Fish Biology

Most fish species breathe in the water through gills. As water passes over the gills, oxygen is extracted from the water. Carbon dioxide is released into the water. A small number of fish are able to breathe air. The skin of most fishes are covered with scales.

Fish are cold blooded. The temperature of the water controls the body temperature of fish. Fish eat plants and other animals. Food is digested in the esophagus, stomach and intestines.

Fish move in the water from the movement of their tails and fins.


Fish are members of the phylum Chordata. There are three major classes of fish:

  • Agnatha: fish that lack a jaw, paired fins and stomach (e.g. lamprey and hagfish).
  • Chrondrichthyes: fish with a skeleton made up of cartilage instead of bone (e.g. sharks, rays, and skates).
  • Osteichthyes: possess a bony skeleton, paired fins and a jaw (e.g. lungfish, eels, sunfish).


Fish can be found in almost every body of fresh and salt water. This includes environments that range from high elevation mountain streams to the great depths of oceanic trenches. Over 32,000 species occupy these diverse aquatic ecosystems.

Fish and Humans

Today, fish make up a significant portion of the human diet. It is a major source of food protein in many countries. Fish are prepared and cooked in a wide variety of ways – from raw to fried to baked.

In some parts of the world, overfishing has threatened some fish with extinction.

Improving Your Reading and Thinking Skills

Think for a few moments. Try to recall the major ideas in this article. When you are done, click the box below for a list of main ideas.


How did you do? With practice, you should be able to identify many of the major ideas. You may even discover some important ideas that aren’t listed.


Classification of Fish, Animal Worlds.


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13 comments on "Chordata Phylum: Classes of Fish"

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Oxygen at nahango mula sa tubig carbon.


Most fish special breathe in the water through gills.

Webster Gabiana

Ang pretekison ng isda ay mga skal.

Webster Gabiana

The protection of fish are skail .
The ader fish are slaimy.
the other fish live in the gross water.

Webster Gabiana

. Fish cold-blooded and aquatic vertebrates.