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10.3: Fish

Difficulty Level: At Grade Created by: CK-12
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Is this animal a fish?

This mudskipper might not seem like a fish. It's not swimming in a lake or an ocean, and it appears to be using its fins like legs. Mudskippers can breathe through their skin and burrow in the mud, essentially living on land for a brief amount of time. These features are not typical of fish, and, yet, mudskippers are still classified as fish. So what features define a fish?

Characteristics of Fish

What exactly is a fish? You probably think the answer is obvious. You may say that a fish is an animal that swims in the ocean or a lake, using fins. But as we saw with the mudskipper, not all fish spend all their time in water. So how do scientists define fish?

Some characteristics of fish include:

  1. They are ectothermic , meaning their temperature depends on the temperature of their environment. Ectothermic animals are cold-blooded in that they cannot raise their body temperature on their own. This is unlike humans, whose temperature is controlled from inside the body.
  2. They are covered with scales.
  3. They have two sets of paired fins and several unpaired fins.
  4. They also have a streamlined body that allows them to swim rapidly.

Fish are aquatic vertebrates, meaning they have backbones. They became a dominant form of sea life and eventually evolved into land vertebrates. There are three classes of fish: Class Agnatha (the jawless fish), Class Chondrichthyes (the cartilaginous fish), and Class Osteichthyes (the bony fish). All have the characteristics of fish in common, though there are differences unique to each class.

The humphead or Napoleon wrasse shows some of the general traits of fish, including scales, fins, and a streamlined body

The humphead or Napoleon wrasse shows some of the general traits of fish, including scales, fins, and a streamlined body.

How do Fish Breathe?

In order to absorb oxygen from the water, fish use gills ( Figure below ). Gills take dissolved oxygen from water as the water flows over the surface of the gill.

Gills help a fish breathe

Gills help a fish breathe.

How Do Fish Reproduce?

Fish reproduce sexually. They lay eggs that can be fertilized either inside or outside of the body. In most fish, the eggs develop outside of the mother's body. In the majority of these species, fertilization also takes place outside the mother's body. The male and female fish release their gametes into the surrounding water, where fertilization occurs. Female fish release very high numbers of eggs to increase the chances of fertilization.

How Big Are Fish?

Fish range in size from the 65-foot, 75,000 pound whale shark ( Figure below ) to the stout infantfish, which is about 0.33 inches (8.4 mm), and the Paedocypris progenetica carp species of the Indonesian island of Sumatra, which is about 0.31 inches (7.9 mm) long, making it also the smallest known vertebrate animal. The second-largest fish is the basking shark, which grows to about 40 feet and 8,000 pounds. Both of the large sharks may look ferocious, and would probably scare anyone who comes across one in the water, but both species are filter-feeders, and feed on tiny fish and plankton. The tiny carp species is unique in that it has the appearance of larvae, with a reduced skeleton which leaves the brain unprotected by bone. The fish lives in dark acidic waters, having a pH of 3.

Whale sharks are the largest cartilaginous fish

Whale sharks are the largest cartilaginous fish.

Exceptions to Common Fish Traits

There are exceptions to many of these fish traits. For example, tuna, swordfish, and some species of shark show some warm-blooded adaptations and are able to raise their body temperature significantly above that of the water around them.

Some species of fish have a slower, more maneuverable swimming style, like eels and rays ( Figure below ). Body shape and the arrangement of fins are highly variable, and the surface of the skin may be naked, as in moray eels, or covered with scales. Scales can be of a variety of different types.

One of the cartilaginous fish, a stingray, shows very flexible pectoral fins connected to the head

One of the cartilaginous fish, a stingray, shows very flexible pectoral fins connected to the head.

Why Fish are Important

How are fish important? Of course, they are used as food ( Figure below ). In fact, people all over the world either catch fish in the wild or farm them in much the same way as cattle or chickens. Farming fish is known as aquaculture . Fish are also caught for recreation to display in the home or in a public aquarium.

Workers harvest catfish from a fish farm in Mississippi

Workers harvest catfish from the Delta Pride Catfish farms in Mississippi.


  • Fish are ectothermic, have a streamlined body covered with scales, and have two sets of paired fins and several unpaired fins.
  • To obtain fish for food, people catch fish in the wild or farm them.

Explore More

Use the resources below to answer the questions that follow.

Explore More I

  1. Why do gills appear bright red?
  2. When a fish breathes, where does it take in water and where does it expel water?
  3. What direction does a fish's blood flow through its gills?
  4. Why does this system in fishes' gills allow for more energetic fish?

Explore More II

  1. How does the lung fish get oxygen?
  2. How does the lung fish escape heat and drought?
  3. Why is the lung fish's behavior adaptive to its environment?


  1. What are three main features that define fish?
  2. How do fish breath?
  3. Explain how most fish reproduce?
  4. How are fish important to humans?




Farming fish for food.


Having an internal temperature that depends on the temperature of their environment.


Organs that take dissolved oxygen from water.

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Difficulty Level:

At Grade

Concept Nodes:


7 , 8

Date Created:

Nov 29, 2012

Last Modified:

Feb 27, 2015
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