11.2 fish

Warm-Up

Write down five different characteristics of fish that you have observed. Once everyone is finished we will discuss them as a class.

SECTION 2 : FISHES

Chapter 11: Fishes, Amphibians, and Reptiles

Characteristics of Fishes

A fish is a vertebrate that lives in water and uses fins to move.

Fish live in water, have fins, obtain oxygen through gills, have scales, and most are ectotherms.

Fish are the largest group of vertebrates; almost 50%.


Fish get oxygen from water. To get oxygen, they open their mouth, let water in. The water then goes to the fish’s gills where it will take the oxygen and spread it to the rest of the fish. Then the water is sent out of the body.


Once oxygen has entered the fish’s gills it will go in a loop through the fish’s body until it has spread all of the oxygen evenly.

A fish has a closed circulatory system like most vertebrates.

The blood in a fish goes from the gills to the heart, and back again.


All fish have fins and these help fish to swim.

A fin provides a large surface to push against the water; it’s kind of like an oar.

Each fin has thin membrane stretched across bony supports.


Most fishes have external fertilization. This means that they lay eggs outside their body. Once the eggs are laid a male fish will come and fertilize them by spreading a cloud of sperm over them.

Some fishes, like sharks, have internal fertilization. This means they carry their young until they are born.


Fishes have nervous systems and sense organs that help them find food and avoid predators.

Fish may have whiskers that help them smell, taste, and feel. This can help them find food and it can help them know when to avoid a dangerous predator.

Jawless Fishes

The major groups of fishes are jawless fishes, cartilaginous fishes, and bony fishes.

Jawless fishes are unlike other fishes since they have no jaws or scales.

Cartilage is what makes up a jawless fishes skeleton.

Hagfishes and lampreys are the only kinds of jawless fishes that exist today.

Cartilaginous Fishes

Cartilaginous fishes have jaws and scales. They also have skeletons made of cartilage.

Sharks use currents and moving to get oxygen whereas rays will move less and get water through holes behind their eyes.

Cartilaginous fishes are usually carnivores.

Bony Fishes

A bony fish has jaws, scales, a pocket on each side of the head that holds the gills, and a skeleton made of hard bones.

Bony fish have a swim bladder which is an internal gas filled sac which helps the fish stay stable at different depths in the water.

95% of all fishes are bony fishes.

They can live in salt and fresh water.

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11.2 fish

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