I suspect that maybe the fishmen can understand animals languages (like Chopper), surely the animals of the sea:
- Kokoro (mermaid) that speak and understand the language of Yokozuna,
- in the Davy Back Fight Kapoty (fishman) the same,
- Arlong speak to Momu and the "cowfish" seems to understand perfectly.
Hasn't all theese pairs had some real history together?
couldnt it just be that they "understand" each other like all pet-master relationships?
Also if mer-/fish-men can understand animals living in the water couldnt it be so that they only can commmunicate properly while both parties is submerged? and in that case Chopper could be unable to understand animals underwater which would make chopper and any fishman complete each other rather then beeing carbon copies.