Fish Anatomy

Fish are cold blooded animals. What this means is that fish can't regulate their body temperature like warm-blooded animals can. This means that a fish's body temperature is dependent on the temperature of the water it is in.

Fish are able to swim through the water by contracting their body and tail muscles. They use their fins to help steer them through the water and also for balance.

Most fish have scales, but not all of them do. All fish produce mucous on their bodies that helps to protect them from bacterial and fungal infections.

Ever wonder what the internal organs of your aquarium fish look like? Below is a diagram that shows the basic anatomy of a teleost fish. Just match the corresponding letters with the fish body part.

Fish Anatomy
  • 1. Liver
  • 2. Stomach
  • 3. Intestine
  • 4. Heart
  • 5. Swim bladder
  • 6. Kidney
  • 7. Testicle
  • 8. Ureter
  • 9. Efferent duct
  • 10. Urinary bladder
  • 11. Gills

Many of the internal organs in fish are also common to many other animals, including humans. These organs serve the same function in the fish as in other animals and so I won't go into any detail on these.

However, fish also have gills and a swim bladder, which humans and other land animals don't have, and so I'll go into a little more detail about these.


The purpose of a fish's gills are to enable it to obtain oxygen from the water. A fish's gills contain special membranes that absorb oxygen and also release carbon dioxide. Basically a fish's gills has the same function as a person's lungs - the gills are where gas exchange occurs.

Water has oxygen dissolved in it, which the fish needs to survive. The fish takes water in through its mouth and this water passes over the gills. The gills contain lots of blood vessels. Oxygen is absorbed as the water passes over the fish's gills, and this oxygen enters the fish's bloodstream so the fish can use it for metabolic processes. Carbon dioxide is a waste product of the fish's metabolic processes and it diffuses out through the fish's gills.

The gills are located underneath an opening called the operculum. Basically, the operculum is the "gill cover."

Fish are unable to obtain atmospheric oxygen through their gills. A few fish have developed a labyrinth organ that allows them to breathe atmospheric oxygen, however, these fish must obtain oxygen from both sources - from the water through their gills and from the atmosphere through their labyrinth organ. This organ evolved in fish species that live in low oxygenated water, such as bettas and gouramis.

Swim Bladder

A fish's swim bladder is basically a sac filled with air. Fish are able to regulate the amount of air in the swim bladder so that they can adjust their position in the water. Sometimes you'll see an ill fish that is no longer able to regulate the amount of air in the swim bladder and they will have odd swimming movements or they will swim at an unusual angle.

Not every fish has a swim bladder, but most fish do.

Nostrils or Nares

There are two small holes on each side of a fish above its mouth. These are the fish's nostrils or nares. Fish don't breathe through these, but they can detect odors through them. These nares allow fish to smell odors.

Lateral Line

The fish's lateral line is located on each side of the fishes body right in the center (along the path of the fish's spine). The lateral line serves as another sensory organ for the fish. Along the lateral line are specialized cells called neuromasts that can detect vibrations in the water as well as the direction the vibrations are coming from.

Fish also have an inner ear that allows them to hear sounds. There are bones in the inner ear called otoliths that move in response to vibrations in the water allowing the fish to hear sounds.

So, now you know a little about your fish's anatomy. This usually doesn't come up for most aquarists, but it is still useful information to know.