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More specifically, those little sand crabs that people can find at the beach where the tide comes in?
I've caught a few and have been wondering if they are edible for my RES.

If they're wild caught' I'd say probably not.You don't know what type of parasites or bacteria they come along with, and it's not something a RES would find in the wild, so they wouldn't have any tolerance to any hitchhikers coming along with the crabs.

I agree, no wild caught munchies for your RES. People have had their turtles eat their expensive cray fish tankmates, so I wouldn't bother with it.