Lobsters don’t grow the way people do. Unlike humans, a lobster has a rigid exoskeleton that it must get rid of before it can grow any larger. The act of escaping from the old shell is called ecdysis (from the Greek, ekdysis meaning getting out) or more commonly known as shedding. The term molting is used to refer to the entire cyclical process of preparing for, undergoing, and recovering from ecdysis (http://www.lobsters.org/tlcbio/biology3.html).
Before molting, lobsters reduce the size of their extremities by drawing water from them, so their old shell isn’t too tight. Then, the shell breaks between the tail and the body (called the carapace). Lobsters will flex their bodies back and forth and eventually back out of their old shell. Without its shell, a lobster is soft and squishy. Lobsters that have just molted are called “jellies”.
Video of lobster molting:
During the molting and post-molt period, it is vital for lobsters to stay hidden because they are unable to protect themselves when they lack a hard exoskeleton. There is no definite period for how long it takes the new shell to harden, but there are some factors that influence the speed of calcification. The water temperature affects the lobster’s growth rate and also the rate a new shell will harden; warmer water promotes growth while cold water slows it down. Sometimes lobsters will eat their old shells to obtain more calcium, speeding up the hardening of their new shell. Before a new shell hardens, lobsters will absorb extra water to make sure the new shell is big enough to allow room to grow.
Within the first five to seven years of its life (approximately the time it takes to weigh a pound), a lobster may shed its shell up to 25 times. However, as the lobster gets bigger, the number of molting events decreases. Adult lobsters only molt about once a year to once every two years. Because lobsters molt, they can regenerate lost limbs. Each time a new molt is formed, the regenerating limb looks more like the original. Lobsters can even regenerate their eyes and antennae.