Sacculina is a genus of barnacles that are parasitic on crabs. The adults bear no resemblance to the barnacles that cover ships and piers; they are recognised as barnacles because their larval forms are like other members of the barnacle class Cirripedia.
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 bump on the underside of the crab's rear thorax, where the crab's eggs would be incubated.
The male Sacculina looks for a female Sacculina adult on the underside of a crab. He then enters and fertilizes her eggs. The crab cares for the eggs as she would have her own, having been spayed by the parasite.
