Sacculina carcini is a species of parasitic barnacle that bears no physical resemblance to its free-living relatives. Instead of building a hard shell on rocks, a female Sacculina larva injects a microscopic blob of cells into the joint of a green crab. Once inside, it grows a root-like network called an interna that spreads through every cavity of the crab's body, absorbing nutrients and hijacking its physiology.
Crucially, Sacculina targets the host crab's androgenic gland and gonads, inducing complete morphological and hormonal castration. The host loses its ability to reproduce and diverts all metabolic energy into feeding the parasite. If the host is a male crab, Sacculina's hormonal manipulation feminizes it: the male's abdomen broadens, its claws shrink, and it begins exhibiting maternal instincts.
Eventually, the parasite pushes an external egg sac called the externa through the crab's underbelly. Believing the parasite's egg sac is its own offspring, the castrated crab obsessively grooms, protects, and vents the parasite's larvae into ocean currents. Through total physiological control, Sacculina turns its host into a living vessel dedicated solely to the parasite's reproductive success.