Hello,
This is easy to answer. None of the commonly available fresh or brackish water crabs is completely aquatic. All need to come out on land to some extent. If you don't let them come out onto land, they will spend all of their time trying to escape, e.g., by climbing up cables. Furthermore, none can really be trusted with fish. They will either try and eat the fish (if the fish are small) or get eaten by the fish when they are moulting (if the fish are big). Finally, almost all the commonly traded species are, to some degree, brackish water animals. While some can live for many months in freshwater, mortality is high when kept in freshwater particularly during moulting, and they only breed in brackish water.
Crabs that come out onto land are quite distinctive, as they usually have quite "boxy" shells. This is to do with the way they breathe out of water. If you think about crabs that you eat, which are marine and fully aquatic, these are much less thick. So, amphibious crabs are round and ball-like, aquatic crabs are flat and plate-like.
The commonest "freshwater" crab seems to be a small, reddish-brown species,
Sesarma bidens. It has distinctive red claws and comes from South East Asia.
An excellent web site for identifying crabs and other freshwater crustaceans is
this one.
Cheers,
Neale
