Sternum

As a rule, the gill area is greater in fast-moving crabs (Portunids) than in sluggish bottom dwellers; decreases progressively from wholly aquatic, to intertidal, to land species; and is greater in young crabs than in older crabs. Often the gills are enclosed in protective chambers, and ventilation is provided by specialized appendages that create the respiratory current. As in cephalopod mollusks, oxygen utilization is relatively high—up to 70 percent of the oxygen is extracted from the water…