Grades 10 - 12
This lesson is an introduction to the crayfish. Using the video
as a starting point, students should learn about the crayfish, its habitat,
and its anatomical features and functions including the reproductive process.
The students should also be able to classify the crayfish and identify its
place in the food chain. Following the video, students will dissect a crayfish.
NATURE WATCH DIGEST #20: Crayfish
Students should be able to:
- identify the habitat in which the crayfish lives
- differentiate between crayfish and lobsters
- classify crayfish
- know the external anatomy and the functions of the external anatomical
parts of the crayfish understand the crayfish's reproductive process
- place the position of the crayfish in the food chain
- estimate the size of the larva based on the swimmerets length
Per group of 2 students:
- 1 crayfish per group (Crayfish can be purchased from most biological
- 1 metric ruler.
Exoskeleton - a hard external encasement that provides
the animal with protection and a place for muscles to attach. It is made
Chitin - tough polysaccharide (sugar).
Free oxygen - oxygen that is dissolved in water that is available
for animals to use for respiration gills.
Respiration Gills - external structure used for extracting
free oxygen from water walking legs.
Water Walking Legs - legs used for walking, food gathering,
and moving water over the gills.
Antennae - long thin anterior structure used to taste food, touch,
and maintain balance.
Antennule - short thin anterior structure used to taste food.
Compound eye - objects are seen as a mosaic similar to the
way insects see.
Cheliped - large anterior pinchers used for protection and hunting.
Regeneration - ability to regrow dismembered body parts .
Mandible - mouth structure used to grind food.
Maxilliped - legs surrounding the mouth used to hold and push food
to the mandibles.
Milt - sperm.
Swimmerets - thin leg-like structures used to swim, move water over
the gills, and hold larva.
Cephalothorax - fused head region and thorax region.
Carapace - name for the exoskeleton covering the cephalothorax.
Herbivores - animals that eat plants.
Carnivores - animals that eat meat.
Omnivores - animals that eat both plants and animals.
Predator - kills prey.
Scavenger - eats already dead animals.
This is an introductory lesson so the video provides the starting
point for the discussion. Before viewing the video, the teacher should review
with the students the process by which life is classified. Review the five
kingdoms of life and have the students determine where crayfish belongs.
The teacher should look over the vocabulary list and review with the students
the terms that have already been introduced in previous lessons. The teacher
needs to have the crayfish and rulers ready to do the lab at the end of
To give a specific responsibility while viewing tell the students,
as you watch the video it is your responsibility to determine in what habitat
the crayfish lives; differentiate between crayfish and lobsters; classify
the crayfish into its appropriate class; know the crayfish's external structures
and their functions; understand the crayfish's reproductive process; and
be able to place the crayfish in the food chain.
START the video at the beginning where there is a picture
of a stream.
PAUSE the video where the narrator says, "Shallow water, edge
of a lake, pond, marsh, or stream." Ask the students to review the
habitat in which crayfish live. Do you have any such habitat near where
you live? RESUME the video.
PAUSE the video where the narrator finishes discussing the characteristics
that distinguish the crayfish from the lobster. Review with the students
the distinguishing characteristics. (Crayfish live in fresh water and only
grow to about 12 centimeters; whereas, lobsters live in salt water and grow
up to 1 meter.) RESUME the video.
PAUSE the video where the narrator says, "Crayfish belong to
a group of animals known as crustaceans which also includes crabs and shrimp."
Key out the crayfish for the students. (Crayfish belong to the Kingdom Animalia,
Phylum Arthropoda, Subphylum Crustacea, and Class Crustacea). Other animals
that belong to this class are the lobster, crab, shrimp, water flea, sowbug,
and the barnacle. All members of the Class Crustacea have a fused head and
thorax. This is called a cephalothorax. RESUME the video.
PAUSE the video where the narrator says, "They have a skeleton
on the outside of their body. This is called an exoskeleton." Review
with the students the term exoskeleton and add that the exoskeleton is a
defining characteristic of all arthropods. The exoskeleton is made of a
material called "chitin" which is a polysaccharide. In other words,
it is a very hard sugar. Tell the students it is the material that makes
insects "crunch" when you step on them. Ask the students what
they think the function of the exoskeleton is. RESUME the video.
PAUSE the video after the narrator says, "The exoskeleton provides
protection." Ask the students if they were correct. (The exoskeleton
provides protection and a place for the crayfish's muscles to attach. It
is probable that you will have to tell the students about the muscle attachment.)
Show the students the cephalothorax. RESUME the video.
PAUSE the video after the narrator says, "These appendages move
water over the gills." Tell the students that the name for these four
pairs of appendages is walking legs. Walking legs have three functions_walking,
gathering food, and moving water over the gills. Gills are external structures
that extract free oxygen from the water. Water is composed of two hydrogen
atoms and one oxygen atom. The oxygen atom that is in water is bonded to
the hydrogen atoms, and thus, is not available for respiration. Oxygen molecules
are dissolved in water much as Kool aide mix is dissolved in water. It is
this oxygen that is available for respiration. The reason a fish tank must
have a bubbler in it is to renew the supply of free oxygen to the water.
If new free oxygen is not added, the fish will use up all of the available
free oxygen and suffocate. Ask the students what process adds free oxygen
to ponds, lakes, and streams. (Water movement.) RESUME the video.
PAUSE the video after the narrator finishes talking about the antenna.
Make sure the picture pauses on the two pair of antennae. Tell the students
that the long pair of antennae are called "antenna." These are
used for touch, taste, and maintaining equilibrium. The short pair of antennae
are called "antennules." These are used for taste. RESUME
PAUSE the video after the narrator finishes talking about the eyes.
Make sure the picture pauses on the eye. Tell the students that the crayfish
has compound eyes like an insect. This means that the crayfish sees the
world in a mosaic. RESUME the video.
PAUSE the video after the narrator finishes talking about the "big
pinchers." Tell the students that these large pinchers are call "chelipeds."
The function of the chelipeds is to capture prey and serve as defense weapons.
RESUME the video.
PAUSE the video after the narrator finishes talking about the walking
legs. Since you have already discussed the three functions of the walking
legs, review these with the students. (The functions are walking, gathering
food, and moving water over the gills for respiration.) REWIND the
video to the part where the narrator is discussing the exoskeleton. TURN
THE SOUND OFF. RESUME the video.
As the video plays review the cephalothorax, exoskeleton, walking legs,
gills, antennae, antennule, compound eyes, and chelipeds. PAUSE the
video as needed in order to solidly review the parts and their functions.
RESUME the video with sound after completing the review.
PAUSE the video after the two crayfish fight and the narrator says,
"Simply grows a new one." Tell the students this is a process
called regeneration. RESUME the video.
PAUSE the video after the narrator says, "They eat plants, animals,
and decaying material on the bottom of the stream." Tell the students
this means the crayfish are herbivores_animals that eat plants, and carnivores_animals
that eat meat. This makes them omnivores_animals that eat both plants and
animals. The crayfish is also a predator_kills prey and a scavenger_eats
already dead animals. RESUME the video.
PAUSE the video after the crayfish kills the second fish. Ask the
students what method of feeding the crayfish is exhibiting. (Predator and
carnivore.) Ask the students what appendage the crayfish used to kill its
prey. (Cheliped) RESUME the video.
PAUSE the video after the narrator says, "jaw feet." Tell
the students these leg-like appendages are called "maxillipeds."
They are used to hold food and move food to the mandibles. Have the students
take the word "maxilliped" apart. "Maxilli" means jaw
and "ped" means foot. Have the students look at the two solid
white teeth-like structures. These are the mandibles. They are used to grind
food. Below the mandibles is the mouth. RESUME the video.
PAUSE the video where the crayfish is eating plankton and the narrator
says, "eat plankton." Ask the students what method of feeding
the crayfish is exhibiting. (Herbivore) RESUME the video.
PAUSE the video after the narrator says, "at mating time."
Tell the students to note the deep, dark, red color of the crayfish. This
is an indication that the crayfish are in a reproductive state. RESUME
PAUSE the video after the narrator says, "milt." Tell the
students that milt is another term for sperm. RESUME the video.
PAUSE the video after the crayfish have mated, the eggs are beginning
to come out of the female, and the narrator says, feathery legs called swimmerets."
Ask the students what type of fertilization is taking place. (External because
the eggs are fertilized outside of her body.) Ask the students what other
types of animals have external fertilization. (Fish and frogs will probably
be common responses.) Tell the students that the swimmerets have three functions.
They help the crayfish swim, they move water over the gills for respiration,
and on the female they hold the larva. RESUME the video.
PAUSE the video as the female with the eggs "dangling like grapes"
starts to walk. Ask the students why she is swinging the eggs around so
much. (The fertilized eggs must have a supply of free oxygen in order to
live.) RESUME the video and have the students see if they were correct.
PAUSE the video after the mother crayfish places her brood on top
of the dead fish. Ask the students what is the method of feeding? (Scavenger)
RESUME the video.
PAUSE the video after the narrator says, "carries 800 larva."
Ask the students how many? Why would one crayfish have so many larva? (Fish
and birds eat them.) RESUME the video but PAUSE the video
after the narrator give the answer of fish and birds eating the baby crayfish.
This is where the bird is standing in the water. Ask the students what role
the baby crayfish are in the food chain? (Prey) Have the students draw some
conclusions about the crayfish's position in the food chain. (The young
are prey. The adults are predators, herbivores, and scavengers. Therefore
they serve as food for others, hunters, and the cleanup crew.) RESUME
PAUSE the video after the narrator says, "most crayfish spend
their life under water." Ask the students why. (Respirations. They
need oxygen dissolved in water to respire.) RESUME the video.
STOP the video after the narrator says, "burrow in the mud or
live in wet meadows." Ask the students why. (Respirations. They need
oxygen dissolved in water to respire.)
After viewing and adding in the discussion topics, you may want
to go back and review parts of the video with the sound turned off. At the
end of the lesson, students will be able to discuss the crayfish at a higher
level than the video. This is an excellent method of review and reinforcement.
Note to the teacher: It is time to do the lab. The post-viewing lab and
the extension I am suggesting, involve the dissection of the crayfish. Some
people are opposed to this; therefore, you may want to substitute a crayfish
dissection computer simulation or a plastic model.
Have the students go to the meat section of the grocery store.
Talk to the butcher. Find out the number of lobsters that are sold. If crayfish
are sold, find out how many of them are sold. How much do they sell for?
Ask the butcher to explain how they are prepared for sale. Have the butcher
give you cooking preparation instructions. Share the information and recipes
with the class via an oral presentation. If available, a computer generated
slide show would be a nice visualization. Based on the class data, have
the students develop a data table and a chart showing the prices. Calculate
the mean price.
Have the students go to a restaurant where lobster or crayfish are on the
menu. Lobster tends to be sold as lobster, whereas crayfish tend to be in
something. Talk to the chef. Find out what dishes are prepared. Find out
how many of each dish is sold. How much does each cost to prepare and how
much is it sold for. Share the information with the class via an oral presentation.
If available, a computer generated slide show would be a nice visualization.
Based on the class data, have the students develop a data table and a chart
showing the prices. Calculate the mean price. Be sure to compare like items.
Continue with the dissection by conducting a lesson on the internal
structures of the crayfish and the functions of the internal structures.
Dissect the crayfish and study the structures and review the functions.
During the dissection, open the stomach and see if you can determine what
the crayfish was feeding on at the time of death.
Have the students search the Internet for information on crayfish. Have
the students assemble an oral report based on their findings. This may include
pictures and scientific data that they down load from the Internet.
Have the students go to the library and research the crayfish. Have them
complete a word processed term paper following the guidelines for writing
a term paper found in the MLA handbook.
If live crayfish are available, the students could perform some simple quantitative
scientific research and analyze their results statistically. Some possible
topics include are crayfish attracted or detracted by light, what foods
do they prefer, and can the crayfish distinguish between colors. The students
must determine how to set up the experiment and how to quantify their results.
The teacher will have to provide help as related to the proper statistical
tests to perform on the data and how to perform the specified statistical
test. Library research can be done to determine if the student's results
match published results. Students can then prepare a word processed scientific
research paper. There are several regularly published scientific journals.
The student should pick one of these and follow the prescribed format for
writing a scientific research paper.
VIDEO AVAILABLE FROM
Taping it off of the air. See your local PBS ITV block feed schedule.
Lesson Developed by Master Teacher John Lindley, Marsing High
School, Marsing, Idaho.
Click here to view the
worksheet associated with this lesson.
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