11. Life Cycle of a Mole Crab

Lesson at a glance: Students will explore the life cycle of a crab.

Goal: Students will be able to explain the different parts of the crab life cycle.

Oregon Content Standards:

Science

2.2 Interaction and Change: Living and non-living things change. .

2.2L.1 Describe life cycles of living things.

Ocean Literacy: Essential Principles and Fundamental Concepts

5. The ocean supports a great diversity of life and ecosystems.

5.a. Ocean life ranges in size from the smallest virus to the largest animal that has lived on Earth, the blue whale.

5.d. Ocean biology provides many unique examples of life cycles, adaptations and important relationships among organisms (symbiosis, predator-prey dynamics and energy transfer) that do not occur on land.

Materials:

• Life Cycle of a Crab powerpoint, reading sheet, cards, etc.

Estimated Time: 35 minutes

Activity:

1. Use the accompanying text to have students individually, group or class read through the life cycle of a crab.

2. Use questioning strategies to identify adaptations for survival during each stage.

a. Examples: Why so many eggs? Why an exoskeleton? Why would you float with the water? Why claws? Why would you molt? Etc.

Eggs

A male and a female crab will mate and make more than 200,000 eggs. That’s a lot!

The female crab will carry the eggs for about two weeks until they hatch. After the crabs hatch from their egg, the parents don’t take care of their young.

Metamorphosis (MEH-tuh-MOR-fuh-sis)

When crabs first hatch from their eggs, they don’t look like mature crabs. They will go through several changes before they are adults. This is called metamorphosis. Can you think of another animal that goes through changes as it grows?

• A caterpillar turns into a butterfly
• A tadpole turns into a frog

Exoskeleton

Crabs are very different from you and me. They have no bones. Instead their skeleton is a hard shell on the outside of their body called an exoskeleton!

When a crab grows, it gets too big for its hard exoskeleton. A crab will shed its old exoskeleton and grow a new one. This is called molting. It takes a day for the new shell to harden and be ready to go.

Imagine you molted as you grew. How might this change your lifestyle?

Larva (LAR-vuh)

When a young crab first hatches from its egg, it is very tiny and is called a zoea (ZO-ee) larva. A crab zoea will swim and float around in the ocean as plankton. A zoea will eat as much as it can to grow.

Zoea don’t look much like adult crabs. They will go through several molts as their body grows and they reach their next life stage.

Megalops (MEG-ah-lops)

After being a zoea, a crab becomes a megalops. When a young crab becomes a megalops it will sink down to the bottom of the ocean. Here it will live for the rest of its life.

Fish eat many megalops because they are bigger than most plankton. Megalops are also easy for fish to see because they are brown.

A megalops larva will only need to molt one more time to look like an adult crab.

Adult Crab

The last life stage of a crab is being an adult crab. When a crab is 3 or 4 years old, they will find another crab and mate. Making more eggs and starting the cycle all over again.