A-z - Animals

Are fish mammals?

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key point:

  • Fish are not mammals for the following reasons: they are cold-blooded, cannot breathe air, and cannot breastfeed.
  • Many people are confused by this question because scientists used to think that certain mammals were fish, such as dolphins, whales and seals.
  • Fish are divided into three groups: jawless, cartilaginous, and bony.

Are fish mammals? Fish are not mammals, but they are such a diverse group that lumping them all into one category is inefficient. They used to be grouped under the class Pisces , but are now divided into three classes and multiple subclasses, clades, orders, suborders, tribes, superfamilies, families, genus, and species.

Are fish animals? In fact, fish are among the most diverse group of vertebrates, or vertebrates, on Earth, although at least one species, the dreaded hagfish, doesn't really have a backbone.

They come in various forms such as sunfish, great white sharks, stunningly beautiful mandarin fish, long-horned dolphins and small guppies. Some belong to clades that contain animals that are not fish at all. However, no matter what biologists know about fish, they do know that fish are not mammals.

is-fish-mammal-1
There are more than 30,000 species of fish.

©V_E/Shutterstock.com

Why are fish not mammals?

Fish are not mammals because most of them are not warm-blooded, with the exception of some sharks and tuna. They have no limbs, fingers, toes, fur or hair.

Most of them cannot breathe air because they have no lungs, but lungfish and snakehead are exceptions. Most have gills that allow them to absorb oxygen from the water. They can only survive in water.

They lay eggs or give birth live, but no fish nurse their young with milk, a behavior that separates mammals from all other types of animals. Even pigeons that feed their young on crop milk, or tsetse flies that feed their young on something like milk in the womb, don't count as mammals.

Why do people think fish are mammals?

People might think that fish are mammals, because not so long ago scientists believed that many mammals were fish. These mammals spend most or all of their lives in water, including whales, seals, sea lions, and even hippos. These animals are mammals, but not fish. Even whales, dolphins, and porpoises (creatures that look a lot like fish) are warm-blooded and feed their young with milk. Although they can hold their breath underwater for long periods of time, they still need air to breathe.

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Many fish also care for their young with the dedication that mammals typically possess. Male jawfish, bettas, and arowanas incubate their eggs in their mouths, which is known as mouth brooding. Of course, they can't eat while holding the egg. Seahorse fathers are known to conceive first and give birth later. Other fish hatch their young in their skin or gills, and some baby fish first eat the slime on their parents' skin. Some cichlids protect their young by using movements to warn them of danger, while others protect their pups even after they are sexually mature. They're still not mammals.

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

©Noheaphotos/Shutterstock.com

What kind of fish are there?

Scientists have divided these animals into three categories. In fact, there are more than three types, but the other types are all extinct. The three existing classes are:

  • Agnatha : These are jawless fishes, namely lampreys and hagfish.
  • Cartilaginous fish : These fish have skeletons made of cartilage rather than bone. These are sharks and rays. By the way, the skeleton of all mammals is mainly made of bones.
  • Osteichthyes : The skeletons of these fish are made of bone. They include those fish with fleshy fins, such as coelacanths and lungfishes, and those with ray fins, which is pretty much every other fish that doesn't fall into the other two categories.

is fish an animal

Fish can be collectively referred to as "sea animals" along with mammals such as whales, dolphins and sea lions. But as we've already discussed, mammals and fish don't share the same traits. However, they do share this – they are all vertebrates. Fish, like mammals, have backbones, or spines.

But are fish animals? Any organism that has a backbone, is able to move, and must find and digest food is classified as an animal. So yes, a fish is an animal.

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blue and yellow fish swimming
blue and yellow fish swimming

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about the author

Read more  Don't Feed These Animals: An Essential Guide to Wildlife Safety

heather ross


Heather Ross is a middle school English teacher and mother of 2 people, 2 tuxedo cats and a golden doodle. In between taking the kids to soccer practice and grading homework, she loves reading and writing about all things animals!

FAQ (Frequently Asked Questions)

Which fish are endangered?

Unfortunately, too many species are endangered. Fish appear to be particularly vulnerable to extinction as overfishing, habitat destruction, pollution and global warming heat the waters in which they live. Warm water contains less oxygen than cold water, and oceans, lakes and rivers have increasingly dead zones. Whether in polar regions, tropical and subtropical regions, or in fresh, brackish or brackish water, fish are endangered. Some endangered ones include:

• Southern bluefin tuna. This tuna lives in the waters of the Southern Hemisphere and is critically endangered.

• Crystal Dartfish. The tiny creature was found in the rapids of the Mississippi and Ohio River basins, though it has since become extinct from much of the Ohio River. It is considered vulnerable.

• Atlantic cod. Atlantic cod was nearly extinct in the 1990s and has yet to really recover. It is listed as vulnerable.

• Beluga Sturgeon. This ancient, long-lived creature has been hunted and poached for its eggs so much that it is now critically endangered.

• Zebra sharks. The shark, which lives in coral reefs in the Indo-Pacific region, is endangered due to habitat loss and hunting for its fins, liver and meat.

How many kinds of fish are there?

There are many more types of fish than mammals. There are approximately 33,000 species of fish compared to 6,400 species of mammals.

Are fish omnivores, herbivores, or carnivores?

They can be all three! Herbivorous fish include parrotfish, certain types of butterflyfish, surgeonfish, certain catfish, and damselfish. Because they feed on plants, these animals live in shallow water where sunlight can penetrate and allow plants to photosynthesize. They eat seaweeds such as glassweed, latok, Halimeda and Sargassum. They also eat corals, such as Turbinaria. Of course, corals aren't technically plants, but fish don't know that. Mullets feed on small aquatic animals as young and algae as adults.

Almost every freshwater fish is an omnivore, meaning they ingest plant and animal material, including algae, aquatic organisms they can handle, parasites, insects, and eggs found on other fish. Bonnethead sharks are also omnivorous, and they are the only omnivorous shark known. With the exception of bonnetheads, truly carnivorous fish such as tuna and sharks may accidentally ingest plants, but plants are useless to them, nutritionally. They evolved to eat meat and benefit from it.

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Are fish in danger?

While most are not dangerous to humans, there are some species that are extremely dangerous. Chief among them is the great white shark, which is responsible for more fatal shark attacks than any other shark. Humans are not usually prey for sharks, but in the ocean they may resemble seals, which are. A shark may take a tentative bite at a swimmer, but this is enough to cause serious or even fatal injury. Bull, tiger, hammerhead and mako sharks have been known to attack humans.

Other fish are dangerous because they are poisonous. This includes beautiful lionfish, rockfish, striped catfish and stingrays. Some have sharp spines that, while non-venomous, can cause stab wounds. In addition to striped eels, these creatures include triggerfish, surgeonfish, and various catfish. Other foods are poisonous, such as certain species of puffer fish or fish contaminated with mercury or ciguatoxin. Ciguatoxin fish are not naturally poisonous, but absorb the toxin from an organism found in coral reefs. Like mercury, the toxin is more concentrated in larger fish.

Other fish may pose a danger to divers or swimmers because of their aggressive bite and strong bite, such as barracudas, wolf eels and monospotters.

How long do fish live?

The range of how long a fish can live is mind-boggling. In any other group of vertebrates, the distance between the shortest-lived and longest-lived animals is greatest. For example, guppies have a lifespan of two years. Greenland sharks can live up to 500 years. The orange stickleback eaten as a dish can live more than 200 years, the great white shark can live about 30 years, the Nassau grouper can live about 16 years in the wild, and the popular channel catfish can live up to 40 years.

What do humans use fish for?

Humans mostly eat them, which is probably why reports of shark attacks are so alarming, despite their rarity. Humans should eat fish, not the other way around! Unfortunately, overfishing has endangered many species, such as cod and yellowfin tuna. Fish are farmed like tilapia and hunted like marlin and other billfish. People also keep fish as pets and display them in public aquariums. Some fish are easy to care for, such as goldfish, while others require very specific conditions to thrive, such as poop.

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