Are Fish Reptiles Or Mammals?

All living things are classified into different groups, according to scientists. They normally use a systematic method to classify and organize them differently for ease of study. Fish are found in both fresh and marine water bodies among the species of vertebrates in the world. Only one species, the opah, is warm-blooded, while most of them are cold-blooded. Have you ever wondered which class they belong to scientifically? Do they fall under reptiles or mammals? In this article, we take you through the fine details as we unveil where Fish is classified. Please stick with us to the end.

Fish are neither reptiles nor mammals, although they belong to the same kingdom and phylum. They are ectothermic gill-bearing aquatic animals lacking countable limbs.

Well, I am sure you have come across this question as a debate between different people or just a thought that came through your mind when you hear anything about Fish. But before we get into it, let’s first understand the two classes of reptiles and mammals to be in a position to figure it out.

What Are Reptiles?

What Are Reptiles

Reptiles are a class of animals covered in special skin made up of bony plates, scales, or both; these are air-breathing vertebrates. The temperature of the environment determines their metabolism rate since they do not maintain a constant body temperature. They move into the shade or sun as required because they cannot cool off in hot weather without sweat glands or even gasp for air. At the same time, they are also unable to stay warm on a cold day without feathers or fur for insulation. Their heat-seeking behavior and slow metabolism make them cold-blooded as they are as well inactive during cooler seasons of the year. 

Some examples of reptiles are;

1. Lizards

2. Crocodiles

3. Snakes

Similarities Of Fish And Reptiles:

Most Fish and reptiles share several similar traits despite being separated by millions of years of evolutionary history. Numerous exceptions exist as both groups are very speciose. They are similar in the following ways;

1. Although exemptions exist in both groupings, most reptiles and Fish have the scaly outer covering.

2. Both of them are vertebrates possessing a series of bones that enclose and protect the spinal cord. Some groups, such as the rays and sharks, are still firmly nested within the vertebrates’ family, replacing the bone with cartilage over their ovulatory history.

3. They have gills, although, at some point in their lives, the reptiles lose them while still in the embryonic stage, whereas Fish retain them.

4. Both have a central nervous system comprising a brain and a network of nerves that relays information about the external world to the animal’s brain. 

5. Their body temperatures fluctuate with that of the environment as most of them are ectothermic. Many reptiles must bask in the sun to raise their body temperature, whereas Fish match the ambient temperature by altering their behavior. 

6. At lower temperatures, they become lethargic and stop feeding, but in higher temperatures, they exhibit higher activity levels of more feeding and growth. This excludes some fish like mackerel sharks, tuna, and moonfish that can maintain elevated body temperatures adaptively. Insulating them very well, leatherback sea turtles also have a thick layer of fat and a low surface area to volume ratio.

What Are The Differences Between Fish And Reptiles?

Take a look at the table below;

Respire through the gills.Respire through the lungs.
Have fins as their modified limbs.Species like snakes do not have limbs, while others have fore-limbs and hind limbs for movement.
Their scales are bony and skinny.Their skins have no glands and are rough and dry.
Live in mainly freshwaters and rivers.Both land and water are their habitats.
The gills are their mode of excretion.Have kidneys that function well for a well-defined excretory system.
Have a two-chambered heart.Have a three-chambered heart.

What Are Mammals?

What Are Mammals

Constituting the Class Mammalia, mammals are characterized by mammary glands for feeding their young ones. They belong to the group of vertebrates with three middle ear bones, fur or hair, and a region of the brain called the neocortex. With some possessing large brains, the ability to use tools, and self-awareness, most of them are very intelligent. Including the production of ultrasound, echolocation, singing, alarm signals, they can communicate and vocalize in several ways. 

Some examples of mammals are;

1. Dogs

2. Horses

3. Human beings

Similarities Between Fish And Mammals:

For all the obvious differences, it is easy to miss the similarities. But you can get them right if you look beyond the ability to breathe underwater and the scales. They include;

1. Both can feel pain and also learn how to avoid it.

2. Both have a very good memory, and Fish, just like mammals, and can remember events up to 40 years ago in human time. For instance;

– The Australian crimson-spotted rainbow fish could still remember how they           

  escaped from their tanks even after eleven months. 

– the goldfish can also remember things for up to five months.

3. They use the tools: Fish use tools to crack open clam shells just as mammals do. 

What Are The Differences Between Fish And Mammals?

They are also significant because these are two large and distinct groups of animals. 

Let’s have a look;

Their bodies are covered with scales.Hair or fur is the covering on their bodies.
Breathe through the gills.Breathe through the lungs.
Live entirely in the water.The habitat is mostly on land apart from whales and dolphins that are modified for a marine locale.
They have laterally flattened bodies and are cold-blooded.Their bodies are not flattened and are warm-blooded 
Locomotion around their habitat is through the fins.Have legs to enhance their ability to move on land.
The young ones are usually hatched from eggs.They reproduce and have live births.

Are Whales And Dolphins Reptiles Or Mammals?

These are classified as mammals but not Fish despite living in the oceans all the time. What distinguishes them from Fish include;

1. Breathe air through the lungs and not gills. To catch a breath, dolphins have to make numerous sprees out of the water.

2. They are warm-blooded, unlike Fish.

3. Feed their young with milk from the mammary glands and take care of them. 

4. Give birth rather than laying eggs.

5. A little different from the fish scales, dolphins have a tiny amount of hair right around the blowhole, whereas other whales have them near the mouth or on top of their heads, though many of the species lose the hair before they are born.


Fish share certain features with other vertebrates as members of the Phylum Chordata and Kingdom Animalia. They belong to the same kingdom and phylum as the reptiles and mammals but have different classes in the taxonomic classification unit. These dissimilarities come about from their characteristic bodily features and the adaptive features to different environments. 

After reading through all the facts and information provided in this article, we are sure you can now answer whether Fish are reptiles or mammals. You can also tell the difference between the two and easily come up with their relation to fish through the similarities that have been pointed out. With all this information at hand, you do not have to wonder whether Fish are classified as reptiles or mammals. Go ahead and share this information with those close to you and help us educate the world. All the best!

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