Research using mitochondrial DNA to study the genetic variability of Petrolisthes armatus has led to the conclusion that it is monophyletic, a single species with an exceptionally wide range.

[3] The carapace of Petrolisthes armatus is roughly oval with a bluntly pointed front, and is granulated and covered with shallow, narrow ridges.

The chelipeds are composed of four rather than five segments and the chelae are long and have a distinctive orange spot that becomes visible when they are parted.

Petrolisthes armatus is a very small species, varying in length between 6 and 8 mm (0.24 and 0.31 in) with a weight of about 0.5 g (0.02 oz).

[2] It seems likely that the larvae of Petrolisthes armatus have been carried to new locations in ballast water, or it may have been introduced during the seeding of oyster or other shellfish beds during aquaculture procedures.

When it does this, it positions itself so that the water current produced by the sponge passes its own feeding apparatus where it can intercept food particles.

When they are ready to undergo metamorphosis, the larvae settle on the sea bed, possibly attracted to a particular location by pheromones released by other species.

Infestation by a juvenile isopod occurs at an early age and host and parasite grow synchronously.