There are quite a few really freshwater crabs, I suggest you try google. South America has many freshwater crabs, everything from swimming crabs to rock crab type crabs and even these things http://www.planetinverts.com/aegla.html many salt water crabs such as Blue crabs can not only live in freshwater they are being raised in freshwater ponds as a food crop here where I live in NC. Crabs can be very tough and some like blue crabs actually leave the ocean in favor of freshwater. I catch them routinely in freshwater along side blue gills and bass. So don't cross off a crab because technically it is brackish, brackish isn't a specific salinity, many of the brackish animals need such a tiny amount of salt that you couldn't taste it if you tried and few freshwater animals or plants would be affected by it. A great many people add more than enough salt to their other wise freshwater tanks to qualify them as brackish.