This tiny hermit crab is a Paguritta gracilipes from Indonesia. It is very very small, living in the former home of christmas tree worm. In the above picture, you can see the claws of a neighboring hermit crab in the foreground, so territorial issues don't appear to be a problem. It is unclear if they kill the worm to get the home, or if they move in once the hole is vacant. They don't seem to be picky eaters, and have been observed to eat brineshrimp and whatever floats by. My guess is that the long antennae act as nets to catch small particles of food.
Photos by Trevor Shirk (Steel Phoenix)