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Fish Classification

Explore the five classes of fish and their physiological characteristics.

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Bony Fish Classification

What type of animal is a sea horse? Is it actually a fish?

It is, and there are about 50 species of seahorses. Although they are bony fish, they do not have scales, but rather a thin skin stretched over a series of bony plates arranged in rings throughout their body. Each species has a distinct number of rings. Seahorses have a coronet on their head, which is distinct to each individual, much like a human fingerprint. Seahorses also swim upright, a characteristic not shared by other fish. Seahorses are poor swimmers, so they are most likely to be found resting, with their prehensile tails wound around a stationary object. They have long snouts, which they use to suck up food, and eyes that can move independently of each other. Seahorses eat small shrimp, tiny fish, crustaceans, and plankton.

Classification of Bony 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 the table in the previous lesson).

Ray-Finned Fish

Ray-finned 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

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-finned 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.


  • 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.


Use these resources to answer the questions that follow.

  1. Describe the body style of agnathans.
  2. Describe the body covering of cartilaginous fish.
  3. Compare rays to skates.
  4. Compare and contrast ray-finned fish to lobe-finned fish.
  5. What animals evolved from lobe-finned fish?
  1. Describe the main differences between the two groups of jawed fishes living today.


1. 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.

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

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