Fish Characteristics

Greetings friend of the fish, welcome to our article on the characteristics of fish where you will learn about the peculiarities of the first vertebrates that originated on Earth.

The characteristics of fish are determined by their physical environment, which is water. That is, the physical characteristics of the water determine the morphology of the fish.

Water is denser, absorbs more light than air, and has less oxygen. The constitution of the fish, fully adapted to this environment, allows it to fully dominate the aquatic environment.

Introduction to fish morphology

We have already seen in everything about fish the three large groups of fish differentiated by the jaw, the skeleton and the fins:

  • Agnates: Fish that do not have jaws or scales.
  • Gnathotonus: jawed fish
    • Chondrichthyans: They are characterized by a cartilage skeleton and visible gill slits.
    • Osteichthyos: Characterized by their bony skeleton and the operculum that protects their gills.
      • Sarcopterygians: They have lobed fins.
      • Actinopterygians: Their fins are provided with rays.

Let us now see the constitution of the fish in parts, stopping at the characteristics of the most important fish.

Main characteristics of the fish

The head and digestive system of the fish

In the digestive system we find one of the characteristics of the fish that separated them into two large groups:

A very primitive 1st group of jawless species of which few species remain today. For example the lamprey, whose mouth is shaped like a disk.

A 2nd group with the species that developed a jaw and accessed a greater «gastronomic variety» that included the intake of plants and other organisms.

The mouth of this second group can be found in three main arrangements:

  • Terminal: the mouth is at the anterior end of the head.
  • Superior: the mouth is directed upwards.
  • lower: -subterminal- the mouth is directed downwards or under the fish.

Regarding the teeth, many fish have them in their mouths, and some in their throats (pharyngeal teeth).

Another curious fact about the characteristics of fish is that shark teeth are modified scales.

There are fish adapted to search for food on the bottom, such as loaches or coridoras, and they have their mouths in a ventral position and those fleshy protuberances called beards, barbels or mustaches, and also protrusions or spines.

the body of the fish

There is an enormous variety of shapes in the fish, although the most common is the fusiform. We have the filiform (eel-shaped) or vermiform (worm-shaped), and also the laterally or vertically “crushed” ones.

One of the characteristics of the fish that is common to all of them is that they have the Lateral Line on both sides.

And what is the lateral line of the fish?

It is a line of sensitive cells that act as a detector of vibrations and nearby currents.

This sensory organ allows rapid detection of movements and vibrations through pressure changes generated by other organisms. It also makes navigation and orientation easier.

The appearance of the fish says things:

  • Flat, slender fish such as angelfish often live among dense vegetation.
  • Fish with sharp teeth correspond to large predators with powerful jaws such as piranhas.
  • Fish with downturned mouths and flattened bellies indicate that they live and feed on the bottom, such as catfish.
  • Fish with flattened backs and upturned mouths, usually live on the surface and feed on insects.

Among all the characteristics of fish, one of the ones that we like the most is that some fish have photophores, which are light-emitting organs that look like luminous points. The purpose is to confuse the predator or attract the prey.

Characteristics of fish skin

Beneath this mucous layer, many fish are covered with scales, although not all. The so-called naked fish do not have an outer covering on the skin.

Regarding the scales we find different types:

  • Placoid: Also known as dermal denticles, they are tooth-like (made of enamel covered dentin.) Typical of rays and sharks.
  • Ganoids: flat and overlapping. Typical of Polypteridae and Lepisosteus.
  • Cycloids: oval and small with growth rings. Typical of Perciformes and Amiiformes.
  • Ctenoids: the same as the Cycloids except for the spines that cover one of the edges. Halibut has this type of scale.
  • other types: there are less common types of scales such as scutum, which is a thickened shield-like scale, which can be turned or spined. The monocentridae is an example.

The locomotor system of fish

The fish uses its hydrodynamic body and fins to move perfectly through the water, and the swim bladder to stabilize itself at any depth. These two characteristics of fish are the most popularly known.

Swim Bladder

The swim bladder is the organ that gives the ability to control buoyancy. It allows the fish to stay in one position, descend or ascend without having to swim.

This organ is absent in fish that are characterized by swimming very fast, families of Tuna and Mackerel, for example.


The fins are one of the most remarkable characteristics of the fish morphology. You can find both fish with an even number of fins, and fish with an odd number of fins.

Fin is each of the external membranes that serves the fish to achieve movement and direction. These fins are moved by the powerful muscles that fish have for swimming. And each fin has its own functions

The most commons are:

Paired Fins

  • Pectorals: located behind the operculum, they have the function of creating dynamic lifting force, helping stabilization.
  • Pelvic or ventral: behind the pectoral fins. Particularly the Gobiidae have a sucking disc formed by the union of the pelvic fins that allows them to «stick» to surfaces.

odd fins

  • Dorsal: located in the back area, they stabilize and direct. They can have up to three dorsal fins.
  • Anal: next to the anus, they have a stabilizing function during swimming.
  • Flow: the tail fin is in charge of the impulse.

There are other more specific types of fins such as the adipose fin, the caudal keel or the finlets. Very curious is the dorsal fin of the Lophiiformes, modified into an illicium and an esca, or what is the same, its own rod and bait.

There is also a classification of fish according to the shape of their caudal fin, but we leave the topic of fins here so as not to extend this post too much on the characteristics of fish.

Thorns and Lightning

In most of the fins of bony fish we can find spines or rays, being able to find fins with:

  • Thorny rays.
  • Soft rays.
  • Combination of both.

The spines are usually stiff and sharp, and although there may be species with flexible spines, they will always be “in one piece” and never segmented.

The spines have different uses, such as defense in catfish, or as an anchoring system in cracks for balistidae.

The rays are usually flexible, soft, segmented and can also be branched.

fish respiratory system

The respiratory system is another of those peculiar characteristics of fish. A fish uses gills, respiratory organs that extract dissolved oxygen from the water and expel carbon dioxide.

The respiratory system of the fish sucks in water and it passes through the gills, which are made up of the gill filaments. These filaments are the ones that carry out the gaseous exchange between oxygen and carbon dioxide.

Most protect their gills by the operculum, which is a bony covering. Few others, such as sharks, have several gill slits.

fish excretory system

Another of the main characteristics of fish is that most expel nitrogenous waste in the form of ammonia. A part is expelled through the gills and the rest through the kidneys, which are responsible for filtering and cleaning the blood.

In the functioning of the kidneys we also find a notable difference that divides marine fish, freshwater fish and those that can adapt:

  • saltwater fish: the kidneys expel as little water as possible, since they tend to lose water due to osmosis.
  • Freshwater fish: The kidneys expel large amounts of water.
  • Adaptable fish: kidneys that change their function, allowing changes between freshwater and saltwater.

The nervious system

Many fish have highly developed sense organs of taste and smell, and nearly all have good color vision.

Most have ears, but they are not very sensitive. Some have sensory organs on the lips, head, and barbules.

Another lesser-known characteristic of fish is that there are some fish that have organs that detect small electrical currents, such as the puffer fish. There are others capable of generating their own electricity, such as the well-known electric eel.

Do fish feel pain?

Dr. William Tavolga conducted experiments that proved that fish responded to pain and fear using electric shocks.

Scientists at the University of Edinburgh, in 2003, also came to the same conclusion.

This time they injected bee venom into the lips to see how they rubbed similarly to a mammal to relieve their pain. In addition, it was shown that in the face of pain, neuronal functioning had a model similar to that of humans.

It is also true that there are scientists who do not hold the same opinion, such as
Professor James D. Rose, who in short will say that he maintains that the fish is not aware of pain since their brains do not have a neocortex.

The reality is that it is still a controversial issue, with conflicting opinions.

To give you an idea, the RSPCA considers it unlikely that fish feel pain like people, but since there is evidence indicating their ability to feel pain, they prosecute those who are cruel to fish.


Fish are dioecious animals whose reproduction is sexual through eggs.

Most fish are oviparous, that is, they expel their eggs into the natural environment and, as they do not have copulatory organs, fertilization is external. For this reason, reproduction is determined by external factors such as temperature, oxygen, light, predators…

On the other hand, there are also two other well-identified reproductive patterns, viviparous and ovoviviparous, which are characterized by requiring internal fertilization and giving birth to already formed offspring.

The reproductive cycle depends on the species and occurs in certain periods: once a year, every month, etc.

There are species that form pairs for life, even letting themselves die when the pair is missing.

Although most fish are dioecious animals, there are species whose individuals can undergo a sex change. It is called hermaphroditism and as an example you have Xiphophorus helleri or the grouper. We can do the following division:

  • Synchronous hermaphrodites: have testicles and ovaries at the same time.
  • Sequential hermaphrodites: They have both types of tissue in their gonads, with one type predominating while the fish is of the corresponding genus.

Incubation time is determined by species and temperature. The higher the temperature, the less time.

Oxygen is also decisive, since if there is not enough of it, the eggs will not reach term. There are species that oxygenate their eggs by making bubble nests, such as the betta fish.

The newly hatched young, called fry, bear the yolk sac and do not resemble the adults. During growth, the fins and the reproductive system appear.

Sexing or distinguishing sex is not easy. Outside the breeding season, it is difficult to differentiate between males and females, since sexual dimorphism is sometimes negligible. Sometimes the coloration, sometimes a bulging belly, the shape of the fins, the behavior…

Conclusions on the general characteristics of the fish

Well, let’s end this post here. There are many things that we have left in the inkwell, but we want to offer light articles that are not very technical or dense. Something easy to read, as well as educational.

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