When lobsters are cooked, heat breaks down these bonds, freeing the astaxanthin so that it reverts to its normal red color. A lobster's hard outer shell does not grow. Homarus can only increase its size by molting periodically. In preparation for molting, the lobster lays down a new, soft shell underneath its old shell. Over a period of several hours after molting, the lobster swells to a larger size and the shell begins to harden.
Lobsters are able to detach and discard legs or claws by a process called autotonomy.
A lobster may also detach a claw if it is unable to withdraw from its old shell during molting. Autotomized limbs can be regenerated. After the next molt they appear fully formed but smaller than usual, and after sever molts they are full sized. Homarus can mate only when the female is soft after molting. The male stops his usual aggressive behavior towards the female, and the mating pair begin a courtship dance with their claws held closed.