Den Den Mushi are snails used for communication in the One Piece universe.
I'm pretty sure trying to make it exactly as in One Piece would be absurd, but here's what a Den Den Mushi should be (imho) in order to be able to be considered so:
- Ability to connect with other Den Den Mushi it already meet.
- Connection should be reciprocal.
- Once connected every sound one Den Den Mushi hears is reproduced by the other, as similar as possible.
- A human should be able to force and terminate those connections (maybe having pheromones or something else from the Den Den Mushi he/she wants to connect with?).
How could a Den Den Mushi do so and how would it evolve to be such a curious creature?