As I understand it, they are omnivorous. I have been feeding a mix of fish flake and pellets in an attempt to discover what they prefer. I have found that they don't eat as much as one might suspect based on their size. I initially had a lot of excess food building up, so I cut their feedings down in frequency and quantity. What I discovered is that they adore guppy grass (Najas guadalupensis). Instead of eating the offerings I provided, they have been steadily demolishing the bunches of this plant that I added to the tank for cover and visual appeal.
I have already had the pleasure of watching one of them molt. It was a slow process in which the crab slowly extricated itself from its existing shell through the back, leaving a perfect hollow replica of itself. A friend of mine, Don, informed me that it is best to leave the empty shell in the tank with the crabs as they will consume it and reclaim some of the calcium and other minerals present in the exoskeleton.
I hope to capture the act in video and post it to the site. I will work on getting some more photos as well.