Are Fish Mammals? Exploring the Diverse Classes of Fish

Are fish mammals? Fish are not mammals, but they are such a diverse group that putting all of them into one class is inefficient. Some species were once classified alongside animals that are not fish at all. However, whatever biologists know about fish, they know that they’re not mammals. Here’s why.

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 its 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. Historically, some aquatic mammals were grouped with fish due to their similar habitats, but they have since been correctly classified as mammals. 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.

Many species of fish also care for their young with a 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, and 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.

What Kinds of Fish Are There?

Scientists have divided these animals into three classes. There are actually more than three, but the others 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.