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Egg
All crustaceans are oviparous, or egg-laying, creatures. How each species' sperm fertilizes its eggs varies tremendously from crustacean to crustacean. Many release their gametes into the surrounding water and have no further contact with their young. Other species require more direct contact between male and female crustaceans to fertilize the eggs. Once fertilized, eggs may float freely or affix themselves to a stable object. In some cases, that object is a parent; while no known crustacean cares for its young, some species do keep their eggs attached.
Larva - Nauplius
Larval crustaceans undergo a nauplius phase first. This free-floating larval organism maneuvers itself toward food with its antennae. In turn, the almost microscopic creatures form the basis for other creatures' diet; nauplii have no defenses against predators. At this stage, nauplii resemble their adult forms so little that researchers cannot always ascertain which species is which just on sight. In some species, the nauplius phase occurs within the egg; this happens in freshwater species such as crayfish.
Larva - Protozoea and Zoea
Shrimp and crabs undergo a protozoea or zoea phase, respectively. Both of these organisms spend their second life phase in a similar state to their nauplius phase, drifting freely in ocean currents and consuming the smaller protists and plants they encounter. Although the tiny translucent creatures look fragile, their exoskeletons prevent them from growing without molting, so they shed their external plating five to seven times throughout this phase. At this point, the protozoea or zoea measures around a millimeter in length.
Larva - Mysis and Megalops
Larval shrimp, crayfish and some species of lobsters undergo a mysis phase in which the organisms first gain significant control over their movement. Scientists term the analogous form in crabs the megalops phase; the name means "big-eyed." Megalops larvae more closely resemble shrimp or crayfish than crabs with their segmented tails and prominent eyes. The parallel mysis phase that shrimp undergo resemble their adult forms more closely.
Juvenile
After they move out of the larval phase, crustaceans reach the juvenile phase of their life cycle. Juvenile crustaceans look like their parent generations on a miniature scale. In some species, a post-larval form that resembles the adult form but with extra limbs for moving through the water occurs briefly during the first juvenile molt. Juvenile crustaceans measure from a few millimeters in length to a few centimeters, depending on how large the adult organism grows.
Adult
Adult crustaceans have stable forms throughout their adult phase. Each molting session allows the animal to grow as it sheds its confining exoskeleton. Unlike much of terrestrial life, crustaceans do not stop growing throughout their lifetimes; the oldest of them grow the largest. By the time they become adults, crustaceans have gone through numerous dramatic shape shifts. Some species undergo more than a dozen phases in their development, while others' life cycles encompass only two or three.