Sacculina carcini is a common species of barnacle that not only attaches itself to various species of crab, forcefully castrating males and turning them into females willing to care for and release their eggs.
Most barnacles spend their entire existence attaching themselves to underwater rocks or the bottoms of ships and just filtering food from water, but sacculina carcini has evolved into a parasite that seeks out unsuspecting crabs, usually green crabs, and becomes the bane of their existence. It all starts with a microscopic Sacculina larva that attaches itself to the most vulnerable part of the crab, the membrane at the base of one of its hairs, and injects a microscopic blob called ‘vermigon’ into the crustacean’s bloodstream. This blob then grows into a parasitic barnacle that uses its host both for protection and reproduction, often changing its sex to achieve its insidious goal.
