Question

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

Answer 1

Fiddler Crabs eat microscopic critters such as: 
Ciliaphora 
Vorticella 
Paramecium 
Frontonia 
Urocentrum turbo 
In fact, to stay healthy, hermit crabs need to eat just like we do if we are maintaining a nutritional diet.  Simply put, hermit crabs are omnivores, just like we are.  They can eat meats, they can have fish, they can certainly have vegetables, and you can even give them fresh fruit for dessert.  Whenever possible, these foods should be organic, or at least unprocessed.  Everything needs to be washed thoroughly before being served.  After that, especially with vegetables and fruits, let the food dry.  Hermit crabs also eat foods rich in tannin.  This includes oak tree leaves and even bark.  These too need to be washed and left to dry. humans eat them

Answer 2

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