The Anatomy of Fish

Fish swimming in an aquarium tank
  • 01 of 09

    Body Temperature

    cold blooded fish and temperature

    By definition and nearly always, fish are cold-blooded creatures. This indicates that they maintain a temperature that is close to that of the water around them. In contrast, water-dwelling mammals like the whale and water rats, which are similar to us, typically maintain a significantly greater internal temperature than the water they are in, despite the fact that the water may be much colder than what would be necessary for a mammal to exist. Hypothermia occurs when a mammal is exposed to chilly water for a period of time that is greater than its ability to regulate body temperature will allow. The mammal's heart slows down throughout this process, its internal core temperature drops, and finally it dies.

    Like mammals, all have a certain temperature range in which they can survive. The fish have no internal temperature regulation, thus they will simply perish if the tank temperature veers much outside of this range. Depending on the typical temperatures of their native waters, several fish species have varying temperature requirements.

  • 02 of 09

    How Fish Breathe

    Aquarium fish have a vertebral column, which they share with humans and other mammals. Fish and mammals have the same basic structure and basic system of bones and organs. Even more interestingly, many times, certain fish species display parental behavior, communicate a feeling of family, recognize particular individuals, and even display emotional cues. Although there is still much we don't know about aquarium fish, we should nevertheless appreciate and care for them since they are more than simply ornaments.

    The gills, which are leaf-like structures that are typically found four on each side of the neck in a pouch covered by the operculum, or bone gill cover, are what fish use to breathe, however they typically only collect oxygen from solution in water. (The labyrinth fish, like the gourami or the betta, are an exception since they really breathe air using a specific organ called a labyrinth.) Water is ingested from the mouth and pushed across the gills, exiting via a slit between the operculum and the body. The gills are richly supplied with blood vessels.

    The fish's requirement for oxygen and the amount of oxygen present in the water around them influence how quickly they breathe. Of course, the aquarium has to have enough oxygen in the water itself, not simply on the top. The fish cannot absorb oxygen from water bubbles; the gills can only do so when there is a chemical in the water that contains oxygen.

  • 03 of 09

    Basic Body Parts

    Here is a picture guide labeling the different parts of a freshwater aquarium fish.

  • 04 of 09

    Internal Organs

    Like with mammals, there are numerous exceptions to the norm in the world of fish, but in general, the common species seen in freshwater tanks have typical anatomy.

    The fish's body is mostly made up of two massive lateral muscles on either side of the spine, which are split into segments that correspond to the vertebrae by sheets of connective tissue. Almost each fish that is prepared in a restaurant or shown in a book exhibits this morphology. The primary organ for swimming is this one. Since the internal organs frequently take up very little space at the front, the fish's apparent trunk is actually more of its tail (not to be confused with the tail fin).

    The forward placement of the anal fin's commencement, which denotes the end of the digestive system, points to the location of the organs. With the absence of the lungs and chest cavity, fish have all the typical organs seen in humans, including the stomach, intestines, liver, spleen, kidneys, and so on.

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  • 05 of 09

    Skin and Scales

    Fish do have skin that is coated in scales, and that skin may suffer from skin disorders, be harmed by sharp things, and be impacted by sunlight in addition to serving as protection for the internal organs. The skin may be completely bare or covered in scales or bony plates, which themselves are protected by an outer layer.

    The fish's appearance and color may be caused by skin pigments rather than scale color or formations, as in the case of the calico goldfish, if the scales are transparent as opposed to opaque. The Corydoras, or South American armored catfish, has bony plates (Plecostomus).

  • 06 of 09


    Fish have two paired and three unpaired fins nearly always. The paired pectoral and pelvic (ventral) fins attach to bone girdles in the body that are analogous to our own pectoral and pelvic girdles and represent, respectively, the arms and legs of humans.

    The dorsal, anal, and tail or caudal fins are the unpaired fins, as may be seen in the accompanying illustration. These fins are supported by rays, which are occasionally formed of cartilage and occasionally of bone. The dorsal fin of certain families is completely divided into two halves, the forepart having spiny rays and the rear part having split rays. There is a little adipose fin in the characins and certain other species that is made of fatty substance and lacks fin rays.

  • 07 of 09

    Air (Swim) Bladder

    The air bladder is another distinctive organ seen in fish. This is a lengthy bag that is inside the human cavity and is filled with gas. It may be completely sealed off, or it may use a duct or tube to interact with the digestive system. It can occasionally be split into two quite different sections that talk to one another. The fish's air bladder regulates its specific gravity, much like a submarine's drive tanks control its buoyancy, or more accurately, the opposite.

    Additionally, the center of gravity can change in fish with split bladders. The so-called lungfish, which suck air into their air bladders and breathe genuine air in a manner like to that of mammals, hint at the fact that it truly correlates to the lungs of higher vertebrates.

    These so-called "lungfish" are actually "Labyrinth Fish," members of the Anabantid and other fish families. The gills, which these fish also have, are a completely distinct organ from the labyrinth organ. When the water these fish swim in gets so polluted that it has little to no oxygen or when they are stranded in a puddle that is too tiny to hold enough oxygen to preserve their survival, the labyrinth is deployed in nature. The air is carried via or into the labyrinth organ, which is located near the gills, and out through the mouth or the gill slits, giving the impression that the fish are gasping at the water's surface.

  • 08 of 09

    Lateral Line

    The majority of freshwater aquarium fish have a line that runs from the head along the side of the body, which may be seen with close observation. This is a group of tubes with stiff bristles at the base and a sticky secretion within. This "lateral line" is a specific component of the lateral line system that helps fish detect impediments that are difficult to notice from a distance, warn of danger, aid in distance perception, and avoid predators.

    Continue to 9 of 9 below.
  • 09 of 09


    Fish do really have what appear to be nostrils, despite the fact that this may seem strange. Fish really frequently have four nostrils. As they do not open to the mouth or gills, they are really scent organs and have no role in breathing.

Last Word

Much can be learned about fish anatomy and it is a fascinating subject. We hope you have learned something by unlocking some of the mysteries of those pets we call freshwater aquarium fish.


"What is Hypothermia? Centers for Disease Control and Prevention. " ;