Ocean crabs are a member of the crustacean family. Crustaceans have hard outer shells, or exoskeletons, and have one pair of pincher-like claws. The crab is closely related to the lobster and shrimp.
There are about 5,000 types of crab. About 500 of those are hermit crabs, which are not considered "true" crabs because they do not grow their own permanent shells.
Crabs can move quickly by walking or running in a sideways motion. They are also strong swimmers and are known to burrow under the ocean sand.
True crabs, as opposed to hermit crabs, generally have a compact abdomen that tucks under the body and is safe from predators. They have five sets of walking legs, two antennae and two eyes.
Crabs have large claws on their front legs that they use for fighting, feeding and display. They have complex nervous systems and adapt easily to extreme environments, allowing them to live in varying conditions.
Crab behavior includes complex mating rituals and communication techniques that include drumming or waving their claws to signal other crabs. Crabs tend to be aggressive toward one another, especially males, and they are generally solitary creatures.