AbstractThe respiratory system of the crab Cancer magister has been investigated in unrestrained specimens fitted with electrodes for monitoring heart rate, branchial pressure catheters for monitoring scaphognathite rate, a ventilation mask containing an electromagnetic flow probe for measuring instantaneous branchial water flow and water sampling catheters for determining gill oxygen extraction and oxygen consumption. In animals recently stressed by physical restraint and air exposure, heart rate, branchial water flow and oxygen consumption were elevated and both scaphognathites pumped continuously in a predominantly forward direction, whereas in well‐acclimated animals ventilation rate and oxygen consumption were low, gill oxygen extraction was elevated, unilateral ventilation predominated and both ventilatory pauses and periodic reversals of scaphognathite beating were common. Analysis of the relationship between branchial water flow and scaphognathite rate reveals that the scaphognathite operated at a constant stroke volume for forward pumping but not for reverse pumping. Furthermore, the stroke volume was significantly lower for the latter over the normal range of scaphognathite rates in this mode. Reverse pumping and ventilatory pauses are behaviors that often occurred in the apparent absence of extrinsic stimuli in this species, thus their possible roles in gas exchange have been analyzed and are discussed.