Behavioural responses of the Dungeness crab, Cancer magister, during feeding and digestion in hypoxic conditions

The Dungeness crab, Cancer magister, inhabits areas that are frequently subject to periods of hypoxia. This species can employ physiological mechanisms that allow it to cope with acute hypoxic episodes. When crabs feed there is a general increase in physiological variables; these may pose an additional physiological burden on crabs already attempting to maintain adequate oxygen uptake in hypoxia. In Barkley Sound, British Columbia, the inshore habitats of C. magister ranged in dissolved oxygen from 28 kPa at the water surface to less than 1.0 kPa just above the sediment-water interface. During short-term hypoxic events, crabs reduced both the amount of food eaten and the amount of time spent feeding. Crabs tended to cease feeding below 3.2 kPa oxygen, but resumed feeding when the dissolved oxygen tensions were rapidly raised to 6 kPa. In a high (10.5-21 kPa) oxygen gradient, both unfed and fed crabs showed no preference for any area of the gradient. In a low (2.5-10.5 kPa) dissolved oxygen gradient, both unfed and fed crabs preferred the highest oxygen regime. In the laboratory, crabs were less likely to enter hypoxic waters (below 3.2 kPa oxygen) to obtain and consume food; those that did moved the food to a higher oxygen regime prior to feeding and settled in higher oxygen regimes for digestion. Crab behaviour was also monitored in the field. Fed and unfed crabs were fitted with ultrasonic telemetry tags and tracked during a tidal cycle. Unfed crabs remained mobile, travelling up to 1,370 m within 6 h, while postprandial crabs settled in areas of high oxygen and moved very little during the first 48 h after release. The present study suggests that C. magister exhibits behavioural responses in order to minimise the use of physiological mechanisms, and maximise foraging and digestive processes. Thus the nutritional state of the individual may be important in regulating both its behaviour and distribution in its natural environment.