The Anatomy of Fish

Body Temperature

Fish, by definition and almost without exception, are cold-blooded vertebrates. This means that they remain at approximately the same temperature as the water surrounding them. In contrast to the mammals who are water-dwelling such as the Whale or Water Rat, which like ourselves, usually maintain a much higher internal temperature than the water that surrounds them even though the water may be a much colder temperature than a mammal could otherwise survive. When the water becomes too cold, or exposure to the cooler water becomes longer than the mammal’s internal temperature control can keep up with, this is called hypothermia. During this process, the mammal’s internal core temperature cools and the mammal’s heart slows and eventually, the mammal dies.

All freshwater aquarium fish have, much like the mammal, an exact temperature range within which they can survive. If the temperature of the aquarium strays far out of this range, the fish have no type of internal temperature control and will simply die. Different fish species have different temperature needs based on the traditional temperatures of the waters of origin of those species.

How Fish Breathe

Aquarium fish share with humans and other mammals the possession of a backbone, or vertebral column. Fish are built on the same fundamental plan, having the same basic system of bones and organs as mammals. Even more surprisingly, in many cases, some species of fish exhibit parental behavior, express a sense of family, show signs of recognition to specific humans and even exhibit signs of emotion. There is much we have yet to learn about aquarium fish, but they are not just decoration, they are pets that deserve our respect and care.

Fish breathe oxygen, but it is usually absorbed only from solution in water by the gills which are leaf-like organs, normally four on each side of the neck in a pouch covered by the operculum, or bony gill cover. (An exception being the labyrinth fish, such as the Gourami or Bettas which actually breathe air with the aid of a special organ called a labyrinth). The gills are richly supplied with blood vessels, and water is swallowed from the mouth and forced over the gills, leaving by a slit between the operculum and the body.

The rate of the fish’s respiratory movements is partly determined by the need for oxygen and its concentration in the surrounding water. Needless to say, the aquarium must have an ample supply of oxygen within the water, not just on the surface of the water. Bubbles going through the water are of no use to the fish, the gills can only absorb oxygen if it is a chemical within the water. 

Basic Body Parts

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

Internal Organs

There are many exceptions to the rule in the world of fish, as there are in the world of the mammal but, in general fish, the common fish found in freshwater aquariums are standard anatomy.

The fish body is composed mainly of a large lateral muscle on each side of the backbone, divided by sheets of connective tissue into segments corresponding to the vertebrae. This anatomy can be seen in almost any fish cooked in a restaurant or pictured in a book. This is the main organ for swimming. The internal organs often occupy a very small volume, toward the front, so that much of the apparent trunk of the fish is really its tail (not to be confused with the tail fin). 

The area of the organs is indicated by the forward position of the beginning of the anal fin, which marks the end of the digestive tract. Fishes possess the usual organs familiar to students of human anatomy, with the exception of lungs and chest cavity; they have a stomach, intestines, a liver, a spleen, kidneys, and so forth.

Skin and Scales

Fish do have skin that is covered in scales, and that skin can have skin conditions, and be affected by sunlight, damaged by sharp objects and protects the internal organs. The skin may be naked, or it may be covered by scales or by bony plates which in turn have an outer layer over them.

The scales may be opaque or transparent; if they are transparent, the appearance and color of the fish may be due to skin pigments, not to scale color or formations, as in the calico goldfish. Bony plates may be seen in the Corydoras, or South American armored catfish (Plecostomus).

Fins

Fish in almost all cases have two paired and three unpaired fins. The paired pectoral and pelvic (ventral) fins correspond, respectively, to the arms and legs of human beings and connect with bony girdles in the body that correspond to our own pectoral and pelvic girdles.

The unpaired fins are the dorsal, the anal and the tail or caudal fins, as shown in the accompanying figure. These fins are supported by rays, sometimes bony and sometimes made of cartilage. In some families, the dorsal fin is split entirely into two parts, the forepart with spiny rays and the hind part with split rays. In the characins and some others, there is a small adipose fin, composed of fatty material with no fin rays.

Air (Swim) Bladder

In addition, fish often possess a characteristic organ, the air bladder. This is a long bag filled with gas and lying in the body cavity. It may be entirely closed, or it may communicate with the alimentary tract by means of a duct, or tube. Sometimes it is divided into two rather distinct parts, which communicate with each other. The air bladder controls the specific gravity of the fish, as the driving tanks of a submarine govern its buoyancy, or vice versa really. 

In fish with divided bladders, the center of gravity can be altered too. It actually corresponds to the lungs of higher vertebrates, and this fact is foreshadowed in the so-called lungfish, which takes air into their air bladders and breathes actual air in a similar fashion to the way mammals do.

These "lungfish" are actually called “Labyrinth Fish” and are of the Anabantid and other families of fish. The labyrinth organ is an entirely different and separate organ from the gills, which these fish also possess. The labyrinth is used in nature when the water these fish swim in becomes so fouled it contains little or no oxygen or they are trapped in a puddle too small to contain sufficient oxygen to sustain their life. The labyrinth organ is situated near the gills, and the air is passed through or into it and out through the mouth or the gill slits, making it appear that these fish are gasping at the surface of the water.

Lateral Line

Upon careful examination of the majority of freshwater aquarium fish, a line may be observed running from the head along the side of the body. This is a series of tubes filled with a gummy secretion and with stiff bristles at the base. The function of this “lateral line” is to direct vibrations of low frequency, and act as the fish’s ear, it is a specialized part of the lateral line system to warn of danger, help with distance, sense obstacles that cannot be easily seen and avoid predators. 

Nostrils

It may seem odd indeed that fish have what seem to be nostrils, but they do have nostrils nonetheless. In fact, fish often have 4 nostrils. They are actually organs of smell and do not perform any function in breathing since they do not open to the mouth or gills.