Fish Anatomy & Physiology
Veterinary & Aquatic Services Department, Drs. Foster & Smith

There are few creatures on earth that have developed such an interesting and unique set of physical characteristics as the fish. Their special adaptations have allowed them to survive in an environment completely different than humans. Water is 800 times denser than air. Water also contains less than 2% of the oxygen contained in the air. Sound and light are very distorted in the water. Yet despite these difficult obstacles in life, fish have some unique anatomical adaptions that allow them to flourish. If aquarium owners have an understanding of these basic adaptations, it will help them to better care for their fish.

The skin

Unlike most land dwelling animals, fish have evolved several unique traits in their outside coverings including scales, fins, and protective mucous. Most fish have 7 fins, although some have 6 and some 8. These fins allow fish to steer and move forward and backward. Because water is 800 times denser than air, fish require a tremendous amount of muscle strength and coordination. The powerful tail fin coupled with a large efficient muscle-filled body gives the fish the necessary strength. On the outside of the skin most fish have compact rows of protective scales. These scales are firmly attached to the skin and are primarily made of hard calcium. The scales offer unsurpassed protection against injury and infection. Catfish have evolved without scales, but some have hard bony plates and others have sharp spines in some of their fins that help keep predators at bay. Over the tops of their scales, fish secrete a mucous covering. This mucus is very effective at trapping and immobilizing bacteria and viruses and even contains antibacterial-like agents that will help kill the trapped bacteria. Another important trait of the mucous is to reduce friction and allow the fish to move through the water more easily.

The swimbladder

The swimbladder is a unique organ found only in fish and is sometimes called the 'air bladder.' It is a smooth, gas-filled organ found in the abdomen of most fish. A fish will either add to or decrease the amount of air in the bladder to help it move up or down in the water. Without the air bladder, the fish would have to swim continuously to keep from sinking to the bottom. By adjusting the amount of air in the bladder, fish can adjust the depth at which they float and extend very little energy in the process. One other benefit of the air bladder in some species is that it can be used to receive or emit sounds.

Maintaining salt balance

As in people, maintaining the proper level of salt in the fish's body is critical to proper health. Fish face an exceptionally difficult challenge because they live in a watery environment and tend to 'leak' a lot. The concentration of salt in a freshwater fish is much higher than in the surrounding water, so salt is constantly leaking out of the fish into the water. To compensate for this, fish have developed several solutions. The first and foremost is that they ingest a very large quantity of water and as a result produce a large quantity of urine (10-20 times as much as land mammals). Their kidneys extract the small amount of salt from the ingested water and put it back into the fish's bloodstream. The other thing they have is an ion pump in their gills that actually helps take salt out of the water and discharges ammonia and other undesirable products. Saltwater fish have the same problem in reverse. For saltwater fish, the sea water contains a much higher concentration than what is in their bodies. As a result, salt leaks in and the fish has to use its kidneys and ion pumps to excrete extra salt.

Breathing underwater

Breathing underwater is one of the most interesting adaptations of fish. Fish need oxygen just like people, but the trick is getting the small amount of available oxygen out of the water and into the bloodstream. The way that fish accomplish this is through their gills. The gills contain thousands of tiny capillaries (blood vessels). Water is constantly pumped over the gills through a combination of opening and closing the mouth and muscular contractions that force water over the gills. The oxygen is absorbed out of the water and goes directly into the bloodstream.

Water temperature can greatly affect the concentration of free oxygen in the water. As the water temperature increases, the free oxygen concentration decreases. Stagnant or poor quality water also contains less oxygen. A fish that becomes stressed or overly excited in water that has low oxygen content can have severe respiratory difficulty and can die. Because the air contains much more oxygen than the water, some people question why fish cannot breathe out of the water. The reason is that the small delicate lamellae (thin branching structures) in the gills will collapse when the fish is out of the water. This will make it impossible for oxygen to be absorbed. To determine how long a fish can be kept out of the water, hold your breath when you take your fish out of the tank. When you need to breathe, then so does your fish.

Sight and sound

Many fish have an excellent sense of sight and fish see colors. They also have ears, however, they do not have external openings. The ears pick up vibrations and help the fish hear and navigate. Fish also have a unique navigational aid unlike anything found on mammals. The structure is called the lateral line and runs along the side of the fish. The lateral line contains small sensory hairs that can detect even tiny vibrations. This extra organ allows fish to navigate and hunt prey even in low light or cloudy water conditions.


Fish have many unique adaptations that help them to flourish in their watery world. By better understanding their basic differences in anatomy and physiology, we can better appreciate and care for these unique creatures.

References and Further Reading

Bailey, M: Burgess, P. Tropical Fishlopedia. Howell Book. New York; 2000.

Burgess, P; Bailey, M; Exell, A. A-Z of Tropical Fish. Howell Book. New York; 1998.

Burgess, W; Axelrod, H; Hunziker, R. Dr. Burgess's Mini Atlas of Marine Aquarium Fishes. TFH. Neptune City, NJ; 1997.

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