Fish are not mammals, but there are so many different types of fish that it’s ineffective to lump them all into one category.
In the past, they were all grouped together in the class Pisces, but now they’re classified into three classes and a bunch of subclasses, clades, orders, suborders, tribes superfamilies families genera and species.
The debate of whether fish are animals has been around for years, but most people don’t know that a fish is classified as a vertebrate.
Vertebrates are Animals with backbones, and there are over 50,000 different species! That includes the ghastly hagfish, which doesn’t have a backbone.
There are many different types of fish, from bream to the great white shark to the shockingly beautiful clownfish.
Some even belong in clades with animals that are not fish at all. However, no matter what biologists know about fish, they know that they’re not mammals.
Quick Facts
- Fish are not mammals because they lack the three defining characteristics of mammals: being warm-blooded, breathing air, and nursing their young.
- In the past, scientists believed that some mammals were fish due to their appearance and similarities. These creatures included dolphins, whales, and seals.
- Agnatha, chondrichthyes, and osteichthyes are the three classes of fish.
Why Are Fish Not Mammals?
- Fish are not mammals because most of them cannot maintain a constant internal body temperature, though some sharks and species of tuna can. They have no limbs with fingers or toes, nor any fur or hair.
- The exceptions to this rule are the lungfish and the snakehead, who can both breathe air. Most fish have gills that enable them to extract oxygen from water instead of lungs. If they left the water, they would die quickly.
- They lay eggs or give live birth, but no fish nurses its young with milk. This separates mammals from every other type of animal. Even pigeons who feed their young crop milk or tsetse flies that imbibe something like milk in utero don’t count as mammals.
Why Do People Assume Fish Are Mammals?
Scientists used to believe that many mammals were fish because they spend the majority of their lives in water. This led people to confusion, as they would label creatures such as whales, seals, and sea lions as fish. Even the hippopotamus was once believed to be a part of this group.
Even though these animals look similar to fish, they are mammals. This means that they are warm-blooded and feed their young with milk. Even whales, dolphins, and porpoises need to breathe air periodically even though they can hold their breath for long periods of time under water.
Father fish protect and incubate their eggs with the same level of care as mammal mothers. Male jawfish, bettas and arowana carry their offspring in their mouth (a process called mouthbrooding), during which they cannot eat. And similarly to human seahorse fathers, male seahorses give birth to live young after carrying them throughout gestation.
Some fish incubate their eggs by holding them in their mouth or skin, while others live with and care for their young after they’re born.
Some species of fish communicate danger to juveniles using different movements, while others protect them until they are old enough to do so on their own. Cichlidsare a type of fish that protects its offspring in this way. Even though they aren’t mammals, parental protection is important to these animals.
What kinds of fish are there?
Scientists have currently divided these animals into three classes, However, there are actually more than three. The other classes became extinct over time. The three that still exist today are:
- Agnatha: These fish lack jaws, which include the lampreys and hagfish.
- Chondrichthyes are fish whose skeletons are made mostly of cartilage, which distinguishes them from other types of fish. Sharks and rays are both examples of chondrichthyes. All mammals have skeletons made mostly of bone.
- Osteichthyes: These fish have bone-based skeletons. They can be identified by their fleshy fins (like coelacanths and lungfish) or their rayed fins (which is a defining characteristic of nearly every other non-osteichthyan fish).
Is a Fish an Animal?
Although fish and mammals such as whales, dolphins, and sea lions are both considered “sea animals,” they couldn’t be more different.
As we’ve already talked about, mammals and fish don’t have any overlapping traits. Although, there is one similarity between the two groups, they’re both vertebrates. This means that just like mammals, fish also have backbones or spines.
So, a fish is technically an animal. Anything that has a backbone, the ability to move, and needs to forage and eat food is classified as an animal by biologists.
