Zombie animals and the parasites that control them

A female Sacculina begins life adrift in the sea, but when the parasite picks up a crab's scent, it sneaks inside the crab's shell and makes itself at home. Once attached, the barnacle sends root-like tendrils all throughout the crab's body.

Those tendrils allow the parasite to draw nutrients from the crab — and take over its mind. From then on, the crab lives only to serve its master — it no longer molts, mates, or re-grows broken appendages, because those activities would take energy away from the barnacle. And when the parasite is ready to reproduce, the crab — even a male one — will care for the barnacle larvae as its own.