Sharks are interesting combinations of fierce and dignified. As such, they can fascinate and terrify people, even sea animal lovers.
These aquatic predators have been around for millions of years, and with so many species (over 400 of them), they have come to stay.
Controversies about their classification pop up occasionally because of their size and the way they give birth.
Some people consider the shark a mammal rather than a fish, and the same debate surrounds the whale. But are sharks mammals or fish?
Not all underwater animals are fish, but the shark should be considered a fish and not a mammal for many reasons.
There are several criteria an animal has to meet to be classified as a mammal, that the shark doesn’t. The common characteristics of sharks classify them as fish, not mammals.
Let’s look into the facts.
What is a Mammal?
Mammals can be found everywhere and come in many sizes.
Mammals are highly adaptable, and they share four common traits: “hair, mammary glands, a hinged jaw, and three tiny middle ear bones.”
As animals, mammals fall under the class Mammalia and are from different families.
The word itself comes from the Latin word mamma, which signifies breast, and as NatGeo pointed out, mammary glands are one defining characteristic of mammals.
They are also all vertebrates but are different from other vertebrates, like reptiles.
Below are some other characteristics of mammals:
- They are viviparous, which means they give birth to live young ones and not in eggs.
- The mammary glands produce milk.
- They breathe through the lungs.
- They are warm-blooded.
Examples of mammals are monkeys, rats, cats, and dogs. Biology-wise, humans are also classified as mammals.
What is a Fish?
The first common characteristic of fish is their aquatic habitat. Fish can’t survive for very long on land because of different factors.
Fish belong to different classes, and they have distinctive qualities.
Some have scales, others have slimy bodies, and some fish even have lungs and can survive outside water longer than others.
The diversity of fish is what makes their classification so complex. As we’ll soon see, some “fish” are mammals.
Fish are so diverse because of how widespread water is, covering up 70% of the planet.
Fish have also lived for a very long, dating back to an impressive 500 million years. There are more fish in the world than other vertebrates.1
Are Sharks Mammals or Fish?
To determine whether a shark is a mammal or a fish, let’s look into the defining characteristics of a shark:
- Sharks belong to the Chondrichthyes class.
- They are cold-blooded, which means they can’t regulate inner body temperature.
- They are both oviparous and viviparous, depending on the species. Some lay eggs and others give birth live.
- They breathe with gills.
- They are large in size and highly carnivorous.
Going by these characteristics, you can’t call a shark a mammal.
Not only is it not classified as a mammal, but it also has qualities that disqualify it from being a mammal. It breathes with gills, is cold-blooded, and doesn’t have hair.
Why are Sharks Considered Mammals?
If there are obvious differences between a shark and a mammal, why are sharks considered mammals by some?
Turns out that this is an understandable misconception.
For starters, the viviparous nature of some shark species is similar to mammals. However, not all sharks are viviparous.
Shark species like the cat shark give birth by laying eggs and hatching them in an egg sac.
There is also a third category of sharks under are ovoviviparous. They produce eggs inside the body, then give birth live.
Another subtle reason why sharks are considered mammals is because of a few warm-blooded species like the shortfin mako shark and the great white shark.
However, even these aren’t mammals because they lack other mammal traits.
The third reason sharks are considered mammals is because of marine mammals.
Are Sharks Marine Mammals?
Marine mammals share all the characteristics of terrestrial mammals, but they live underwater. Whales, dolphins, sea otters, sea lions, and seals are good examples.
The shark’s structure is similar to that of a dolphin, and that leads to it being confused for a mammal.
However, sharks differ from marine mammals and should not be classified with them.
Marine mammals are warm-blooded, live close to water surfaces, and have a special type of lung instead of gills. They also produce milk, which sharks don’t do.
There are generally four family groups of marine mammals:
- Pinnipeds (seals, sea lions, walruses)
- Cetaceans (whales, dolphins, porpoises)
- Sirenians (manatees, dugongs)
- Marine fissipeds (polar bears, sea otters).
The pinnipeds live in both water and land; cetaceans and sirenians are always underwater but can easily come up on the surface, while the fissipeds live on land but stay very close to water.
Sharks differ from any of those in that they live in water.
What type of Fish is a Shark?
Knowing that the shark is a fish, what type is it?
There are five classes of fish in which all species can be classified.2 They include:
1. Hagfish (jawless fish)
Hagfish are distinguished by a lack of scales and fins. They also don’t have backbones but are still vertebrates because of their cranium.
In place of backbones, they have notochords. Hagfish produce fluids that make them slippery, a defense mechanism against predators.
2. Lampreys
Lampreys and Hagfish fall under the same class (Agnatha); like hagfish, lampreys have no scales.
However, lampreys differ from hagfish with the presence of fins and a backbone. Still, fins and backbones don’t define lampreys.
They are more defined by the round sucker that makes up the mouth and is fully teethed.
Those suckers are used to feed on the blood of other fish. Lampreys are the vampires of the sea.
3. Ray-finned fish
A large number of fish species fall under this category, some of which are goldfish, tuna, salmon, perch, and cod.
The distinctive quality of ray-finned fish is the webs on their fins. Their skeleton is also bony, and they have a swim bladder.
4. Lobe-finned fish
Lobe-finned fish have often been compared to ray-finned fish because of their similarities, but their fins are different.
The lobe-finned fish has a stronger fin. Lobe-finned fish can also have lung-like organs, making them live out of water for long.
5. Cartilaginous fish
The cartilaginous fish is defined by its jaws, and most of them are predators. Also, their skeletons are made of cartilage instead of bones.
Sharks have large jaws and cartilage, so they are cartilaginous fish.
Summary: Is a Shark a Fish or a Mammal?
It is normal to think a shark is a mammal because of the species that give birth live and the resemblance sharks have with some other marine mammals like dolphins and whales.
However, the shark is a true, cartilaginous fish of the Chondrichthyes class.
It lives underwater, has gills instead of lungs, is cold-blooded (for the most part), doesn’t have hair, and doesn’t produce milk.
Check out these interesting articles:
- Are Sharks Tetrapods?
- Do Sharks Have Emotions, Feelings, Or Feel Affection?
- Are There Sharks In Antarctica (And Arctic Waters)?
References & Notes
- About Fish. National Geographic.
- Fish Classification. CK-12 Foundation.
