This barnacle castrates a crab, then turns it into a babysitter for its eggs

Sacculina larvae inject themselves into a crab and grow into a root-like network through its entire body. The parasite sterilizes its host, and if the crab is male, reshapes its abdomen to look female. Fooled into treating the barnacle's egg sac as its own brood, the crab spends the rest of its life grooming and aerating eggs that aren't its own.

— Wikipedia, Sacculina — Sacculina carcini