The female cypris larva locates a crab, injects a small mass of undifferentiated cells through a soft joint, and abandons everything a barnacle normally has. Inside, the cells grow into an interna, a branching root network that spreads through the host and draws nutrients directly. Eventually an externa, the reproductive sac, emerges under the crab's abdomen where a female crab would carry eggs.
The host stops reproducing. Gonads are suppressed and moulting ceases, so the crab never grows again and never regenerates lost limbs. In infected males the abdomen broadens toward the female form and the crab performs female brooding behaviour, grooming and aerating the externa and, at hatching, climbing to a high point and fanning the larvae into the current exactly as it would release its own.
The reputation as a biological control agent has not survived testing. Proposals to release Sacculina against invasive green crabs foundered because the parasite infects other decapods, including native species, and because infection rates achievable in the field were unpromising. It remains a textbook case of parasitic manipulation rather than a management tool.