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Embryos
Adult male and female crabs get together, usually in the spring to early fall, depending on the species, and spawn. Up to nine months later, the females lay their eggs and attach them to their bodies, either on their backs or bellies. These fertilized eggs are carried around by the females for a couple of weeks to one month. When the larvae are ready, they will hatch from these eggs into the ocean.
Zoea
Newly hatched crabs are larvae in a stage called zoea. These little larvae look nothing like adult crabs. Frequently, they are too small to be seen, at 0.25 millimeters in length and look more like shrimp. This stage takes about 31 to 60 days depending on the species of crab. Larvae in the zoea stage are filter feeders and eat plankton. They typically float along ocean currents, letting the water carry them. Larvae of most crab species in this stage will move through different stages of growth until they reach about three times their birth size.
Megalops
Larval crabs next enter their final stage of growth called megalops, and resemble a minute lobster. These larvae in this stage are able to swim freely, but they still rely on the ocean currents to carry them close to where they will spend their adult lives. This stage is shorter for some crab species, but generally takes between 31 to 49 days. These larvae eat plankton and also begin to prey on other minute creatures. The second half of the megalops stage is also called the first crab stage, which is the first time a larva looks like a crab.
Juveniles
After the first crab stage, the larvae officially become juveniles. They are still extremely small at the beginning of this stage, usually around 2.5 millimeters. Over a period of around 12 months, depending on the species, these juveniles change into adult crabs. Juveniles move to shoreline areas during this period, where they'll spend their time as adults. There are several growth stages during this time, up to as many as 21 stages for the blue crab.
Adults
The final stage of the crab life cycle is the adult stage. Adults are fully capable of reproduction and further molts will only result in a slightly larger shell as the crab grows over time. Some species are air breathers and can only remain under water for a certain period of time. Adult crabs feed on small mollusks, fish and shrimp. They bury themselves in mud during cold weather. Each species of crab varies in life span, but it is generally numbered in terms of several years.