The female Sacculina larva finds a crab and walks on it until it finds a joint. It then molts, injecting its soft body into the crab while its shell falls off. The Sacculina grows in the crab, emerging as a sac, known as an externa, on the underside of the crab's rear thorax, where the crab's eggs would be incubated.
When a female Sacculina is implanted in a male crab it will interfere with the crab's hormonal balance. This sterilizes it and changes the bodily layout of the crab to resemble that of a female crab by widening and flattening its abdomen, among other things. The female Sacculina has even been known to cause the male crabs to perform mating gestures typical of female crabs.
After this invasion of the Sacculina, the crab is now unable to perform the normal function of molting. This would result in a loss of nutrition of the Sacculina and impair its overall growth. The natural ability of regrowing a severed claw that is commonly used for defense purposes is lost after the infestation of Sacculina. Although all energy is directed the Sacculina, the crab develops a nurturing behavior typical of a female crab. The natural hatching process of a crab consists of the female finding a high rock and grooming its brood pouch on its abdomen and releasing the fertilized eggs in the water through a bobbing motion. The female crab stirs the water with her claw to aid the flow of the water. When the hatching parasite eggs of the Sacculina are ready to emerge from the brood pouch of Sacculina, the crab performs a similar process. The crab shoots them out through pulses creating a large cloud of parasites. The crab then uses the familiar technique of stirring the water to aid in flow.
The male Sacculina looks for a female Sacculina adult on the underside of a crab. He then enters and fertilizes her eggs. The crab (male or female) then cares for the eggs as if they were its own, having been rendered infertile by the parasite.