The Anatomy of Fish: Classification, body parts and more

Fish are aquatic vertebrate animals, usually ectothermic and with gill breathing. They are commonly covered by scales, and are endowed with fins which allow their continuous movement. However, if you want to learn more about the anatomy of fish then don´t miss this article.

Before knowing everything about the anatomy of fish, we should know about….

Types of  Fish.

Chondrichthyan fish

Chondrichthyes ( from the Greek khondro, «cartilage» and ikhthys, «fish») are cartilaginous fishes, which include the subclasses Elasmobranquios (sharks, rays) and Holocéfalos (chimeras).

These fish have a very hydrodynamic body supported by a cartilaginous skeleton that despite being less resistant than bone, is enough strong to sustain them.

In the same order of ideas it is worth mentioning that they have rigid fins, rough skin with placoid scales and powerful jaws provided with an endless supply of grinding teeth that are not fused to the jaw. Chondrichthyes lack an operculum, so it’s  necessary that they swim to maintain the flow of water through the gills.

They also lack a swim bladder and instead have a liver with high concentrations of lipids that facilitates their flotation. Chimeras, sharks and rays belong to this category.

Bony Fishes

They represent the bigger family of fish, and their main feature is given by their ossified skeleton. Their body is fusiform and usually somewhat compressed in the caudal region, although this feature doesn´t extend to all species for which their anatomies vary considerably.

Their mouth is usually located at the end of their head and their fins vary according to their shape and since in many species are completely atrophied.

Regarding  their scales  there are two types: those that have a diamond shape, while the others are characterized by being circular, usually covered with hard enamel and turned as if they had the form of a mosaic that is attached to the body.

In general, bony fish are carnivores being the crustaceans, other fish such as mollusks, or even worms their main preys. In the case of the bony fishes that live in fresh water, they consume huge amounts of crustaceans, and also aquatic insects, tadpoles, and the insect larvae.

Just as there are carnivorous species, there are also many others that feed mainly on vegetables.  Due to such circumstances their teeth and digestive system are linked directly with their type of feeding habits.

Carnivorous fish have sharp teeth and a rather short intestine, while herbivorous species tend to have teeth that grind everything besides a much longer intestine since vegetables are much more difficult to digest.

Ciclostomata Agnatha Fish

These fish have quite archaic forms and are also considered as fish without a jaw. Ciclostomata include Petromizontidos and mixinoideos. Moreover, they are characterized by the simplicity of their skeleton, which corresponds to that of a very primitive vertebrate.


They have an ell-like body and despite having an immobile mouth, they are voracious predators since this structure has the shape of a sucker, with very sharp horny teeth. As if this were not enough, they have glands in their mouth whose secretion prevents the blood from coagulating, with which the predated fish bleeds very quickly.



The mixinoideos or mixinos also possess a long and cylindrical body, although more similar to the one of a worm that to the one of an eel. They live on the seabed, even at a great depth, where they bury half of their body, leaving only the nostril and mouth outside, prepared to catch food.

This can be either remains of other animals that fall to the bottom of the sea, as well as live fish with difficulty moving

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The Anatomy of Fish

The anatomy of fish  is determined by the characteristics of water which is much denser than air, has less dissolved oxygen and a greater absorption of light.

Almost all fish have an aerodynamic body, which is divided into head, trunk and tail, although the dividing points are not always easy to distinguish.

Their head includes the snout, from the eye to the most advanced point of the upper jaw, the operculum or gill cover, and the cheek, which extends from the eye to the preoperculum.

The lower jaw defines a chin. The head can have several fleshy structures known as barbels, which can be very long and resemble whiskers.

We will now know the main aspects  of the  anatomy  of fish

External Anatomy

The fish have fins and a spine and most of them have scales and breathe through their gills. As for their fins they are appendices that they use to maintain their position, move, direct or stop.

There may be individual fins located along the body of each species, such as the dorsal fin (back), caudal fin (tail) and anal fin, as well as paired fins such as the pectoral fins (breast) and the pelvic fins (hip)

Fishes like catfish have a fleshy lobe behind the dorsal fin, called adipose fin.  The dorsal and anal fins mainly help the fish not to roll on their sides.

The tail fin primarily meets propulsion functions so that the fish can move forward. The paired fins help with steering, stopping and floating

Scales on most bony fish (most freshwater fish other than ganoid-sized fish, and catfish that do not have scales) are ctenoid or cycloid .The first ones have irregular edges that have small projections called Ctenii on their posterior edge. The cycloid scales have smooth rounded edges. The bass and most other spined fish have ctenoid scales

The bass and most other fish with spines have ctenoid scales. Most fish also have a very important mucus layer that covers their body which helps them prevent infections.

In many freshwater fish, the fins are supported by rigid spines that are usually quite sharp playing a defensive role. The dorsal and caudal fins are constituted by rays, which are less rigid and frequently branched.



The gills are feathery organs filled with blood vessels that make up the fishes´ respiratory system. They are covered by a flexible bone plate called «operculum».

The water is «inhaled» by the fish through the mouth, passes over the gills and is «exhaled» under the operculum. As water passes over the thin gill walls, dissolved oxygen enters the blood and travels to the fish cells.

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Depending on the species of fish, the eyes can be located in lateral or dorsal position, giving them a monocular vision. There are even fish with atrophied eyes, others that lack them, and it is noteworthy those that have each eye divided into two parts which helps them to maintain a simultaneous view above or below.

Some fish may have other structures in the head, such as whiskers or beards that perform sensory functions to find food even when there is not enough light.   

They carry out gustatory and tactile functions; characteristic very common in the fish that feed on the water substrate. Taste is also present in the form of buttons inserted in the mouth, even in the esophagus or larynx. Smell, on the other hand, is represented in homonymous pits.

One of the organs located on the outside of the fishes ‘body is the lateral line, which is a sensory organ that detects water movements and vibrations.


The mouth´s shape is a good clue to know what the fish eat; the bigger it is, the larger the prey they can consume. Fish may or may not have teeth, depending on the species. Fish like the chain pickerel and gar have canine-shaped teeth.

Other fish have less obvious teeth, such as the cardiform teeth in the catfish, or vomerine teeth that are small patches of teeth on the roof of the striped bass´s mouth. The Grass carp and other small fish have pharyngeal teeth located in the throat that change from their branchial arches to grind them.

The anatomy of fish: Internal Anatomy


This is the fishes´ primary structure on which their body is built. It is connected to the skull in the front of the fish and to the tail in the back. The vertebral column is made up by numerous hollow vertebrae, which harbor and protect the delicate spinal cord.

Spinal cord

It connects the brain with the rest of the body and transmits sensory information from the body to the brain, as well as the brain’s instructions to the rest of the body.


This is the control center where automatic functions such as breathing and also higher behaviors occur. All sensory information is processed here.

Side line

It is one of the fishes´ main sensory organs since it detects vibrations under water and is able to determine the direction of their origin.

Swim Bladder

This hollow, gas-filled balance organ allows a fish to conserve energy by maintaining neutral buoyancy (suspension) in the water. Fish trapped in very deep water sometimes need air to be released from their swim bladder to return to shallow waters, due to the difference in atmospheric pressure at the surface of the water. Species of fish that do not have a swim bladder sink to the bottom if they stop swimming.


They allow fish to breathe underwater.


They filter the liquid waste materials from the blood, which are then expelled from the body. The kidney is also extremely important in regulating the concentrations of water and salt in the body of the fish, which allows certain species to exist in fresh or salt water, and in some cases in both, such as snook or tarpon.

Stomach and Intestines

These organs break down food and absorb nutrients. Fish, like bass, which are piscivorous, have very short intestines because such foods are easy to decompose and digest chemically.

Fish like tilapia that are herbivores require longer intestines because the plant material is usually hard and fibrous and more difficult to decompose into usable components.

Pyloric caeca

This organ with projections similar to fingers is located near the junction of the stomach and intestines. Its function is not completely understood, but it is known to secrete enzymes that aid in digestion. They can work to absorb digested foods or do both.


This is the waste disposal site of the fish body. It is also the outlet for eggs or sperm during spawning.


This important organ has several functions. It helps digestion by secreting enzymes that break down fats and also serves as a storage area for fats and carbohydrates. The liver is also important in destroying old blood cells and maintaining adequate blood chemistry, in addition to playing a role in the excretion of nitrogen (waste).


It circulates the blood throughout the body. Oxygen and digested nutrients are sent to the cells of various organs through the blood, and the blood carries waste products from the cells to the kidneys and liver for disposal.

Gonads (reproductive organs)

In adult bass females, the mass of bright orange eggs is unmistakable during the spawning season, but is still generally identifiable at other times of the year. The male organs, which produce milk to fertilize the eggs, are much smaller and white, but are in the same general location. The eggs, of certain fish are considered a delicacy, as in the case of sturgeon caviar.


They provide movement and locomotion. This is the part of the fish that is normally eaten, and makes up the fish fillet.

How Do Fishes Reproduce?

The fish´sexual life is much more active than you may think. The vast majority are oviparous, which means that the female lays a number of eggs, which the male then fertilizes dispersing the sperm into the water. For example, the sea bream or sea bass reproduces in such a way.

But there are also ovoviviparous fish, in which fertilization occurs via a copula. Then the female lays the eggs already fertilized and after a while are the juvenile are born. This is the case of the cat sharks (Scyliorhinus canicula) of the Galician coast.

There are also viviparous fishes, in which the sex identification is not very difficult since differences between male and female are very notable. The female usually tends to be larger than the male and during incubation, her abdomen turns swollen; the urogenital orifice area becomes dark and ultimately acquires a more square profile.

After internal fertilization fry develop inside the mother nurturing from the placenta. It may happen that female stores sperm in her body, so several fry can be born in a same mating (between 30 and 200 specimens)

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How Do  Fish Swim?

Fish displace by moving the tail on both sides, from left to right and from right to left. When they move their tail, a volume of water is displaced backwards which causes a frontwards propelling reaction.

The propulsion is perpendicular to the tail, and therefore the fishes only take advantage of the component projected on advance line. They counteract the rest by moving their head from side to side.

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Types of Fish According to their Diet.

From the point of view of food there are different groups, depending on what they eat. There is the group that includes carnivorous fish, aquatic herbivorous vertebrates, the so-called omnivores and other fish with more sophisticated alimentations habits.

Carnivorous Fish.

Carnivorous fish feed on other smaller fish, but also on a large number of invertebrate animals. The most common are marine insects, polychaete worms, mollusks and crustaceans.

Sharks and tunas are examples of carnivorous fish. Carnivorous fish employ different capturing strategies since each species has perfected its technique depending also of which is its objective and of the habitat in which they live.

However, there are two general behaviors that can summarize the way in which predatory fish are able to capture food. The first is active hunting or persecution. With this method the fish must be able to develop high speeds to prevent the prey from escaping.

In many cases what they do is attack banks of smaller fish, such as sardines, and thus ensure that they will be able to catch one among many specimens. The second method is stalking, in which the fish hides, camouflages or uses lures to attract its prey and catch them.

In this case the fish must also be very fast, but more in explosive movements and not as durable as in the fish that use active hunting.

Carnivorous fish have certain adaptations that allow them to eat whole animals. For example, they have small teeth that allow them to hold the prey and chop it into pieces to improve digestion.

Larger predators such as the aforementioned sharks or tunas have teeth that are much more developed than the others.

Herbivorous Fish

They obtain their nutrients from foods of vegetable origin like plants and algae, depending on the depth in which they live. This type of animal needs to be feeding continuously since every time they do it they really get few nutrients.

In addition, the digestion of plant-based foods is much more complicated than that of animal products, so herbivorous fish are animals that have their digestive system functioning almost all day.

A characteristic related to these fish, is that most of them also consume animal foods, for what it is complicated to speak of strict herbivorous fish, since very few species feed only on plants.


Omnivorous fish.

There are many fish that belong to this group, since they can feed on both plants and animals, depending on the circumstances. Omnivorous fish are much more opportunistic animals and able to adapt to the environment they inhabit.

These animals don´t eat the same preys than carnivorous fish, since in most cases their intake is limited to small invertebrates and no other fish.

There are many examples of omnivorous fish, one of the most surprising being that of piranhas. These fish live only in fresh waters of South America and, despite their reputation as merciless carnivores; they also frequently feed on marine vegetables.

Filtering  Fish.

This is an exclusive pattern of some fish. This process consists in intaking and filtering large amounts of water to obtain food from microscopic organisms or of very small size that can inhabit the vital fluid.. The microscopic living beings in the water are known as plankton and there are two basic types, zooplankton (of animal origin) and phytoplankton, (of plant origin).

Since the filter fish don’t  distinguish between one type of food and another, this group has to be considered within the omnivores.

The filtration process is quite diverse and depends on the species. A typical example is found in animals such as sardines and anchovies.

In these fish there are formations derived from the gills known as gill rakers that make up the filter where the plankton is trapped, to be later taken to the esophagus and thus begin the digestion.

Feeding by filtration, in addition to being very common in fish is also common in many other invertebrates and is even found in aquatic mammals such as baleen whales.

Detritivore Fish.

In the wide field of fish feeding we must not forget the so-called detritivore fish, that is, opportunistic aquatic vertebrates that feed on the waste of other fish. It is common to see them, in aquariums since they comprise a wide range of species integrated in the family of Callichthyidae, better known as armored catfishes.

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