Parasyte -the maxim-

One day, without warning, parasitic aliens arrived on Earth, invading human hosts by taking over their brains. These creatures seize complete control of their victims, shapeshifting as needed to hunt their prey. Sixteen-year-old Shinichi Izumi becomes a target, but the parasite fails to reach his brain-instead, it gets stuck in his right hand. Trapped, the parasite, called Migi, has no option but to depend on Shinichi for survival. Forced to cooperate, the two must fight off hostile parasites that see them as a danger to their kind.