chondrichthyes” which means “Cartilage fish”.  A shark has no bone like a normal fish. Their skeleton is made of cartilage – a substance much lighter and flexible than bone. The sharks do not have any swim bladders like bonny fishes. The light weight of the cartilage skeleton helps them to be afloat in the water.

Sharks are not mammals

  • We have learned that sharks have no air bladders to increase their buoyancy. But their liver helps them in that. The liver is filled with special oil which is lighter than water.
  • Cartilage is flexible which helps the sharks to turn very quickly and bend their body easily. That is helpful for a predator to catch preys. Sharks need strong teeth. So, there teeth is calcified cartilages which is stringer yet lightweight.
  • The skull of bonny fish is a complex structure made of 63 bones. But the skull of a shark is relatively simple as it has only 10 cartilages.
  • There is no rib cage in a shark. The bonny fishes all have rib cages.
  • The upper jaw of a bonny fish is fixed with the skull. But the sharks have a movable upper jaw. It helps them to bite with more force.
  • What feature of a shark looks odd to you? Yes, the mouth. It is not in front of the head like bonny fishes. It is situated at the ventral or belly side.
  • Sharks are more sensitive than most other fishes. They have a special organ named ampullae of Lorenzin. That organ can detect the slight electrical fields produced by movements of other animals in the water. It can also detect the heartbeats of another animal from a certain distance. No common fish has such a fancy tool.
  • Fish is a cold blooded animal. They cannot warm the blood inside their body internally. But sharks can regulate their internal body temperature to some extent. It is not accurate as humans but much better than bony fishes. The muscles of their body keep the blood warm.
  • We know that fishes lay eggs. Some sharks also lay eggs in the ocean. These eggs remain protected inside a shell. But many species of sharks (like the Great White Sharks, Whale Shark or Dogfish) are viviparous or ovoviviparous. That means the egg hatches inside the mother’s womb and the shark gives live birth after a certain period. Is not it a different characteristic than common fishes?
  • Most bony fishes are covered with scales. These scales grow with them. So, one can estimate a fish’s age by just examining its scales. But sharks have no such scales. Their skin is covered with dermal denticles. These denticles are small but hard scale like things which makes the skin of a shark very rough and tough.
  • The kidneys and genitals of a shark open in a single opening. Unlike bonny fishes, they have no separate rectum and anus.
  • Another difference between a common fish and shark is urea in the blood. The blood of a shark has very high level of urea concentration. Sharks do these to keep the salt and water balance controlled. The process of Osmoregulation helps aquatic creatures to keep the balance between water inside their body and outside. By using urea some sharks like the Bull Sharks can live in both salt water and freshwater comfortably.
  • Bonny fishes grow very quickly. They attain maturity is within a few months or even days. They produce thousands of eggs at a time. From these eggs numerous offspring are born.  But Sharks grows slowly like most advanced mammals. They need many years to be mature enough to reproduce. They also do not produce too many offspring.
  • The fins of a bonny fish are more advanced than a shark. The fish fins are lined with spines. There are also many muscles to control its movement. So, a fish can go both forward and backward. But the shark fins are spineless. There are also not many controlling muscles. So, sharks can only go forward.
  • The pectoral fin of a shark is bigger than bonny fishes. It is more developed for giving them speed and agility.
  • The shark blood is more efficient in carrying oxygen than most other fishes. Their blood has a higher level of hemoglobin to do so. It makes them more agile.
  • The gills of a bonny fish are covered by bony plates. But the gills of a shark have no hard cover. These remains exposed to water.
  • Fish has a weakness. Their eyes are not protected. But sharks have eyelids or special eye protection layers. Some sharks can roll their eye back into the socket for protection.
  • Sharks and common bonny fishes belong to two distinct classes. Sharks belong to the class Chondrichthyes and bonny fishes belong to the class Osteichthyes. These branches are separated from each other some 4 billion years ago. Sharks are related to rays and chimeras.

From these points it can be concluded that sharks are a special category of fish. They can have some specialties different from common fishes. But characteristically and biologically they are closer to common fishes than mammal or other animal groups.

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