A crab's skeleton is on the outside of its body, therefore the only way for a crab to grow is to shed the old shell. For one to three months after shedding the shell, a crab is soft-shelled. After shedding its shell, a crab will expand its body mass by swallowing water then forming a new shell over this larger mass. Once the new shell is in place, the crab grows to fit the new size. For the first two years of life, a crab sheds its shell several times a year.
Crabs are vulnerable during the soft-shell periods.
Predators of crabs include dogfish, hake, halibut, lingcod, sculpins, wolf eels and octopus. A common assumption is that crabs walk sideways, yet they can walk and run in any direction to escape from harm. They may also bury themselves to evade predators. They prefer muddy to sandy ocean bottoms near eelgrass beds. Rock crabs, however, thrive in rocky areas.