# Fish

Fishes are animals that live in the water. All fishes are vertebrates and have a spinal column made of either cartilage or bone. Fishes breathe in the water through structures called gills, which are located on either side of the head.
Gills act as the "lungs" of fishes. Gills are fine structures with lots of tiny blood vessels, called capillaries, close to their surface. From these capillaries fish take in oxygen, and release carbon dioxide and other waste products when water passes over them.
Most fishes are covered in protective scales. These are different than the scales of reptiles and birds. Fish scales actually grow inside the skin of the fish, while reptile and bird scales grow on the surface of the skin.
Most fishes are ectothermic, meaning they rely on the environment around them for their body temperature. Some fishes like tuna are endothermic, and produce their own internal body heat.
Fishes have a simple two-chambered heart with one atrium and one ventricle. Blood is pumped from the heart to the gills.
In addition to gills, some fishes can take in oxygen from the air. These include lungfishes, gouramies and bettas, and some catfishes. They may swallow air and take in oxygen through their intestines (catfishes), or have specialized organs that are similar but no the same thing as our lungs.

## Species of Fish

There are over 30,000 known living species of fish.
Some commonly known species of fish include:
* Anchovy
* Clownfish
* Salmon
* Sharks
* goldfish
* guppies
and many others.

## Fish-Human Relationships

Fish and humans have been interacting for many thousands of years.
The scientific study of fishes is called ichthyology, and a scientist that studies fishes is called an ichthyologist.
Fish are a very important food source for humans. In many times and locations, fish have provided the majority of the animal protein that people have access to.
Fish have been kept in captivity by humans for several thousand years. They have mostly been kept as a ready supply of food.
In the last few hundred years, fish have also been kept as ornamental pets. Goldfish, the first fish known to have been kept for ornamental purposes, have been kept and bred for their beauty in China for about 1000 years.
Angling, or fishing, has been practiced as a recreational sport for about 500 years, though no one is certain how it developed from a technique solely for collecting food into the funs sport it has become in many parts of the world today.
With human population growth, in some places more fish have been captured for food than wild populations can replace. Some species of food fish are threatened or in danger of extinction.

## Fish vs. Fishes

In English, the word "fish" is properly used to refer to one individual fish, or many of the same kind of fish. For example, a salmon is a fish, and salmon are fish.
The word "fishes" refers to a group of fish of more than one species. For example, salmon and anchovies are fishes, and a group of salmon and anchovies are fishes.

## See Also

* Aquaria - Includes additional fish species descriptions and how to care for them.

## External Resources

* [FishBase] with information on almost every known species of fish.
* [How to become an ichthyologist] with information on how to have a career working with fishes.
* [Fish] on Wikipedia.