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Is a Fish an Animal? Here’s What You Need to Know

The classification of living organisms into groups can be tricky. You may wonder, is a fish an animal? Or are they from a different animal class?

Biology uses classifications that divide the world into kingdoms. There is an animal kingdom and a kingdom for plants, bacteria, and microorganisms.

Fish account for about half of all vertebrate species. Fish have been around the planet for over 500 million years. They existed long before dinosaurs traversed the world.

But the question remains, are fish animals? Let’s find out.

Are fish classified as animals?

According to scientific knowledge, you can classify fish as an aquatic animal. Fish is one of the six major animal classes, including mammals, birds, reptiles, amphibians, and invertebrates. While fish flesh is not termed as meat, fish are animals with brains and the ability to feel pain.

What is a fish?

The term fish refers to a range of vertebrates from various evolutionary lines. It refers to a life form instead of a taxonomic group. Fishes share similarities with other vertebrates as members of the phylum Chordata. Gill slits, a notochord (vertebral covering rod), a hollow dorsal cord, and a tail appear at some point throughout their entire lifespan.

It is challenging to define a fish because it encompasses a diverse range of aquatic animals. Fish have roughly the same number of species as all other vertebrates combined. Approximately 4,500 of the 50,000 species of animals with backbones are mammals. Nine thousand seven hundred are birds, 6,500 are reptiles, 4,000 are amphibians, and 25,000 are fishes.

Through new species discovery, these figures fluctuate. And there has been a discovery of newer fishes more frequent than other new vertebrates. With these discoveries, scientists expect that fish species will surpass other vertebrates.

Fish species have been around for over 450 million years. And they have evolved to fit every conceivable type of aquatic habitat. When fishes colonized land habitats, they evolved into tetrapod (four-legged) land vertebrates.

The common conception of fish as a slippery aquatic animal with fins and gills applies to living fish species. But far more fishes detract from that illustration than conform to it. The body, for example, elongates in many forms and shortens in others.

Also, the body is flattened for some (in bottom-dwelling fishes) and compressed for many others. They can extend their fins, forming intricate shapes, or they can be reduced or lost. And the mouth position, eyes, nostrils, and gill openings differ substantially.

Many fish are colored and shaped, matching their surrounding water. Others are among the most colored organisms, with a diverse range of skin tones. And it often causes astounding brightness on a living person.

The vibrance of pigments may be stimulated by the surface structure of the fish, giving it a glowing appearance. Many fish can change their pigmentation, some for camouflage, others to improve behavioral impulses.

See Related: Are Humans Animals? Things to Know

What kind of animals are fish species?

variety of colorful fish species

Most fish are classified as vertebrates, which means they have a backbone. Fish, like mammals, birds, and reptiles, are a distinct group of aquatic animals.

There are many different types of fish, ranging in size from the dwarf minnow at around 1 cm to the whale shark, which can reach lengths of up to 10 meters. All fish are aquatic creatures with gills, scales, and fins.

You can classify fish into three types:

  • Osteichthyes or also known as bony fish. It refers to a type of fish that includes common species such as salmon, tuna, and even eels.

  • Chondrichthyes are cartilaginous fish due to the Gnathostomata division of their jaws. This category includes sharks, rays, and skates.

  • Agnatha: These are jawless fish called lampreys and hagfish.

See Related: These Are 13 of the Longest Living Animals on Earth

What are the characteristics to be classified as an animal?

You can classify animals through six shared characteristics, according to biologists:

  1. It needs to have multiple cells.

  2. It consumes food to generate energy.

  3. It sexually reproduces.

  4. It is composed of cells that have no cell walls.

  5. It is capable of movement at some point in their lives.

  6. It reacts to external stimuli.

Like other animals, fish meet all the criteria to become an animal. Hence, it is commonly recognized that fish are animals.

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What distinguishes fish from other animals?


Fish are a diverse group of creatures. More than 32,000 fish species exist, more than all mammals, birds, reptiles, and amphibians combined.

Whales, dolphins, starfish, and jellyfish are different species. But you can consider sharks, rays, eels, and sea horses as fish species. So, what distinguishes fish from other animals?

Fish are unique creatures because they have gills and fins. And they only live in water. Invertebrates and amphibians have gills. Several mammals and amphibians also have fins. And many of these species live in water, but only fish have all three characteristics.

Fish, unlike mammals, are cold-blooded (ectothermic). It means they do not have a constant internal body temperature. Instead, their temperature is influenced by their surroundings. True fish have a backbone and also fins. Most have gills for breathing and scales covering their bodies.

Are fish a mammal?

Fish are not mammals, except for sharks and tuna species, because most are not warm-blooded. They lack limbs, fingers, feet, fur, and hair.

Most of them cannot breathe the air because they lack lungs. But lungfish and snakeheads are exceptions. The majority of them have gills that allow them to extract oxygen from water. And they can only exist in water.

They lay eggs or give birth, but no fish nurses newly hatched young with milk. And this trait distinguishes mammals from all other animals.

People think of fish as mammals because scientists thought most mammals were fish. Whales, seals, sea lions, and even hippopotami are mammals that spend most of their lives in water. These creatures are mammals, but not fish. Even whales and dolphins – which resemble fish, are warm-blooded and feed their young with milk. Though they can hold their breath for an extended time underwater, they still need oxygen.

Many fish species care for their young with the same devotion mammals do. Mouthbrooding occurs when male jawfish, bettas, and arowanas incubate eggs in their mouths. And they can’t eat while holding the eggs. Also, seahorse fathers are well-known for gestating their offspring before giving birth.

Other fish brood their young in their skin or gills, and some babies first eat their parent’s mucus. Some cichlids protect their young by moving to warn them of danger. While others protect their young even after reaching sexual maturity. But they are still not considered mammals.

Do fish have emotions?

Because fish have no facial expressions, we assume they are not emotional. But scientific knowledge demonstrates that fish are sentient and emotional beings.

Like other animals, fish respond to negative experiences by altering their behavior and anatomy. Fish learn to avoid things that are harmful to them, which aids in their survival.

A common misconception that fish have is they have short memories. Goldfish can remember the exact colors and locations of particular feeding tubes for a year after exposure.

Fish may not be able to feel the same emotions as humans, such as anguish, delight, worry, amusement, and disgust. However, to say that fish have no feelings would be incorrect.

What are the behaviors of fish animals?

Fish behavior is a complex and diverse subject. Like all animal life, with a central nervous system, you can find out a fish’s response to stimuli by the inherited traits of its nervous system. It can also be known from past experiences.

Fish perceive their surroundings using their normal senses of sight, smell, hearing, touch, and taste, including dorsal fin water-current sensors. Fish with large eyes may have a limited sense of smell, whereas fish with small eyes hunt and eat by smell (such as eels).

Feeding, reproduction, and escape from enemies are the three most important activities in the life of a fish. Sardine schooling behavior on the high seas is primarily a defensive mechanism to avoid enemies, but it is also associated with their breeding and feeding requirements.

Predatory fishes are often solitary, waiting to lunge after their prey. It is an impossible motor skill for beaked parrot fishes, which feed on coral reefs and swim in different groups from one coral head to the next.

Fish Distribution and Abundance

Except for high-temperature lakes and salty-alkaline rivers, all bodies of water support fish life. The flux density of fishes is the result of Earth’s geological history and evolution. It also includes fishes’ ability to evolve and adapt to accessible ecosystems.

Fishes can be seen to be distributed based on habitat and geographical area. Marine and freshwater habitats differ. Most fishes in a marine habitat differ from those in a freshwater habitat. Freshwater habitats can be found in various forms. The biological functions of fish in streams, tropical lagoons, and other waterways differ.

Marine habitats include deep ocean floors, surface oceanic, rough coastlines, and sandy shores. Furthermore, even when ecosystems occur along the same coastline, tropical and temperate coastal waters will have different aquatic fauna.

See Related: How Do Eels Reproduce? Learn the Mysterious Lifecycle

What are fish habitats?

Fish are particularly sensitive to water temperature. And the amount of oxygen is an important factor to know which species can survive in water bodies.

Freshwater Habitat

freshwater fishes
image by Citron / CC BY-SA 3.0

There are over 30,000 freshwater fish species in total. Freshwater habitats are home to the most diverse fish, invertebrates, and aquatic plants. Almost half of all fish species live in freshwater. It means they swim in rivers, lakes, and wetlands that account for less than 3% of the world’s water supply. In North America alone, there are over 800 known freshwater fish species. And there are over 10,000 species worldwide.

Some freshwater fish, such as salmon and trout, are known as anadromous. They hatch in freshwater and swim out to sea, where they live until they can reproduce in freshwater. Catadromous species are migrating in the opposite direction, such as freshwater eels. These creatures hatch at sea. And they also spend the majority of their lives in freshwater and then return to the sea to reproduce.

Overfishing, marine pollution, habitat loss, and dam construction are threats to freshwater fish. About 40% of North American freshwater fish species have recently become endangered. At least 57 freshwater fish species have become extinct in North America since 1900.

Marine Habitat

marine blue fishes
image by Uxbona / CC BY 3.0

The marine ecosystem provides various habitats that support sea life. Marine life is dependent on the saltwater in the sea (“marine” comes from the Latin “mare,” meaning sea or ocean). An ecological or environmental area inhabited by one or more living species is referred to as a habitat.

You can classify marine habitats as coastal or open ocean habitats. You can find coastal habitats from where the tide comes on the shoreline. Even though the shelf area accounts for only 7% of the ocean area, coastal habitats are home to most marine life. Open ocean habitats are found beyond the continental shelf in the deep ocean.

Alternatively, marine habitats can be classified as pelagic or demersal. Pelagic habitats are found near the ocean’s surface or in the open water column, away from the ocean’s bottom. You can find demersal habitats near or on the ocean’s bottom. A pelagic organism is one that lives in a pelagic habitat, such as a pelagic fish. Similarly, an organism living in a demersal habitat, such as a demersal fish, is referred to as a demersal organism. Pelagic habitats are inherently shifting and ephemeral, depending on ocean currents.

What do you need to know about Freshwater Fish?

koi freshwater fish

Freshwater Characteristics

Freshwater fish are aquatic organisms that live primarily in rivers, lakes, and lagoons. This habitat is relatively small but large enough for them to live comfortably. Freshwater fish have special gills that aid in temperature regulation and their bodies are covered in scales that regulate low temperatures. They also have scales throughout their anatomy that allow them to move more freely.

Freshwater Anatomy

The anatomy of freshwater fish is determined by the water in which they develop. Because not all waters are the same, some are denser than others, to which each type of fish adapts.

They have the same body structure as most vertebrates. But their characteristics differ depending on the species they belong to. They have a head, trunk or abdominal area, and tail.

Their fish brains have branchial areas that allow them to store oxygen and breathe in the water. It’s worth noting that their trunk or abdomen houses the majority of their organs, including pair of fins that allow them to move as freely as possible in the water.

The tail is an additional component to its anatomy because it provides all movement to freshwater fish, allowing them to reach the necessary speed to move along and across the waters where they coexist.

Most freshwater fish are covered by scales, allowing them to resist temperature changes. Their skin, like that of other vertebrates, contains special characteristics. Although not all species have scales on their bodies, some freshwater fish species have a structure that is like scales and serves the same purpose.

Carnivorous Freshwater Fish.

These species feed on smaller fish, but they also eat species that live in the water, such as insects and mollusks. But, the latter is uncommon among them and occurs only when necessary.Various fish develop characteristics that allow them to ingest their food completely. And many are considered carnivorous fish only for ingesting aquatic insects.

Herbivorous Freshwater Fish

It is common for freshwater fish because algae and plants are abundant in this type of water. And it provides them with the nutrients they need to survive.

Although this type of food is abundant for these fish, it is difficult for them to digest. It is because the content is so heavy that it causes the digestive system to work all day.

According to research, most freshwater fish that consume this type of food supplement their diet with live species. It implies they can become carnivores to improve digestion and gain mobility in the water.

Omnivorous Freshwater Fish

Most freshwater fish seek to supplement their diet with plants and living things. And this represents their nutrition. These fish do not consume the same foods as carnivorous fish. And they prefer to feed on other species rather than other fish.

Piranhas are the best example of omnivorous freshwater fish. Despite it being one of the most ruthless carnivorous species. It is because they supplement their diet with vegetables found in the fresh waters of the Amazon where they develop.

How Do Freshwater Fish Procreate?

The reproductive patterns are the same, whether freshwater or saltwater fish. It happens at regular intervals, which could be every month or every year.

It also depends on the species and external factors in those who develop, such as temperature and food. The distinction of sex between fish is difficult. It is because its reproductive system is not visible, making it difficult to determine whether it is male or female.

Because of various research, it was discovered that you distinguish males and females by the color of their skin or the species’ anatomy.

Types of Freshwater Fish Species


The Goldfish is without a doubt the most popular pet fish. They can be found in Asian rivers because their species originated there. This type of freshwater fish consumes a large amount of oxygen. And it prefers shallow waters with warm temperatures.

They are fish capable of consuming large amounts of food despite their small size. And they are omnivorous fish in general, meaning they can seek food anywhere in the river.

Specialized breeding has resulted in different Goldfish breeds and color forms over the years. Aside from the Common Goldfish, you can also get a Comet Goldfish, Oranda, Fantail, and other fish.

It occurs around two years of age, and for fertilization to happen, they must be in the water with temperatures ranging up to 22 degrees. The female lays her eggs in nests built by the male, and they hatch in three days, at which point they are in grave danger of being eaten by their parents.


male betta fish

When most people think of a pet fish, they visualize a Betta. You can identify a betta fish by its long trailing fins resembling a lady’s dress.

Never keep a Betta in a bowl or a small novelty tank. They will need at least a 10-gallon tabletop tank decorated with underwater plants. Additionally, you should not keep more than one of these fish together.

It is important to remember that Betta cannot breathe underwater, including other fish. They need to come to the surface for air, so keep the surface of your tank clean as well.


When it comes to capturing freshwater fish, angel fish are considered one of the most popular because of their color diversity. They have a unique swimming style since they do not swim like other species. Their swimming is slow and vertical as if floating in the water. Many people believe they resemble sea angels.

In reproduction, they are a freshwater fish species that is difficult to differentiate sex. Females usually lay their eggs in water-based vertical plants. And after that, they stay away because they eat their young without realizing it.

Types of Saltwater Fish Species


clownfish in a coral reef
image by Ritiks / CC BY-SA 3.0

The ocellaris clownfish, also known as the common clownfish, is a popular and one of the most popular types of saltwater fish. Clownfish are saltwater fish that cannot survive in freshwater, unlike other fish species, such as guppies. Because clownfish need a constant salinity level, they can be riskier to care for than freshwater species.

Common clownfishes are slow, waddling swimmers who are generally site-attached. It means they pick a territory and spend most of their time there.


The Royal Gramma is a member of a group of fish known as fairy basslets. This bold and beautiful fish is unique with its bright purple head and yellow body/tail. Royal Grammas are saltwater cave-spawning fish that reproduce in herds. A single male will usually mate with more than one female. The male constructs a nest of macroalgae in which the eggs are laid night after night.


Longnose Hawkfish
image by Crazybee2 / CC BY-SA 4.0

Hawkfishes are primarily found in the Indo-Pacific, with two species native to the tropical western and eastern Atlantic Oceans. You can find most of the species in shallow water, with some reaching depths of several hundred feet.

Hawkfishes have a continuous, 10-spine dorsal fin with both hard and soft rays, some of which have cirri (small pom-poms) at the tips. You can identify pectoral fins by their elongated, unbranched lower rays.

Longnose hawkfish, like most other hawkfish, spend most of their time on a rock, sea fan, or piece of coral, waiting to rush at a food item. Their conical teeth have been modified to grasp benthic and free-swimming crustaceans (their primary diet in the sea).

What is the importance of fish to humans?

Fish are crucial in aquatic ecosystems and the fight against human hunger and malnutrition. Fish is high in minerals, calcium, vitamins, and phosphorus. And there are various benefits besides eating fish because it also provides important health benefits.

Fish can also help control diseases; like malaria, fever, and mosquito-borne diseases. Fish is typically high in healthy fats, providing healthcare benefits to prevent cardiovascular disease. White-fleshed fish, in particular, has the lowest fat content of any animal protein source, and oily fish are high in omega-3 fatty acids, or “good” fats.

Fish considered meat is an important part of the diet because the human body cannot produce significant amounts of these essential nutrients. Furthermore, fish are low in omega-6 fatty acids found in red meat.

We can use fish in various products beneficial to humans, such as skin care, wood polishing, and jewelry box covering. People benefit from the development of fish species and fish industries in terms of job opportunities in the fishing industry.

Frequently Asked Questions (FAQs) About Fish

Which fish species are threatened and endangered?

Unfortunately, multiple species are in danger of extinction. Fish appear vulnerable to extinction because of overfishing, habitat destruction, pollution, and global warming, which heats up the waters in their habitat. Warm water contains less oxygen than cold water, and dead zones are becoming more common in oceans, lakes, and rivers. Fish are endangered in the polar regions, the tropics, and the subtropics, including in freshwater, saltwater, and brackish water. Some of those endangered species are:

Southern Bluefin Tuna. This tuna is found in the southern hemisphere’s waters and is critically endangered.

-Crystal Darter. This tiny creature can be found in fast-flowing streams in the Mississippi and Ohio River basins, though it has become extinct in much of the Ohio River basin. It is regarded as vulnerable.

-Atlantic Cod. The Atlantic cod was nearly fished to extinction in the 1990s and has yet to recover, categorizing it as vulnerable.

-Beluga Sturgeon. This ancient and long-lived creature has been chased down and poached for its role to the point of extinction.

-Zebra shark. This shark, found in the Indo-Pacific coral reefs, is threatened by habitat loss and hunting for its fins, liver, and meat.

Are fish omnivores, herbivores, or carnivores?

They can be any of the three! Parrotfish, butterflyfish, surgeonfish, blennies, and damselfish are examples of herbivorous fish. You can find these animals in shallow water where sunlight can penetrate, allowing plants to photosynthesize.

Glassweed, latok, Halimeda, and Sargassum are among the seaweeds they consume. They also consume corals like Turbinaria. Corals aren’t plants, but the fish aren’t aware of this. Mullets feed on small aquatic animals when they are young and switch to seaweed once they reach maturity.

Almost all freshwater fish are omnivores. It means they eat plant and animal material, including algae and parasites from other fish. Bonnethead sharks are also omnivorous, the only sharks known to be so. Aside from the bonnethead, predatory fish, such as tuna and sharks, may consume a plant that is useless to them. They have evolved to consume and enjoy meat.

What is the average lifespan of a fish?

The life span for fish is astounding. The distance between the shortest and longest-lived animals is the greatest in any other group of vertebrates. Guppies, for example, have a lifespan of two years. The Greenland shark has a lifespan of 500 years.

The orange roughy, a popular dish, can live for over 200 years. The great white shark for about 30 years. The Nassau grouper for 16 years and the popular channel catfish for up to 40 years.

Why fish are not considered an animal?

Fish are considered animals. They are a diverse group of aquatic vertebrates that breathe through gills and have fins for swimming. The idea that fish are not animals is a misconception.

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