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13.2: Fish

Difficulty Level: At Grade Created by: CK-12

Lesson Objectives

  • Identify traits of fish.
  • Explain how fish reproduce.
  • Outline the classification of fish.
  • Describe where fish live and what they eat.

Lesson Vocabulary

  • fin
  • fish
  • gill
  • spawning
  • swim bladder


If the word fish makes you think of cute little goldfish in a tank, check out the anglerfish pictured in Figure below. It’s also called a black sea devil. Surprisingly, goldfish and angler fish aren’t all that different from each other as far as fish diversity goes. Both fish are placed in the same fish class. Clearly, fish are a diverse and interesting group of vertebrates.


What Are Fish?

Fish are aquatic vertebrates. They make up more than half of all living vertebrate species. Most fish are ectothermic. They share several adaptations that suit them for life in the water.

Aquatic Adaptations

You can see some of the aquatic adaptations of fish in Figure below. For a video introduction to aquatic adaptations of fish, go to this link: https://www.youtube.com/watch?v=WrZEViYbkNc.

  • Fish are covered with scales. Scales are overlapping tissues, like shingles on a roof. They reduce friction with the water. They also provide a flexible covering that lets fish move their body to swim.
  • Fish have gills. Gills are organs behind the head that absorb oxygen from water. Water enters through the mouth, passes over the gills, and then exits the body.
  • Fish typically have a stream-lined body. This reduces water resistance.
  • Most fish have fins. Fins function like paddles or rudders. They help fish swim and navigate in the water.
  • Most fish have a swim bladder. This is a balloon-like organ containing gas. By inflating or deflating their swim bladder, fish can rise or sink in the water.

Aquatic adaptations in fish: gill cover; scales; fins

Organ Systems of Fish

Fish have a circulatory system with a heart. They also have a complete digestive system. It includes several organs and other structures. Fish with jaws use their jaws and teeth to chew food before swallowing it. This allows them to eat larger prey animals.

Fish have a nervous system with a brain. Fish brains are small compared with the brains of other vertebrates. However, they are large and complex compared with the brains of invertebrates. Fish also have highly developed sense organs. They include organs to see, hear, feel, smell, and taste.

How Fish Reproduce

Almost all fish have sexual reproduction, generally with separate sexes. Each fish typically produces large numbers of sperm or eggs. Fertilization takes place in the water outside the body in the majority of fish. Most fish are oviparous. The embryo develops in an egg outside the mother’s body.

Spawning and Brooding

Many species of fish reproduce by spawning. Spawning occurs when many adult fish group together and release their sperm or eggs into the water at the same time. You can see fish spawning in Figure below. Spawning increases the changes that fertilization will take place. It typically results in a large number of embryos forming at once. This makes it more likely that at least some of the embryos will avoid being eaten by predators. You can watch trout spawning in Yellowstone Park in this interesting video: http://video.nationalgeographic.com/video/trout_spawning

Adult salmon gather near the water surface to spawn.

With spawning, fish parents can’t identify their own offspring. Therefore, in most species, there is no parental care of offspring. However, there are exceptions. Some species of fish carry their fertilized eggs in their mouth until they hatch. This is called mouth brooding.

Larvae and Metamorphosis

Fish eggs hatch into larvae. Each larva swims around attached to a yolk sac from the egg (see Figure below). The yolk sac provides it with food. Fish larvae look different from adult fish of the same species. They must go through metamorphosis to change into the adult form.

Salmon larvae, each with a yolk sac attached to it.

Classification of Fish

There are about 28,000 living species of fish. They are placed in five different classes. The classes are commonly called hagfish, lampreys, cartilaginous fish, ray-finned fish, and lobe-finned fish. Table below shows pictures of fish in each class. It also provides additional information about the classes.

Class Distinguishing Traits Example
Hagfish Hagfish are very primitive fish. They lack scales and fins. They even lack a backbone, but they do have a cranium. They secrete large amounts of thick, slimy mucus. This makes them slippery, so they can slip out of the jaws of predators.


Lampreys Lampreys lack scales but have fins and a partial backbone. Their mouth is surrounded by a large round sucker with teeth. They use the sucker to suck the blood of other fish.


Cartilaginous Fish Cartilaginous fish include sharks, rays, and ratfish. Their endoskeleton is made of cartilage instead of bone. They also lack a swim bladder. However, they have a complete vertebral column and jaws. They also have a relatively big brain.


Ray-Finned Fish Ray-finned fish make up the majority of living fish species. They are a type of bony fish, with an endoskeleton made of bone instead of cartilage. Their fins consist of webs of skin over flexible bony spines, called rays. They have a swim bladder.


Lobe-Finned Fish Lobe-finned fish include only coelacanths and lungfish. They are bony fish with an endoskeleton made of bone. Their fleshy fins contain bone and muscle. Lungfish are named for a lung-like organ that they can use for breathing air. It evolved from the swim bladder. It allows them to survive for long periods of time out of water.


Fish Ecology

Fish vary in the types of places they live and what they eat. Many fish live in the salt water of the ocean. Other fish live in freshwater lakes, ponds, rivers, or streams. Most fish are predators, but they may differ in their prey and how they get it.

  • Hagfish are deep-ocean bottom dwellers. They feed on other fish, either living or dead. They enter the body of their prey through the mouth or anus. Then they literally eat their prey from the inside out.
  • Lampreys generally live in shallow water, either salty or fresh. They eat small invertebrates or suck the blood of larger fish.
  • Cartilaginous fish, such as sharks, mainly live in the ocean. They prey on other fish and aquatic mammals, or else they eat plankton. Their jaws and teeth allow them to eat large prey.
  • Bony fish, such as ray-finned or lobe-finned fish, may live in salt water or fresh water. They may eat algae, smaller fish like the butterfly fish in Figure below, or dead organisms. To see how one species of predatory bony fish catches its prey, watch this amazing video: http://video.nationalgeographic.com/video/stonefish-predation

Butterfly fish like this one have “fake” eyespots. The eyespots may confuse larger predators long enough for the butterfly fish to escape.

Lesson Summary

  • Fish are aquatic vertebrates. They make up more than half of all vertebrate species. Most fish are ectothermic. They share several adaptations that suit them for life in the water, such as gills and fins.
  • Almost all fish have sexual reproduction, generally with separate sexes. Most fish are oviparous. Many species reproduce by spawning. Eggs hatch to form larvae, which undergo metamorphosis to become adults.
  • There are about 28,000 species of living fish. They are placed in five different classes: hagfish, lampreys, cartilaginous fish, ray-finned fish, and lobe-finned fish.
  • Fish may live in salt water or fresh water. Most are predators.

Lesson Review Questions


  1. Identify aquatic adaptations of fish.
  2. Outline how fish are classified.
  3. Describe how cartilaginous fish such as sharks feed.

Apply Concepts

  1. An unknown fish lives in shallow fresh water and eats small invertebrates. In which class would you place the unknown fish?

Think Critically

  1. Summarize how fish generally reproduce.
  2. Compare and contrast lobe-finned and ray-finned fish.

Points to Consider

Amphibians evolved from lobe-finned fish.

  1. What are some examples of modern amphibians?
  2. What traits of lobe-finned fish made them good candidates for amphibian ancestors?

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