Lesson Three :

Lifecycle & Stages

Year Target : 2-4   Duration: 30 Minutes   

Key Learning Areas: Language, Science

Materials:

Sue Fox's Hermit Crabs: Complete Owner's Guide; Stages of development text such as that below, but modified to suit the level of the class; (optional) worksheet which describes the main features of each stage in which the stages are in the wrong order and need to be cut out and pasted into workbooks or onto plain sheets of paper to order them; glue, scissors and paper if necessary.

Objectives:

To revisit previous lesson in particular the lifecycle of land hermit crabs. Students will view a poster/overhead transparency of the lifecycle and then have a closer look at the anatomy of the land hermit crab. Students will learn to identify the different body parts of a land hermit crab and their uses.

Content:

  • Introduction - revist previous lesson on lifecycle
  • Exposition : anatomy of a hermit crab
  • Exposition : stages of development of a land hermit crab
  • Qng/Recall : Students will be prompted to recall the stages and features of some of the stages
  • Recording: overview of the stages of development recorded
  • Q&A time : Any questions about the lifecycles of hermit crabs or other animals
  • Discussion : Overview of what we have learned today

Sequence of Activities:

* Introduction:

"As we learnt last week, the land hermit crab starts its life in the sea. After hatching from eggs laid in the ocean by their mother, the young become part of the plankton of the sea. A month or so later when they have developed into a crab they crawl ashore in search for a shell. We will now learn a little more about this fascinating lifecycle and then learn the names of the body parts of a land hermit crab, as well as their uses."

* Exposition:

Show labelled image of a land hermit crab out of its shell and point to and say the names of the body parts visible in the image. Also point to where the maxillipeds are and other body parts mentioned in the stages of development below.

* Exposition:

Exposition material is based on information found within "Complete Pet Owner's Guide" by Sue Fox

Make sure to have a labelled image of a land hermit crab out of its shell such as that in the anatomy section of my website, found here.

Stages of Development of the Land Hermit Crab

Eggs and Larvae: "As the eggs develop, they change in color from dark reddish brown to pale blue or gray. After about three weeks, the eggs are ready to hatch. At low tide, the female moves towards the sea, but does not enter the water. using her fifth legs, she picks up small clusters of fully developed eggs from the egg mass attached to her pleopods. She takes the eggs further out of her shell with her maxillipeds and claws. Then she drops or flings the eggs onto the wet rocks where they hatch when the ocean water washes over them"

Zoeae: "When the eggs hatch, the individuals do not look like hermit crabs. Hermit crabs, like other crustaceans, must first go through several larval stages of development before they become adults. The newly hatched hermit crab larvae are called zoeae. They measure about 1/8 of an inch (3 mm) and are visible without microscope. The zoeae have large bulging eyes and long shrimplike bodies. They spend most of their time swimming in the ocean as part of the plankton. Many of the zoea are eaten by larger sea animals such as fish. While they are larvae, hermit crabs are carnivorous and feed on other tiny animals. As larvae, hermit crabs drift on ocean currents. Sometimes they are dispersed to other areas from those in which they where initially hatched. In other cases, the ocean's eddies and currents keep them in the same island location.

Glaucothoe: "The zoeae grow and develop by molting through a number of stages. The number of stages varies with the species of crab. For the purple claw hermit crab, there are usually between four and five, although sometimes there are up to six. Each zoeal stage can last almost a week. At each molt, the zoeae grow larger and sometimes add more appendages. The zoeal larval development takes about 26 days, after which a transformation point is reached and the larvae molt to a postlarval stage called the glaucothoe or megalops. At this stage, the animal looks more like an adult hermit crab and both swims and walks about. After at least another month, the glaucothoe metamorphoses to become the juvenile land hermit crab. While still living in the ocean, the glaucothoe begin to search for shells in which to live. Newly metamorphosed juvenile crabs can find their first shells underwater by detecting clues from dead snails. Tiny crabs that crawl ashore without shells will usually die. When the Ecuadorian crabs first come ashore, they are transparent and measure about 1/5 of an inch (5mm).

Juvenile crabs: "The crabs then move to the land, where they mainly lead a nocturnal existence. During the day, they seek shelter in cracks under ledges or logs, or bury themselves in the sand. Sometimes they are active during the day, such as in humid conditions or in rain. After a few more molts, the little crabs can move further and further away from the ocean. Typically, purple claw hermit crabs are ready to breed by their second year. By this time they have passed through a molt called the puberty molt, where the pleopods and other structures needed for reproduction are fully developed."

Fox, S. (2000, p. 155)

* Questioning/Recall:

"Today we have learned that hermit crabs go through many stages before they become fully formed. How does this compare with other animals we have studied, the (such as frogs, butterflies etc)."

OR

"Who can name some of the stages the hermit crab goes through before they move to land?"

"In which stage would the mother crab flick the eggs into the sea?"

"In which stage did the hermit crab move on land?"

"What was the importance of finding a shell?" OR "What would happen if a juvenile hermit crab found a shell before making its way onto land?"

* Recording:

Write an overview of the above lesson on the blackboard/whiteboard or prepare a simple worksheet for students to glue into their books/put into folders. If you have time, try to make up a lifecycle puzzle in which the students put the lifecycle stages into order and then glue them into their books.

* Conclusion :

Overview of what we have learned today and return workbooks, glue and scissors

Extentions:

Have students draw/paint/write about the lifecycle and compare with the lifecycle of a frog or other animal. If possible, find photos of plankton magnified for viewing and research animals that eat plankton and how hermit crab zoea are part of the food chain for many marine life.

Reference:

Fox, S. (2000) Complete Pet Owner's Gude : Hermit Crabs. Barron Books

Notes:

You will most probably have to adapt the stages text to the level of your class. Use the glossary to find the meanings of the words and make sure to have the clearly labelled diagram of a land hermit crab out of its shell so that you can point to each body part that is mentioned to give the students an understanding of the description. Have a blown up version made into a poster on the wall and visible at all times so that the words will become more familiar, and if possible incorporate some of those words (based on the level of your class) into the spelling words.

[Lesson 1] [Lesson 2] [Lesson 3] [Lesson 4] [Glossary] [Hermit Crab Books]

http://users.tpg.com.au/users/vanessap/hermit/lesson2.html

 
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