Are Fish Considered to be Mammals, Reptiles, or Amphibians? Why?

The world of organisms is vast and confusing at the same time. Most people believe fishes belong to mammals or reptiles. Some think they are amphibians. But is that the truth? Let’s find out:

Fishes may be considered to be mammals, reptiles, or amphibians because of a few similar characteristics, but they have their own unique properties. Besides a few exceptions (mudskippers, live bearings, and lungfishes), all fishes have common traits differentiating them from mammals, reptiles, and amphibians.

Want a detailed answer? Let’s explore the characteristics of fish and how and why they should not be considered and confused with mammals, reptiles, or amphibians.

Let’s go!

What Makes A Fish “Fish”?

The oceans never run out of fish, and the diversity of this organism is uncertain. But for an estimation, more than 30,000 fish species have been identified, according to Joseph S. Nelson’s Fishes of the World – a reliable and valid reference source on the classification of fishes.

With so many fishes in the world, it must be exciting to understand how they all are connected. And thus, before we dive deep into which category – mammals, reptiles, amphibians – fish belongs to, let’s first learn what makes a fish “fish.” The following are the common characteristics found in all fishes:

  1. Fishes are Cold-Blooded Animals: Fishes are cold-blooded organisms because their body temperature adapts to the environment. Because their bodies cannot maintain a constant temperature, the changes in water temperature can make them dormant or fast during winters or summers, respectively.
  2. Fishes Are Water Habitat: It’s the most apparent characteristic in fish; they live in water. Note that the differences come in the time a fish can spend outside of water.
  3. Fishes Move With Fins: Because they do not have any arms and legs, they use their fins for movement, unlike other organisms. It may include a tail fin, anal fin, side fins, and dorsal fins.
  4. Fishes Breath With Their Gills: Like humans have lungs, fishes have gills. They take in oxygen and release carbon dioxide.
  5. Fishes Have Swim Bladders: A swim bladder has air that saves a fish from sinking to the bottom of the water. Sometimes, the fish swallows water which is saved in the bladder to ease the breathing pattern in waters with less oxygen.
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What are Mammals?

Mammals are warm-blooded animals and typically have the following few of the many common characteristics:

  • Their body has fur or hair (fur, quills, whiskers, horns, etc.)
  • The female mammals have mammary glands to produce milk to feed the baby mammals
  • A warm-blooded metabolism helps them maintain a constant body temperature regardless of the season.
  • A mammal baby is born alive; they aren’t hatched by first producing the eggs.

Examples of mammals are bears, elephants, giraffes, zebras, bats, and rats.

●    Are Fish Mammals?

Now that we’ve learned the characteristics of both fishes and mammals, it’s not hard to conclude whether fishes are mammals or not. The simple answer is:

Fishes are NOT mammals because they are cold-blooded, while mammals are warm-blooded. Also, fishes do not have any fur or hair on their body; instead, they have scales. Fishes do not feed their young with milk, and they lay eggs. The body temperature of fishes changes with water and doesn’t remain constant, unlike mammals.

The essential thing to remember here is that whales and dolphins are exceptions. They are aquatic creatures, but they do not come under the category of fish since they are warm-blooded, breath through their lungs, and get air both in water and above the water, which makes them mammals.

Also, the term “livebearing” is associated with fishes that give live birth. This includes plates, swordtails, and guppies. They do not lay eggs and hatch them but give birth to young ones. Still, they are not mammals, given they don’t feed their young will milk or do not fit the other criteria for mammals.

What are Reptiles?

Reptiles are animals with the following characteristics:

  • They are air-breathing vertebrates
  • They are cold-blooded animals.
  • Their bodies have dry scales or scutes.
  • Reptiles breathe through the lungs.
  • Reptiles can live in both water and on land. They are found in oceans, deserts, underground burrows, rainforests, and treetops.
  • They give birth to live young or lay shelled eggs that are hatched.
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Examples of reptiles are lizards, turtles, snakes, crocodiles, etc.

●    Are Fish Reptiles?

Considering the reproduction and cold-blooded traits, we can say that fishes are reptiles. But is that all? The truth is:

Fishes are NOT reptiles because reptiles live both in water and on land, while fishes can only survive in water. Plus, fishes have wet, slimy scales while reptiles have dry scales. Fishes do not breathe in air, and those who do aren’t fishes but mammals (whales and dolphins).

Also, fishes have gills to breathe and fins to move, but reptiles breathe through lungs and use their forelimbs or hindlimb to move (some reptiles like snakes do not even have any limbs; they crawl instead).

The shocking thing here is that though fishes are water habitats, some can (and do) survive out of water. Mudskippers are a great example as they can crawl on the land. However, they still do not count as reptiles because they have fins and gills. Though they can breathe in air and come out of the water, the absence of lungs makes them fish and not a reptile.

Lastly, reptiles breathe with lungs, and fishes do not; they have gills instead. But as the world is full of mysterious creatures, we also have fishes who have lungs called “Lungfish.” These fishes use their swim bladder for breathing, and the organ is considered a lung because it allows them to get oxygen from the air. Yet still, lungfishes are “fishes” and not reptiles because they just have lungs and no other characteristics of reptiles.

What are Amphibians?

Also known as “ectothermic,” amphibians:

  • are cold-blooded vertebrates
  • have a diverse habitat (meadows, streams, forests, ponds, lakes, rivers, etc.)
  • breathe through their lungs, gills, and skin
  • breathe both in water and on land
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Examples of amphibians are frogs, toads, salamanders, newts, etc.

●    Are Fish Amphibians?

The correct answer is, again, NO. Even though fishes are most closed to amphibians and sometimes can be confused with amphibians (given they are cold-blooded), you should remember that:

Fishes are NOT amphibians because the former survives in the water while the latter can live on both land and in water (mudskippers are also fishes, although they can crawl on land).

Similarly, fishes have only gills to breathe, but amphibians have lungs, gills, and skin – all to breathe (again, lungfishes are still fishes although they have lungs).

Fishes cannot breathe in air, but amphibians can. Also, fish have scales, but amphibians have typically moist, smooth, slimy skin with no scales. And fishes can survive both in saltwater and freshwater, but amphibians mostly survive in freshwater and less in saltwater.

Most people believe fishes are amphibians because of the similarity both the animal groups have. However, at the core, the two are not the same and possess their unique traits. Fishes can be considered closed to amphibians but not completely amphibians, given they have differences.

So, What Exactly Are Fish?

A fish is simply a “fish” with its unique characteristics. It doesn’t come under any category and has its own traits, just like other groups in animal classification like mammals, reptiles, birds, and amphibians.

Easy, isn’t it? And in case you want to revise quickly, here’s a simplified table with all the necessary information!

 MammalsReptilesAmphibiansFish
Examplesbears, elephants, giraffes, zebras, bats, and ratslizards, turtles, snakes, crocodilesfrogs, toads, salamanders, newtsGoldfish, tuna, salmon, shark, mudskippers
Bodyfur/hairDry scalesMoist skin; no scalesWet, slimy scales
Temperaturewarm-bloodedcold-bloodedcold-bloodedcold-blooded
HabitatSurvive on both water and landSurvive on both water and landSurvive on both water and landSurvive in just water (mudskippers are an exception)
ReproductionGive live birth,Lay eggs and hatch them, also give birth to live young (some snakes and lizards but cannot be called fish)Lay eggs with no hard shells and hatch themLay eggs with no hard shells and hatch them (livebearing are an exception)  
MovementHave limbs, fins, wingsHave forelimbs or hindlimbs (snakes crawl)Have forelimbs and hindlimbs (some crawl)Have only fins
BreatheThrough lungs, both in water and landThrough lungs, both in water and landThrough lungs, gills, and fins, both in water and landOnly gills, only  habitat is water (except Lungfish)

Bottom Line

Hopefully, now you know why fishes are considered to be mammals, reptiles, or amphibians. And that though each animal group does share similar characteristics, fishes are unique with their own fishy traits. They are not mammals, reptiles, or amphibians and make one of the fifth distinct animal groups in animal classification!