What do fiddler crabs and hermit crabs eat? who eats them?

Fiddler Crabs usually eat microscopic dead organic matter, plants, algae, and other small animals. They usually find their food in the sand or mud. 
land Hermit crabs usually eat a variety of things, such as fallen fruits, decaying wood, leaf litter, plants and grasses and items washed ashore by the tide, and they might even eat feces. 
we, Humans do eat fiddler crabs and most of the fish will inhale these tiny crabs.
again, humans eat hermit crabs too, most fishes, crabs, octopuses and birds eat them too