Crustaceans:
Crustaceans are a group of animals whose skeletons are on the outside of their bodies. There are about 31,000 species of crustraceans, including barnacles, crabs, lobsters, and shrimps. They live in saltwater and breathe through gills. They have many pairs of legs. Their bodies are jointed, like your elbow. Crustaceans use feelers for touching, feeling and smelling.
We'll be studying crabs. There are over 4,500 different species of crabs. Crabs are divided into two groups:
- True crabs
- Hermit crabs
Anatomy:
A crab's body has a head, thorax, and abdomen. The hard shell covering the head and thorax is called a carapace. They have five pairs of legs. The chelipeds (two front legs) each have a powerful pincer. The other legs are smaller.
Moving About:
If you've ever seen a crab walking, you probably noticed that they walk sideways. They do this because their legs are so close together on each side of their body, that the legs would trip over each other if the crab walked forward. Crabs can also propel themselves through water.
Diet:
Most crabs are omnivores. They will eat almost anything, plant or animal. Crabs use their pincers to catch their prey and pick up their food.
Reproduction:
Most crabs mate in salt water. The female carries more than a million eggs under her belly. The eggs hatch out as tiny larvae that look like tiny shrimp. It changes shapes many times. As it continues to grow, it becomes too big for its shell and has to molt, shedding it's shell. The new skin hardens to make another shell. The crab is very vulnerable during this time.
Defending Themselves:
Crabs have a number of ways to defend themselves and escape trouble:
- Pincers and hard shells - Crabs have built in weapons. A crab can pinch really hard. Some crabs will shed its pincer, and it does not let go!
- Camouflage - Some crabs blend in with their background. Some crabs will actually take seaweed or sponges and place it on its back.
- Hiding - Many crabs will burrow into the sand or mud. Some live with sea anemones on their shell to disguise them or live inside the shell of a living mollusk.
Extension:
You think you might want a pet crab? Learn about Land Hermit Crabs.
