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9.14: Crustaceans

Difficulty Level: At Grade Created by: CK-12
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What are shrimp?

Shrimp are an example of crustaceans, one group within the arthropods. Shrimp live on the ocean floor in many parts of the world. Shrimp are not only food for humans; they are also an important food source for larger marine animals.


Crustaceans are a large group of arthropods, consisting of almost 52,000 species. The majority of crustaceans are aquatic. Some live in the ocean, while others live in fresh water. A few groups have adapted to living on land, such as land crabs, hermit crabs, and woodlice (Figure below). Crustaceans are among the most successful animals, and can be considered the dominant aquatic animals. Though small, crustaceans are numerous enough to be the main source of energy for large ocean mammals. They are found as much in the oceans as insects are on land.

A terrestrial arthropod, a species of woodlice

A terrestrial arthropod, a species of woodlice.

Classes of Crustaceans

Six classes of crustaceans are generally recognized (Table below).

Class Characteristics Examples
Branchiopoda Mostly small, freshwater animals that feed on plankton and detritus. Brine shrimp
Remipedia A small class of blind organisms found in deep caves connected to salt water. Nectiopoda
Cephalocarida Small crustaceans, with an eyeless head covered by a horseshoe-shaped shield; has two pairs of antennae and two pairs of jaws. Horseshoe shrimp
Maxillopoda Mostly small, with a small abdomen, and generally no appendages. Barnacles, copepods
Ostracoda Small animals with bivalve shells. Seed shrimp
Malacostraca The largest class, with the largest and most familiar animals. This class has the greatest diversity of body forms. Crabs, lobsters, shrimp, krill, woodlice

Can Crustaceans Move?

Remember that crustaceans are an arthropod subphylum, and that arthropod means "jointed feet." As expected, the majority of crustaceans can move. A few groups are parasitic and live attached to their hosts. Adult barnacles (Figure below) cannot move, so they attach themselves headfirst to a rock or log.

Barnacles are non-moving crustaceans

Barnacles are non-moving crustaceans. Many barnacles attach themselves to man-made structures.

Characteristics of Crustaceans

Characteristics of crustaceans include:

  1. An exoskeleton that may be bound together, such as in the carapace, the thick back shield seen in many crustaceans that often forms a protective space for the gills.
  2. A main body cavity with an expanded circulatory system. Blood is pumped by a heart located near the back.
  3. A digestive system consisting of a straight tube that has a gastric mill for grinding food and a pair of digestive glands that absorb food.
  4. Structures that function like kidneys to remove wastes. These are located near the antennae.
  5. A brain that exists in the form of ganglia, or connections between nerve cells.
  6. Crustaceans periodically shed the outer skeleton, grow rapidly for a short time, and then form another hard skeleton. They cannot grow underneath their outer exoskeleton. They are very vulnerable during this time, as they lack their hard shell.

Crustaceans Reproduction

Most crustaceans have separate sexes, so they reproduce sexually using eggs and sperm. Many land crustaceans, such as the Christmas Island red crab, mate every season and return to the sea to release the eggs. Others, such as woodlice, lay their eggs on land when the environment is damp. In some crustaceans, the females keep the eggs until they hatch into free-swimming larvae.


  • Crustaceans include crabs, lobsters, shrimp, krill, and woodlice.
  • Features of crustaceans include an exoskeleton that may be bound together.

Explore More

Use the resource below to answer the questions that follow.

  1. How do some crustaceans strengthen their exoskeleton?
  2. How many antennae do crustaceans have? How does this differ from a butterfly?
  3. How does filter feeding among crustaceans differ from filter feeding among mollusks?
  4. Why do scientists feel filter feeding evolved multiple times among crustaceans?
  5. What are three functions of appendages in the crustacean?


  1. What are three examples of crustaceans?
  2. What are two characteristics of crustaceans?
  3. Why do crustaceans shed their outer skeleton.
  4. What is the carapace?

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carapace Thick back shield seen in many crustaceans that often forms a protective space for the gills.
crustaceans Group of mostly aquatic arthropods that includes lobsters, crabs, and barnacles.
ganglia Network of nerve cells.
gastric mill Specialized organ for grinding food.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Aug 30, 2016
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