Appearance
P. armatus is one of the largest species of hermit crab: adults may reach a carapace length of 43 mm (1.7 in). The legs, including the claws, have bands of colour, in red, orange and white, and the claws bear short spines on the dorsal surface. The eyestalks are short, but bear large black compound eyes. P. armatus can be told apart from the similar P. ochotensis by the spines on its claws, which in P. ochotensis are replaced by granules. The two species are so similar that they have sometimes been considered members of the same species, but are now generally thought to be two distinct species.