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Lesson Objectives

  • List the general traits of fish.
  • Describe the features of jawless fish.
  • List the general features of the cartilaginous fish.
  • Describe the features of bony fish and the significance of this superclass.
  • List some of the reasons why fish are important.

Check Your Understanding

  • What are the unique characteristics of vertebrates?
  • What are the two main groups of vertebrates?


  • aquaculture
  • barbels
  • cartilaginous skeleton
  • ectothermic
  • pineal eye
  • placoid

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. Fish are aquatic vertebrates, which became a dominant form of sea life and eventually evolved into land vertebrates.

They have a number of characteristic traits and are classified into two major groups: jawless fish and jawed fish. Jawed fish are further divided into those with bones and those with just cartilage. Fish, in general, are important to humans in many ways. Can you think of some of these ways?

Some characteristics of fish include:

  1. They are ectothermic, meaning their temperature depends on the temperature of their environment. This is unlike humans, whose temperature is controlled inside of the body.
  2. They are covered with scales.
  3. They have two sets of paired fins and several unpaired fins.
  4. Fish also have a streamlined body that allows them to swim rapidly (Figure below).

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.

How Do Fish Reproduce?

Fish reproduce sexually. They lay eggs that can be fertilized either inside or outside the body. In most fish, the eggs develop outside the mother's body. In the majority of these species, fertilization 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 51-foot whale shark (Figure below) to the stout infantfish, which is about 0.5 inches.

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.

Although most fish live in aquatic habitats, such as ocean, lakes, and rivers, there are some that spend a lot of time out of water. Mudskippers, for example, feed and interact with each other on mudflats for up to several days at a time and only go underwater when digging burrows (Figure below). They breathe by absorbing oxygen across the skin, similar to how frogs breathe.

See http://www.youtube.com/watch?v=ZUsARF-CBcI for further information (5:35).

Unlike most fish, mudskippers spend time out of the water

Mudskippers, shown on the mudflats, spend time feeding and interacting with each other.

Jawless Fishes

Jawless fish, part of the superclass Agnatha, belong to the phylum Chordata, subphylum Vertebrata. There are two living groups of jawless fish, with about 100 species in total: lampreys and hagfish (Figure below). Although hagfish belong to the subphylum Vertebrata, they do not technically have vertebrae.

A hagfish

In addition to the absence of jaws, fish in this class are characterized by the absence of paired fins or an identifiable stomach. Characteristics they do have include:

  1. A notochord, both in larvae and adults.
  2. Seven or more paired gill pouches.
  3. The branchial arches, a series of arches that support the gills of aquatic amphibians and fishes, lie close to the body surface.
  4. A light sensitive pineal eye, an eye-like structure that develops in some cold-blooded vertebrates.
  5. A cartilaginous skeleton, or a skeleton made of bone-like material called cartilage.
  6. A heart with two chambers.
  7. Reproduction using external fertilization.
  8. They are ectothermic.

Many jawless fish in the fossil record were armored with heavy bony-spiky plates. The first armored fish in this class evolved before bony fish and tetrapods, including humans.

Cartilaginous Fishes

So why did fish eventually evolve to have jaws? Such an adaptation would allow fish to eat a much wider variety of food, including plants and other organisms. The cartilaginous fishes are jawed fish with paired fins, paired nostrils, scales, two-chambered hearts, and skeletons made of cartilage rather than bone. Cartilage does not have as much calcium as bones, which makes bones rigid. Cartilage is softer and more flexible than bone.

The 1,000 or so species of cartilaginous fish are subdivided into two subclasses: the first includes sharks, rays, and skates; the second includes chimaera, sometimes called ghost sharks. Fish from this group range in size from the dwarf lanternshark, at 6.3 inches, to the 50-foot whale shark shown in Figure above.

Blood, Skin, and Teeth

Since they do not have bone marrow (as they have no bones), red blood cells are produced in the spleen, in special tissue around the gonads, and in an organ called Leydig’s Organ, only found in cartilaginous fishes. The tough skin of this group of fish is covered with dermal teeth, or placoid scales. In adult chimaera, the placoids are very small, making it feel like sandpaper. The teeth found in the mouth of cartilaginous fish evolved from teeth found on the skin.


The sharks, rays and skates are further broken into two superorders:

  1. Rays and skates.
  2. Sharks (Figure below).

Sharks are some of the most frequently studied cartilaginous fish. Sharks are distinguished by such features as:

  • The number of gill slits.
  • The numbers and types of fins.
  • The type of teeth.
  • Body shape.
  • Their activity at night.
  • An elongated, toothed snout used for slashing the fish that they eat, as seen in sawsharks.
  • Teeth used for grasping and crushing shellfish, a characteristic of bullhead sharks.
  • A whisker-like organ named barbels, a characteristic of carpet sharks.
  • A long snout (or nose-like area), characteristic of groundsharks.
  • Large jaws and ovoviviparous reproduction, where the eggs develop inside the mother’s body after internal fertilization, and the young are born alive. This trait is characteristic of mackerel sharks.

A spotted Wobbegong shark showing skin flaps around the mouth and camouflage coloration.

Bony Fishes

There are almost 27,000 species of bony fish, which are divided into two classes: ray-finned fish and lobe-finned fish. Most bony fish are ray-finned. There are only eight living species of lobe-finned fish, including lungfish (Figure below) and coelacanths (Figure below).

One of the only eight living species of lobe-finned fish, the lungfish.

One of the eight living species of lobe finned fish, the coelacanth.

Most fish are bony fish, making them the largest group of vertebrates in existence today. They are characterized by:

  1. A head and pectoral girdles (arches supporting the forelimbs) that are covered with bones derived from the skin.
  2. A lung or swim bladder, which helps the body create a balance between sinking and floating by either filling up with or emitting gases such as oxygen
  3. Jointed, segmented rods supporting the fins.
  4. A cover over the gill, which helps them breathe without having to swim.
  5. The ability to see in color, unlike most other fish.

One of the most interesting adaptations of these fish is their ability to produce replacement, bone, by replacing cartilage from within, with bone. They also produce “spongy," bone. This means that the fish have a lightweight, flexible bone on the inside, surrounded by stronger and more rigid bone on the outside. This type of bone allows the fish to move in different ways compared to the cartilaginous fish.

How Big Are Bony Fish?

The ocean sunfish is the most massive bony fish in the world, up to 11 feet long, and weighing up to 5,070 pounds (Figure below). Other very large bony fish include the Atlantic blue marlin, the black marlin, some sturgeon species, the giant grouper and the goliath grouper. In contrast, the dwarf pygmy goby measures only 0.6 inches.

An ocean sunfish, the most massive bony fish in the world, up to 11 feet long and weighing 5,070 pounds!

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, a type of farming known as aquaculture. Fish are also caught for recreation to display in the home and or in public aquaria.

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

Lesson Summary

  • The general traits of fish help adapt them for living in an aquatic environment, mostly for swimming, and also for absorbing oxygen.
  • Fish are ectothermic (cold-blooded), although some show warm-blooded adaptations.
  • Jawless fish do not have bone, but they do have cartilage.
  • Fish with jaws consist of both the cartilaginous fish and the bony fish.
  • Cartilaginous fishes include sharks, rays, skates and chimaera.
  • Bony fish form the largest group of vertebrates in existence today, and have true bone that can regenerate.
  • Fish are an important food source for humans.

Review Questions


1. What are three traits that all fish have in common?

2. What is an example of one exception to the general traits of fish?

3. What are three characteristics of jawless fish?

4. List three ways that fish are important.

Apply Concepts

5. Between the hagfish and a cartiliaginous fish, the structure of the jaw evolved? Why do you think this major structure evolved in fish? How does it benefit them?

6. What is the major difference between cartilaginous fish and bony fish? Why do you think the bony fish evolved to be different?

Critical Thinking

7. Mudskippers are an example of a fish species that must absorb oxygen across the skin, instead of through gills, since they spend most of their time out of water. What kind of environment would pressure fish to evolve to breathe in air instead of breathing in the water?

Further Reading / Supplemental Links

Points to Consider

Amphibians are next.

  • How do you think the gills of fish relate to the lungs of other animals?
  • Lungfish have paired lungs similar to those of tetrapods. How do you think the breathing systems of lungfish could be similar to and different from tetrapods in the way they breathe?
  • What structures are different between fish and amphibians, that allow amphibians to live on land?

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Date Created:

Feb 29, 2012

Last Modified:

Sep 02, 2014
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