Does feeding limit cardiovascular modulation in the Dungeness crab Cancer magister during hypoxia?

Decapod crustaceans inhabit aquatic environments that are frequently subjected to changes in oxygen content. The physiological mechanisms that allow them to cope with periodic episodes of hypoxia have been well documented. Most crustaceans exhibit a bradycardia coupled with diversion of haemolymph from digestive organs towards ventral structures. However, all these experiments were conducted on animals that were starved prior to experimentation in order to avoid increases in metabolism associated with digestive processes. The present study sought to determine how the Dungeness crab Cancer magister balances the demands of physiological systems when they feed and digest in hypoxia. Cardiac parameters and haemolymph flow rates through each arterial system exiting the heart were measured using a pulsed-Doppler flowmeter. Scaphognathite beat frequency (ventilation rate) was calculated by recording changes in pressure in the branchial chamber. There was an increase in both cardiac and ventilatory parameters following feeding. Digestive processes were facilitated by an increase in haemolymph flow rates through the anterior aorta, hepatic arteries and sternal artery. Cancer magister showed a typical bradycardia during hypoxia (3.2 kPa). However, food intake caused a significant reduction in this response. Likewise, ventilation rate also showed effects of addivity, increasing in response to both food intake and hypoxia. Digestion during hypoxia was associated with a decrease in both stroke volume and cardiac output. Blood was diverted away from digestive structures, suggesting that blood flow events are prioritized during hypoxia. The changes in haemolymph flow rates paralleled those in previous reports on reductions in protein synthesis in the hepatopancreas during hypoxia. Haemolymph flow rates through the anterior aorta did not change; thus the blood supply to the supraoesophageal ganglion was maintained during feeding in hypoxia. The results show that the nutritional state of an animal is important in modulating its physiological responses to environmental perturbations. This underscores the importance of an integrative approach, studying physiological responses at the organismal level.