Is fish an animal?

In this post, we will answer the question “Is fish an animal?” and understand why fish are animals, as well as all their general biology, ecology, and diversity. 

Is fish an animal?

Yes, fish is an animal. We can say that an organism, to be considered an animal, must be eukaryotic, multicellular, heterotrophic, and not have a cell wall. The fish has all these characteristics, thus, fish is an animal.


Fish are vertebrate animals that live exclusively in the aquatic environment. They have different sizes, shapes, and colours. They are animals of great economic importance since they are used in our food, in practices such as sport fishing, and also in the aquarium hobby.

Fish are vertebrates that dominate aquatic environments, from puddles to frozen water at the poles. Currently, there are more than 55,000 species that live all over the planet and can measure from only a few millimetres to many meters. Most fish are torpedo-shaped (or an aeroplane), have fins, and the body is covered with scales.

Fish can live in different habitats, some of which are quite unusual. For example, there are fish that live at more than 8,000 metres deep and others that live in the Himalayan mountain range at more than 5,000 metres in altitude. Some live in springs in the middle of the desert between Mexico and the United States, others in caves or hot-water underwater volcanoes. 

Some swim close to the surface like tuna and are called pelagic; those who live where the light is dim are known to be mesopelagic like some lantern fish; benthic fish are those that live close to the ocean floor.


Fish can be classified based on their body structure. The fish with the most basic structure are lampreys that stick to larger individuals and suck blood. They have a cylindrical and elongated body, like an eel, have no scales, and a circular mouth with several layers of teeth that help them attach to their prey.

Sharks and rays, instead of a skeleton of bones, have a skeleton of cartilage and their skin looks like sandpaper due to the disposition of the small scales that cover their bodies. Most prey on large animals, but whale sharks and basking sharks feed on tiny plankton. 

Third, in the classification is the lungfish group. They have this name because they have a structure that resembles our lungs and that is why they can use oxygen from the air when they need it or when the environment dries up. 

The fourth group includes sturgeon, fish that produce caviar. They have a body part with cartilage and broad scales. 

Fifth and last in this ranking are bony fish which comprise 96% of all existing fish. As the name suggests, they have a skeleton made up of bones. They have great biodiversity and variation in colours, body shape, scales, and much more.

Circulatory system

The fish circulatory system is closed and a heart with two cavities is observed: an atrium and a ventricle. 

The blood that passes through the heart is venous, in other words, rich in carbon dioxide. Their circulation is classified as simple, as the blood goes through the heart only once to complete the circulatory cycle. It goes in through the atrium and out through the ventricle. Once the blood leaves the heart, it is pumped to the gills, where the gas exchange occurs.

The reproduction of fish

Fish reproduction varies from group to group. In cartilaginous fish, internal fertilization occurs; in most bony fish, external fertilization occurs. 

In bony fish, it is possible to observe, in some species, indirect development, with the formation of larvae, and the development of a fry stage.

Some fish have internal fertilization such as sharks and rays. A part of them are oviparous; but there are also the viviparous that keep the young inside the uterus until the moment of giving birth, just like the mammals; another part of them carry the eggs, with the young inside, in the uterus being known as ovoviviparous. 

Some fish live confined to small ponds and reproduce annually, others travel miles across the oceans to reproduce in specific regions; some spend their adult life at sea and will reproduce in fresh water and others do the opposite, spend their lives in freshwater (anadromous) and will reproduce in saltwater (catadromous).

Sensory systems

Fish have powerful and efficient sensory organs, such as the lateral line. These structures, located laterally in the fish, allow the animal to capture movements in the water and, consequently, avoid predators.

In addition to the lateral lines, the fish have developed olfactory lobes, which allow the perception of smells, and Lorenzini ampoules, which allow the capture of electrical currents produced by other animals. These ampoules are only found in cartilaginous fish.

Ectothermic animals

Most fish are unable to keep their body temperature constant using physiological mechanisms. Some species, however, can keep parts of their body warmer than others (regional heterothermia), managing to raise the temperature through endothermic heat production. Tuna and some sharks have this ability.

Adaptations of fish to the aquatic environment

To live in the aquatic environment, fish have a large number of adaptations, highlighting the presence of gills for breathing. These blade-shaped and richly vascularized organs allow gas exchange between the water in the medium and the animal’s blood, which constitutes gill respiration. The water initially enters the mouth, passes through the slits in the pharynx, travels to the gills, and then leaves the animal’s body. Coordinated movements of the operculum and mandibles allow water to reach the gills. The swimming movement can also be used to ensure ventilation of the gills.

In addition to the presence of gills, the fish has a body with a hydrodynamic shape, which helps to move in the water. Usually fish have a fusiform body, i.e., elongated and with tapered ends, which allows better swimming. Fish with a fusiform shape can achieve high swimming speed.

In addition to their characteristic shape, fish have a large amount of mucus on their skin, which helps to reduce friction with the water. Mucus, like scales, also plays an important role in protecting fish from pathogens.

Also noteworthy is the presence of fins, which vary in shape, size, and position in each species of fish. These structures have as main functions to maintain the fish balance, to help in the change of direction and depth, and to act as propellers, as in the case of the caudal fin.

Fish also have strategies to avoid sinking, since they have a density greater than that of water. Floating is guaranteed in cartilaginous fish by the presence of a developed liver with a large amount of fat. In bony fish, there is a swim bladder, a hydrostatic organ that allows the fish to swim to the bottom and the surface.

Fish breathing

To breathe, fish have special organs called gills, which are generally four pairs, two on each side of the head. They are protected by a membrane (the operculum), which regularly opens and closes. They are attached to the sidewalls of the pharynx.

The water enters the mouth and is driven to the gills in a continuous stream. Then, the mouth closes and the operculum opens, allowing the water to escape to the outside. The exchanges between oxygen and carbon dioxide take place during the permanence of water in contact with the gills. 

Abyssal fish

Abyssal fish are strangely shaped animals, with large teeth, huge mouths, and large, protruding telescope eyes. They live below three thousand meters at temperatures of 0ºC under great pressure, where they survive because the pressure of their bodies is very high, which results in equilibrium with the environment.

Most abyssal fish feed on other fish. Some have organs that produce light, which act as true reflectors. Many of the abyssal fish have yet to be identified.


In this post, we answered the question “Is fish an animal?” and understood why fish are animals, as well as all their general biology, ecology, and diversity.

If you have any thoughts or doubts, feel free to drop us in a comment below!

Frequently Asked Questions (FAQs): Is fish an animal?

What are Oviparous fish?

Most fish have external fertilization and release their eggs into the environment so that they finish developing and hatch. These fish are known as oviparous. Some catfish and seahorses keep these eggs protected in their mouths and a pouch, respectively.

Do fish sleep?

Fish do not sleep similarly to land mammals sleep. They reduce their cerebral activity and metabolism to rest. However part of their body remains alert.

What are vertebrate animals?

Vertebrate animals are the ones that have backbones, as their name suggests. Fish, amphibians, reptiles, birds, and mammals are vertebrates. 

How do fish breathe?

In general, fish breathe using their gills. The water enters the fish mouth and passes through the gills, where the gas exchange occurs and the blood receives oxygen.


Wootton, R. J. (1991). Fish ecology. Springer Science & Business Media.

Winemiller, K. O., Agostinho, A. A., & Caramaschi, É. P. (2008). Fish ecology in tropical streams. In Tropical stream ecology (pp. 107-III). Academic Press.

Gerking, S. D. (2014). Feeding ecology of fish. Elsevier.