Adult Pachygrapsus feed mostly on algal films and diatoms that grow on rocks in the intertidal, which they can scrape off with chelae (appendages with spoon-shaped cups).  They are opportunistic feeders that will scavenge and eat just about anything, ranging from green and red algae and brown seaweed, to dead animals and detritus.  

 

These crabs have also been known to feast on variety of limpets, bay mussels, Littorina and Tegula snails, hermit crabs, and isopods.  By pressing down with their chelipeds, they can pinch the top off the limpet shells and eat them in this way.  

 

Like most intertidal crabs, P. crassipes is eaten by terrestrial, air-breathing predators.  Most common predators for this crab are sea gulls, rats, raccoons, and humans, though they have been found to eat one another particularly after one has molted.  They are quite evasive and can bind themselves into small crevices in the rocks.  When in life threatening situations, as a last-ditch effort, they can autonomize their limbs to save the rest of their body from predation.