another question for me is why unreal mantas are not living in water.
and are biter fishes piranhas ?
Mantas have been always aerial creatures in Unreal from early development. They can go in water, and they swim faster than a Slith (but they don't pose a similar threat).
Biterfish aren't really piranhas. There are the ones who eat and those who are harmless. They'll damage you if touched. Overall they need to die.