We are all somewhat familiar with the Akatsuki members resembling a sort of animal (Kisame being a shark and Sasori being a scorpion for example). There's been some confusion about Hidan's and Kakuzu's. TicoTico theorized that he was an elephant. When he first appeared, I was of the opinion that he was rather octopus or squid-like. However, I have realized what Kakuzu appears to be from what little we've seen of him. He is a parasite. A nematode. A group of roundworms that live in aquatic environments (streams, ponds, rivers, lakes, oceans, WATERFALLS). Some species are parasitic and thrive in vast numbers within the host's body. Many have theorized that his body is a jigsaw puzzled of body parts from ninjas he's killed in the past all stitched together by those black worm-like threads inside his body based on his ability to control individual "worms" and detach his hands from his arms, which are connected by these same "worms". His body seems to be full of these things. Some believe him to be a medical ninja. Either way, this is heavily suggestive of a parasitic theme to his character. Theory: he exists in a symbiotic relationship with these "worms" inside his body. They feed off his nutrients or chakra while he benefits from the enhancements they give his body: self-repair, alteration of tissue material, super-strength, etc.