are red claw crabs strictly freshwater, or are they brackish?

Answer 1

They are brackish. Most pet stores keep them in freshwater because it's more convenient for them (most don't have brackish set-ups or sell brackish fish other than those that can be kept in freshwater tanks), and the employees at most don't know any better.
Besides needing salt (seasalt for the extra trace elemnets, not general aquarium salt), they also need a way to get completely out of the water. This could be a piece of driftwood with an area above the water line at the center of the tank, or a rock "island". The difficulty here is that if you raise the area too high, you can't use a cover on the tank as easily, but if you put an island in an end or in a corner, the crabs will climb and object and possibly escape from the tank (they're good at that!). They can't climb glass though, but be carecul of any power cords or airline tubing running into the tank. These guys almost need a tank to themselves to be kept correctly.

For more on red claw crabs and their care (also photos of tanks set up for them):
http://badmanstropicalfish.com/profiles/profile101.html
http://www.geocities.com/elgecko1989/crab.html

Answer 2

These crabs are usually sold as freshwater crabs. From what little Red Claw Crab care information I could find on the internet, it seems that they are actually brackish crabs.

My tank is a brackish tank with a salinity of 1.004. A ph of 8. I use Reef Crystals salt mix. I have talked to several people on some forums that have tried to keep them in fresh water and had no luck long term. So if you want to try these crabs, I would recommend brackish water

:-)