Barnacles are scary as hell

Isn't nature lovely ?

The microscopic larva of Sacculina seeks out an unsuspecting crab. It then settles on a part of the crab where its armours is most vulnerable, usually on the membrane at the base of one of the crab's hair. The larvae then transforms itself into a facehugger kind of living hypodermic syringe (called a kentrogon). This syringe stabs the base of the crab's hair and injects the next stage of the parasite – a microscopic blob called the vermigon – into the crab's bloodstream. This blob will eventually grow into a parasite that takes over the crab's entire body. Its tendrils spread throughout the crab's insides and the only part of the parasite which is visible on the outside is the externa: the female reproductive organ which protrudes from the crab's abdomen. Sacculina takes over the host in both body and mind – it castrates the crab, then turns it into a doting babysitter that grooms and aerates the barnacle's brood, tending the next generation of baby-snatchers as if they were its own babies.

https://phys.org/news/2014-05-crab-castrating-parasite-zombifies-prey.html