Fish Classification
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: November 10, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
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Chapter 1. Fish Classification
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Fish Classification
Give an overview of the five living classes of fish.
Distinguish between hagfish and lampreys.
Describe unique features of cartilaginous fish.
Give examples of cartilaginous fish.
Distinguish between ray-finned fish and lobe-finned fish.
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 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).
Hagfish
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.
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Lampreys
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 1.1). Lampreys use their
sucker to feed on the blood of other fish species.
FIGURE 1.1
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 1.2). 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.
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 1.3).
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 1.4.
Lobe-Finned Fish
Lobe-finned fish are currently far fewer in number than ray-finned fish. Their fins, like the one shown in Figure
1.4, contain a stump-like appendage of bone and muscle. There are two groups of lobe-finned fish still alive today:
coelacanths and lungfish.
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Chapter 1. Fish Classification
FIGURE 1.2
Cartilaginous Fish. All of these fish belong to the class of cartilaginous fish with
jaws. (a) Oceanic whitetip shark (b) Ray
(c) Ratfish
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.
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FIGURE 1.3
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.
FIGURE 1.4
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)
Summary
• 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.
Explore More
Use this resource to answer the questions that follow.
• http://www.hippocampus.org/Biology → Non-Majors Biology → Search: Fish
1. Describe the body style of agnathans.
2. Describe the body covering of cartilaginous fish.
3. Compare rays to skates.
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Chapter 1. Fish Classification
4. Compare and contrast ray-finned fish to lobe-finned fish.
5. What animals evolved from lobe-finned fish?
Review
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.
3. The majority of living fish belong to what class?
4. Describe the lungfish.
References
1. Courtesy of the US Fish and Wildlife Service. Sucker nouth of a lamprey . CC BY 2.0
2. (a) Image copyright Dray van Beeck, 2014; (b) Barry Peters (petersbar); (c) Linda Snook/MBNMS/US
National Oceanic and Atmospheric Administration. Cartilaginous fish . (a) Used under license from Shutterstock.com; (b) CC BY 2.0; (c) Public Domain
3. Grant Peters. Jaws of a shark . CC BY 2.0
4. (left) Image copyright Zheltyshev, 2014; (right) David Jolivet. Fins of bony fish . (left) Used under license
from Shutterstock.com; (right) CC BY 2.0
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