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Objective Students will understand the anatomy and life history of crabs, and become familiar with the green crab as a non-native invasive species. National Science Education Standards
Abilities necessary to do scientific inquiry (5-8, 9-12) Understandings about scientific inquiry (5-8, 9-12) Organism regulation and behavior (5-8) Population and ecosystems (5-8) Behavior of organisms (9-12) Warm-Up: Ask students to name as many types of crabs as they can. Ask them to describe what adaptations crabs have for living in their environment, and discuss which species are commercially important. If possible, bring in a crab exoskeleton for students to examine. Background: Crabs belong to a group of animals called Crustacea, which also includes lobsters and shrimp. They are decapods, meaning they have ten legs. There are more than 50 families of crabs and thousands of species worldwide, including blue crabs, shore crabs, hermit crabs, and spider crabs, to name a few. They have excellent eyesight and certain species can detect movement from as far as 20-30 yards away. Crabs can hear and make different kinds of sounds which are used in courtship or to intimidate a competitor. Habitat: Only a small number of species live in freshwater. Marine species live in all oceans, and may live anywhere from the shoreline to the very deep parts of the sea. Some crabs can tolerate wide ranges of temperature and salinity, but others need more specific parameters. Molting: When walking along the shore, you have probably seen crab shells washed up on the beach. A crab's hard exoskeleton doesn't actually grow, but a soft shell grows inside it, and when the crab gets too big, the exoskeleton must be shed. This process is called molting or ecdysis (ek`-da-sis). In preparation for growing the new shell, the crab absorbs the calcium from the old shell into its blood, so that it can be used for the new shell. The old, hard shell cracks, and the crab, now soft and wrinkled, frees itself. At this point, the crab absorbs large amounts of water and expands to its new size. While the new shell is hardening, the crab is vulnerable to predators, so it tries to stay hidden, and sometimes will not eat for several days. Molting frequency decreases with a crab's age, and some crabs may molt up to twenty times in a lifetime. Reproduction: Mating usually takes place between a newly-molted, soft-shelled mature female and a larger male with a hard shell. The male carries the female around before she molts, and may continue to do so after mating. This protective behavior not only helps the female, but guards the male's genetic investment, and decreases chances of his being displaced by another male. After mating, the female deposits thousands of fertilized eggs onto her swimmerets. When the eggs hatch, usually in the warmer months, free-swimming larvae spend several months as plankton before they go through a number of molting stages, eventually settling down on the sea bed. Feeding: Crabs are not fussy eaters and will dine on worms, mollusks, other small crabs, algae, decaying fish, or anything else they can catch. The crab has a hearty appetite, and some adults may eat forty half-inch clams per day, and may even eat crabs their own size. It finds food by using smell detectors on its antennae, and other detectors on its legs that tell the crab when it makes contact with a food source.
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