This is the White Speckled Hermit Crab (Paguristes punticeps), a Caribbean species of the Diogenidae family. Adults of this species grow to a length of 3 to 5 inches (about 8 to 13 cm). These reef dwellers inhabit empty gastropod shells, tail end first, so you won't normally get to see the crab's full length.
Without the artificial light produced by the camera strobe, these crabs look dark brown with white speckles. It's also difficult to make out those wonderful bright blue eyes without artificial light and a macro lens. But that's why macro photography was invented, right?
One of the most noticeable morphological features of this species: both of its claws (chelipeds) are approximately equal in size. More commonly, the claws of hermit crab species are of different sizes -- typically the left claw is larger than the right.