Crabs are known as decapods because they have 10 legs. There are over 4500 species of crabs, the smallest being a little over a quarter inch long, and the largest measuring an entire foot from claw to claw. They have hard outer shells made of chitin; these exoskeletons are also called carapaces. These carapaces protect their soft bodies. Male crabs often have larger claws and narrower, more triangular abdomens than females.
Crabs are omnivores that usually walk or swim sideways, and are present in every ocean, in freshwater, and on land, especially in tropical regions. Some prefer solitude, still others live in groups. A group of crabs is called a cast, and casts work together to provide food and safety for their families. Crabs communicate via sound, using their pincers and claws to make the noise.
Life Cycle of a Crab
These creatures start life newly-hatched as larva with spines, and drift on the waters. You've probably heard that whales eat plankton; these crab larva are part of that plankton! Some of these larva still have yolk from their eggs to feed off of, but most have to find food right away in order to fuel their next stages of growth. The creatures at this stage are called zoea.
Moulting is the process by which a crab removes itself from it's exoskeleton. The zoea larvae go through particular numbers of changes and moults before they become megalopa, the precise number depending on the species. Megalopa are one moult short of the next stage: juveniles.
Juvenile crabs look similar to adult crabs, but have several more changes and moults to go. They live on the bottom, rather than float in the water; juvenile land crabs begin their migration to land at this stage.
Many moults and changes later, they finally become adult crabs. Males and females mate, and the females carry the eggs until they are done developing, at which point she releases the newly-hatched larvae into the water, usually timed with the tides. Crabs typically live to be three or four years old.
Crabs in Our World
Crabs are food for more than just humans. Sea mammals such as otters also like eating crabs, as do octopi, some fish, and birds such as herons.
Crabs perform an important job in the ocean: they harvest decomposing plant and animal letter, basically cleaning the ocean floor.
Crabs in Culture
Mankind has seen two crabs in the stars. One is the Crab Nebula, a gorgeous nebula shaped like (surprise!) a crab. The other is Cancer, Latin for 'crab', one of the 12 constellations of the zodiac.
Sebastian from The Little Mermaid and Mr Krabs from Sponge-bob Square-pants are two well-known fictional crabs. A third was called Karcinos, and it helped the Hydra battle Hercules, in exchange for which it was placed in the sky by Hera, hence the constellation.