Like a ghost among a forest of spines, the Urchin Pea Crab (Dissodactylus primitivus), seen in this gorgeous Coral Morphologic video, hides out on a Red Heart Sea Urchin. This crab shares a commensal relationship with the sea urchin. It lives among the spines of the urchin, feeding on detritus and any other food particles that get close enough. The urchin is a sand dweller that feeds on decaying organic matter. It buries itself in the sand, where it hides from predators and feeds on anything it can find.