GO IN DEPTH

How a Blue Crab Changes as It Grows

 
How a Blue Crab Changes as It Grows Photo Credit: Jarek Tuszynski / CC-BY-SA-3.0 [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons.

Purpose

To understand the changes that a blue crab goes through during molting, and why it is important for scientists to understand these changes.


Context

It is important to understand the changes an animal goes through during its life span in order to research what may be causing that animal some difficulty to survive and reproduce. One good example is the blue crab in the Chesapeake Bay, which has been showing a steady decline in population due to many reasons and stresses. This has had quite an impact on the economy of the watermen living on the Eastern Shore of Maryland as well as the people who make their living selling the blue crab.

In this lesson students will be exploring the BlueCrab Archives website. It includes pictures of a blue crab as it goes through the stages of molting. Students often become confused with the term molting since it is used in different ways in the animal world. However, it is important to have them understand that animals go through changes as they grow. Some animals simply become bigger while others have to change a part of their body in order to grow. The blue crab is an example of this.


Motivation

To prepare the students for the changes that a blue crab goes through during molting, you could discuss how the students themselves have changed as they have grown. Have the students brainstorm ideas in cooperative groups. Then call on one child from each group for an idea until all ideas have been elicited. Make a list on the board, the overhead, or chart paper.

If they have cited a variety of changes, you might want to focus their attention to physical changes. If you do not see any ideas related to changes as we grow, you might want to have the groups brainstorm further for more examples of changes or tell them to brainstorm some examples of how we show changes as we grow.

Some questions to ask:

  • How do you know when you have grown? What evidence do you look for that lets you know you have grown?
  • What kind of changes has your body gone through as you have grown?
  • Do you think other living things change as they grow? Why do you think that?
  • Name a living thing and explain how it grows.

Development

Explain to the students that now they will be learning about how one animal grows and why it is important for scientists to understand how it grows.

Go to the BlueCrab Archive website for this part of the lesson. If your students are not familiar with the blue crab or they have only seen these crabs after they have been steamed, you might want to have them observe the photographs in the section: Blue Crab Identification. These photographs are very good at showing male and female crabs and their differences.

Ask the students questions about the crabs' appearances before they change, such as the following:

  • Describe a blue crab-think about the color, body parts, and where those body parts are located on the blue crab.
  • What makes a blue crab different from other animals you are familiar with?
  • Why is it necessary to know what a crab looks like before it goes through any growth changes?

Next, go to the Molting section of the website. You could have the students read in pairs, in small groups, or, if you have a computer overhead, have the class read this section together. After reading this paragraph, discuss the questions below with the whole class.

Note: Before students read the paragraph they could read the discussion questions below, giving them an opportunity to think about their answers before the class begins the discussion.

Discussion questions:

  • What happens to the blue crab when it molts?
  • What happens to the shell?
  • How long does the shell last like this?
  • Why is it important for the scientists to know this?
  • What might the blue crab need to do to protect itself during this time?

Now have students go to Busting out of its shell.

Have the students observe the photographs before they read the information. They can click on each photograph to see the image enlarged. When they are finished with the enlarged image, have them click "Back" on the browser to return to the normal sized image. Have them take notes.

When students have finished taking notes on this section, you might want to have them share their notes with the whole class. This promotes valuing the work of others. Tell students that scientists share and compare notes quite often.

Questions to answer after sharing notes:

  • What happened to the color of the blue crab during the molting? Why do you think that happened?
  • How can you tell that the crab grew? How many ways can you think of and what are they?
  • Where did the molting seem to start? Why would this be important for the crab's survival?
  • Why do you think the molting started in that part of the crab's body?
  • Is the crab's shell hard or soft? What would the crab need to be able to do at this time in order to survive danger? Why would that be important for scientists to know if they are to help the crab population increase?
  • At the end of the photographs, it said that the crab is tired and weak. Why would this make the crab vulnerable to danger?
  • Suppose you were eating steamed crabs. One of the crabs you have just opened did not have as much meat as the other crabs although it was about the same size. What might have happened to that particular crab just before someone caught it? Why do you think that?

Assessment

To assess student understanding, ask questions such as the following:

  • How do we know that a blue crab has grown besides measuring and weighing the crab?
  • What evidence would we be able to find to prove that?
  • Why would scientists need to know about how a crab molts if they are trying to find ways to help increase the crab population?

Have students choose a project from the list below:

  • Construct a booklet explaining the molting process in easy terms.
  • Create a puppet show to demonstrate the molting process.
  • Compare how a blue crab changes as it grows with how a snake changes as it grows.
  • Name another animal that changes as it grows larger. How does it change? Compare this with how the blue crab changes.

Extensions

The following activities can be used to help extend the ideas in this lesson:

  • Look into how scientists are searching for ways to decompose crab shells to be converted into fertilizer.
  • Have the students research how other animals change as they grow.
  • Investigate another animal that is declining in population due to habitat destruction and how that has affected when the animal is ready to grow and change.
  • Invite a local naturalist to your classroom to discuss how the health of the local ecosystem affects the inhabitants' way of life.

The following book resources should be helpful in extending the ideas in this lesson:

  • Animals in the Classroom, by David C. Kramer, 1989, Addison-Wesley Publishing Company.
  • Animals Alive: An Ecological Guide to Animal Activities, by Dennis Holley, 1994, Roberts Rinehart Publishers.
  • Pets in a Jar: Collecting and Caring for Small Wild Animals, Seymour Simon, 1975, Puffin Books.
  • Reptiles and Amphibians: National Geographic Nature Library, Catherine Herbert Howell, 1993, National Geographic

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Lesson Details

Grades Themes Project 2061 Benchmarks
AAAS