What is the difference between a salt water crab and a fresh water crab?

The primary difference between saltwater crabs and freshwater crabs lies in their physiological adaptations to their respective environments. Saltwater crabs have specialized gills that allow them to extract oxygen from the water, while freshwater crabs have modified gills that are better suited for extracting oxygen from air. Additionally, saltwater crabs have a higher tolerance for salinity levels, whereas freshwater crabs require a lower salinity environment to survive. These adaptations enable each type of crab to thrive in their specific habitat.