Are Fish Mammals?

blue and yellow fish swimming

Written by Heather Ross

Updated: October 21, 2022

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Are fish mammals? Fish are not mammals, but they are such a diverse group that putting all of them in one class is inefficient. They used to be grouped in the class Pisces, but now they are classified into three classes and a multitude of subclasses, clades, orders, suborders, tribes, superfamilies, families, genera, and species.

Is a fish an animal? Actually, a fish is a member of the most diverse group of vertebrates, or animals with backbones, on Earth even though at least one, the ghastly hagfish, doesn’t really have a proper backbone.

They come in forms as different as the mola, the great white shark, the shockingly beautiful mandarin dragonet, the long-horned cowfish, and the tiny guppy. Some belong in clades that contain animals that are not fish at all. However, whatever biologists know about fish, they know that they’re not mammals.

There are over 30,000 species of fish.


Why Are Fish Not Mammals?

Fish are not mammals because most of them are not warm-blooded, though some sharks and species of tuna are exceptions. They do not have limbs, fingers, toes, fur, or hair.

Most of them can’t breathe air because they don’t have lungs, though the lungfish and the snakehead are also exceptions. The great majority have gills that let them extract oxygen from water. They can only survive in the water.

They lay eggs or give live birth, but no fish nurses it’s young with milk, an action that separates mammals from every other type of animal. Even pigeons who feed their young with crop milk or tsetse flies who feed their young with something like milk in utero don’t count as mammals.

Why Do People Think Fish Are Mammals?

People may think of fish as mammals because not very long ago scientists believed that many mammals were fish. These mammals spent most or all of their lives in water and included whales, seals, sea lions, and even the hippopotamus. These animals are mammals, but they are not fish. Even whales, dolphins, and porpoises (animals that look very much like fish) are warm-blooded and feed their babies with milk. Even though they can hold their breath for very long periods of time underwater, they still need to breathe air.

Many species of fish also care for their young with the devotion usually associated with mammals. Male jawfish, bettas and arowana incubate eggs in their mouth, which is called mouthbrooding. They, of course, can’t eat while they hold the eggs. Seahorse fathers famously gestate their babies then give birth. Other fish brood their young in their skin or their gills, and some babies first eat the mucus from their parents’ skin. Some cichlids protect their young by warning them of danger through movements, while others protect their young even after they’ve become sexually mature. They are still not mammals.

Most fish do not have eyelids. Sharks are an exception to this.


What kinds of fish are there?

Scientists have divided these animals into three classes. There are actually more than three, but the other classes are extinct. The three extant classes are:

  • Agnatha: These are jawless fish, which are the lampreys and the hagfish.
  • Chondrichthyes: These are the fish whose skeletons are made of cartilage instead of bone. These are the sharks and the rays. By the way, all mammals have skeletons made mostly of bone.
  • Osteichthyes: These are fish whose skeletons are made of bone. They include those with fleshy fins like coelacanths and lungfish and those with rayed fins, which is just about every other fish that isn’t in the two other classes.

Is a Fish an Animal?

Fish can be lumped together with mammals such as whales, dolphins, and sea lions under the term “sea animals.” But as we’ve already discussed, mammals and fish don’t share the same characteristics. However, they do share this–they are both vertebrates. Fish, as well as mammals, have backbones, or spines.

But is a fish an animal? Any living thing that has a backbone, is capable of movement, and must find and digest food is classified as an animal. So yes, a fish is an animal.

Up Next…

Here, fishy fishy! Enjoy fore fish facts!

  • 10 Incredible Fish Facts These facts might make you think twice about eating a fish. They are quite intelligent and amazing!
  • 10 Incredible Flying Fish Facts As if the fact they can “fly” wasn’t enough, check out more fascinating flying fish facts!
  • Pet Fish Want an aquarium in your home? Read about which fish make the best pets.

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About the Author

Heather Ross is a secondary English teacher and mother of 2 humans, 2 tuxedo cats, and a golden doodle. In between taking the kids to soccer practice and grading papers, she enjoys reading and writing about all the animals!

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