Most newly hatched crabs look very different from their parents and must undergo several transformations (metamorphoses) before they take on their distinguishing adult characteristics. Only a few species of crab, usually of the freshwater variety, start life looking like miniature adults, skipping over the larval and post-larval stages.

Generally, a crab larva, called zoea, begins as a transparent microscopic organism with a rounded body and undeveloped legs, spending the early part of its life adrift in currents of water as a participant of the plankton community. If the larva escapes becoming a meal for some other marine creature, it will undergo a series of molts, allowing it to grow and change its structure as it progresses toward the megalops stage. Depending on the variety of crab, the number of moltings, or times that the larva will shed its exoskeleton, is generally anywhere from five to seven.

Upon reaching the second larval stage -- known as the megalops stage -- the young crab has developed a body more closely resembling that of an adult, having evolved little claws and other legs similar to its progenitors. Even so, the immature crab has not yet curled and tucked under its large abdomen and thus appears to exhibit a tail, looking very much like a miniature lobster or crayfish. During this time of its life, the young crustacean continues to grow and is able to swim and crawl along the water's bottom. Eventually, after further molting, the megalops emerges into the first crab stage, looking like a small version of its adult parent.