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19.2: Fish

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • Describe structure and function in fish.
  • Explain how fish reproduce and develop.
  • Give an overview of the five living classes of fish.
  • Summarize the evolution of fish.
  • Outline the ecology of the different fish classes.


ectothermic, aquatic vertebrate with a streamlined body and gills for absorbing oxygen from water
depositing large numbers of gametes in the same place and at the same time by fish or amphibians
swim bladder
balloon-like internal organ in most fish that can be used to move up or down through the water column by changing the amount of gas it contains


Fish are aquatic vertebrates. They make up more than half of all vertebrate species. They are especially important in the study of vertebrate evolution because several important vertebrate traits evolved in fish.

Structure and Function in Fish

Fish show great diversity in body size. They range in length from about 8 millimeters (0.3 inches) to 16 meters (about 53 feet). Most are ectothermic and covered with scales. Scales protect fish from predators and parasites and reduce friction with the water. Multiple, overlapping scales provide a flexible covering that allows fish to move easily while swimming.

Adaptations for Water

Many structures in fish are adaptations for their aquatic lifestyle. Several are described below and shown in Figure below.

  • Fish have gills that allow them to “breathe” oxygen in water. Water enters the mouth, passes over the gills, and exits the body through a special opening. Gills absorb oxygen from the water as it passes over them.
  • Fish have a stream-lined body. They are typically long and narrow, which reduces water resistance when they swim.
  • Most fish have several fins for swimming. They use some of their fins to propel themselves through the water and others to steer the body as they swim.
  • Fish have a system of muscles for movement. Muscle contractions ripple through the body in waves from head to tail. The contractions whip the tail fin against the water to propel the fish through the water.
  • Most fish have a swim bladder. This is a balloon-like internal organ that contains gas. By changing the amount of gas in the bladder, a fish can move up or down through the water column.

General Fish Body Plan. A fish has a stream-lined body with gills and fins. (1) operculum (gill cover) (2) lateral line system (a sensory organ system) (3) dorsal fin (4) fat fin (5) caudal peduncle (narrow body region that connects the main body to the caudal fin) (6) caudal fin (7) anal fin (8) photophores (light emitting organs found on some fish) (9) pelvic fins (paired) (10) pectoral fins (paired).

Fish Organ Systems

Fish have a circulatory system with a two-chambered heart. Their digestive system is complete and includes several organs and glands. Jawed fish use their jaws and teeth to grind up food before passing it to the rest of the digestive tract. This allows them to consume larger prey.

Fish also have a centralized nervous system with a brain. Fish brains are small compared with the brains of other vertebrates, but they are large and complex compared with the brains of invertebrates. Fish also have highly developed sense organs that allow them to see, hear, feel, smell, and taste. Sharks and some other fish can even sense the very low levels of electricity emitted by other animals. This helps them locate prey.

Fish Reproduction and Development

Nearly all fish reproduce sexually, and most species have separate sexes. Those without separate sexes avoid self-fertilization by producing sperm and eggs at different times. Each fish typically produces a large number of gametes. In most fish species, fertilization takes place externally. These fish are oviparous. Eggs are laid and embryos develop outside the mother’s body. In a minority of fish, including sharks, eggs develop inside the mother’s body but without nourishment from the mother. These fish are ovoviviparous.


In many species of fish, a large group of adults come together to release their gametes into the water at the same time. This is called spawning. It increases the chances that fertilization will take place. It also means that many embryos will form at once, which helps ensure that at least some of them will be able to escape predators. With spawning, there is no way for fish parents to know which embryos are their own. Therefore, fish generally don’t provide any care to their eggs or offspring. There are some exceptions, however, including the fish described in Figure below.

Mouth Brooding. Some species of fish carry their fertilized eggs in their mouth until they hatch. This is called mouth brooding. If you look closely, you can see the eggs inside the mouth of the African tilapia fish pictured here.

Fish Larvae

Fish eggs hatch into larvae that are different from the adult form of the species (see Figure below). A larva swims attached to a large yolk sac, which provides the larva with food. The larva eventually goes through metamorphosis and changes into the adult form. However, it still needs to mature before it can reproduce.

Salmon Larva. This newly hatched salmon larva doesn’t look very fish-like. The structure hanging from the larva is the yolk sac.

Classification of Fish

There are about 28,000 existing species of fish, and they are placed in five different classes. The classes are commonly referred to as hagfish, lampreys, cartilaginous fish, ray-finned fish, and lobe-finned fish (see Table above in the previous lesson).


Hagfish are very primitive fish. They retain their notochord throughout life rather than developing a backbone, and they lack scales and fins. They are classified as vertebrates mainly because they have a cranium. Hagfish are noted for secreting large amounts of thick, slimy mucus. The mucus makes them slippery, so they can slip out of the jaws of predators.


Like hagfish, lampreys also lack scales, but they have fins and a partial backbone. The most striking feature of lampreys is a large round sucker, lined with teeth, that surrounds the mouth (see Figure below). Lampreys use their sucker to feed on the blood of other fish species.

Sucker Mouth of a Lamprey. The mouth of a lamprey is surrounded by a tooth-lined sucker.

Cartilaginous Fish

Cartilaginous fish include sharks, rays, and ratfish (see Figure below). In addition to an endoskeleton composed of cartilage, these fish have a complete backbone. They also have a relatively large brain. They can solve problems and interact with other members of their species. They are generally predators with keen senses. Cartilaginous fish lack a swim bladder. Instead, they stay afloat by using a pair of muscular fins to push down against the water and create lift.

Cartilaginous Fish. All of these fish belong to the class of cartilaginous fish with jaws. (a) Oceanic whitetip shark (b) Ray (c) Ratfish

One of the most important traits of cartilaginous fish is their jaws. Jaws allow them to bite food and break it into smaller pieces. This is a big adaptive advantage because it greatly expands the range of food sources they can consume. Jaws also make cartilaginous fish excellent predators. It you’ve ever seen the film Jaws, then you know that jaws make sharks very fierce predators (see also Figure below).

Jaws of a Shark. Sharks have powerful jaws with multiple rows of sharp, saw-like teeth. Most other fish are no match for these powerful predators.

Ray-Finned Fish

Ray-fined fish include the majority of living fish species. including goldfish, tuna, salmon, perch, and cod. They have a bony endoskeleton and a swim bladder. Their thin fins consist of webs of skin over flexible bony rays, or spines. The fins lack muscle, so their movements are controlled by muscles in the body wall. You can compare their ray fins with the fleshy fins of lobe-finned fish in Figure below.

Fins of Bony Fish. The fins of ray-finned and lobe-finned fish are quite different. How is the form of the fins related to their different functions in the two classes of fish? Ray Fin (left), Lobe Fin (right)

Lobe-Finned Fish

Lobe-fined fish are currently far fewer in number than ray-finned fish. Their fins, like the one shown in Figure above, contain a stump-like appendage of bone and muscle. There are two groups of lobe-finned fish still alive today: coelacanths and lungfish.

  1. Coelacanths are ancient fish with just two living species. They are at risk of extinction because of their very small numbers.
  2. Lungfish have a lung-like organ for breathing air. The organ is an adaptation of the swim bladder. It allows them to survive for long periods out of water.

Evolution of Fish

Invertebrate chordates use their gills to filter food out of water, not to absorb oxygen. In the early evolution of fish, there was a switch to using gills to absorb oxygen instead of to filter food. Gills consist of many thin, folded tissues that provide a large surface area for oxygen uptake. With more oxygen absorbed by the gills, fish could become much larger and more active.

Timing of Fish Evolution

Ancestors of hagfish are thought to have been the earliest vertebrates. Their fossils date back to about 550 million years ago. Fossils of cartilaginous fish with jaws, resembling living sharks, first appeared in the fossil record about 450 million years ago. They were followed about 50 million years later by the bony fish.

The Bony Fish

At first, the lobe-finned bony fish were much more common than the ray-finned bony fish that dominate today. Lobe-finned fish were also ancestral to amphibians. Their stump-like appendages and lung-like organs evolved into amphibian legs and lungs. Ray-finned bony fish may have been the first fish to evolve in freshwater. They eventually became the most diverse and dominant class of fish.

Ecology of Fish

The habitats and diets of fish are varied. They live throughout the ocean and also in freshwater lakes, ponds, rivers, and streams.

Fish Food

Most fish are predators, but the nature of their prey and how they consume it differs from one class to another and even within classes.

  • 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 ocean water or freshwater. They either consume small invertebrates or suck blood from larger fish with their sucker mouth.
  • Cartilaginous fish such as sharks may live on the bottom of the ocean. However, most live in the water column. They prey on other fish and aquatic mammals or else consume plankton.
  • Bony fish may live in salt water or freshwater. They consume a wide range of foods. For example, they may eat algae, smaller fish, detritus, or dead organisms, depending on the species of fish.

Fish at Risk

Today, more than 1,000 species of fish are at risk of extinction. This is mainly because of human actions. Specific causes include over-fishing and habitat destruction caused by water pollution, dam building, and the introduction of non-native species.

Lesson Summary

  • Fish are aquatic, ectothermic vertebrates. Many structures in fish are adaptations for their aquatic lifestyle. For example, fish have a stream-lined body that reduces water resistance while swimming. They also have gills for “breathing” oxygen in water and fins for propelling and steering their body through water.
  • Nearly all fish reproduce sexually and have separate sexes. Fertilization is generally external, and most fish are oviparous. Many adults of the same species may come together in a group and release gametes into the water at the same time, which is called spawning. Fish hatch into larvae that are different from the adult form of the species.
  • There are about 28,000 existing species of fish, and they are placed in five classes: hagfish, lampreys, cartilaginous fish, ray-finned bony fish, and lobe-finned bony fish.
  • The evolution of fish included a shift from using the gills for filtering food to using them to absorb oxygen from water. The earliest fish, resembling living hagfish, evolved about 550 million years ago. Adaptations that eventually evolved in fish include a complete vertebral column, jaws, and an endoskeleton made of bones instead of cartilage.
  • Fish live throughout the ocean and in freshwater lakes and streams. Most fish are predators, but the nature of their prey and how they consume it may vary. Many species of fish are threatened by human actions, such as water pollution and over-fishing.

Lesson Review Questions


1. What are gills? What purpose do they serve in fish?

2. Describe fish scales, and state their functions.

3. Describe how fish use their muscles to swim.

4. What is a swim bladder? How is it used?

5. List two ways that fish can sense prey animals.

Apply Concepts

6. Assume that a new species of fish has been discovered deep in the ocean. It has a complete vertebral column made of cartilage. Which class should the new species be placed in? Name one other trait you would expect to find in the new species of fish. Explain your answers.

Think Critically

7. Explain why the practice of spawning is adaptive.

8. Fish with jaws may be very large. Infer how their jaws may be related to their large body size.

Points to Consider

Lobe-finned fish were the ancestors of amphibians, which were the first vertebrates to live on land.

  • What are some examples of amphibians?
  • How do you think amphibians might differ from lobe-finned fish? What adaptations do you think amphibians needed to evolve in order to live on land?

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