CARDIOVASCULAR ADAPTATIONS ENHANCE TOLERANCE OF ENVIRONMENTAL HYPOXIAIN THE CRAB CANCER-MAGISTER

Unrestrained crabs instrumented with probes for ultrasonic measurement of arterial haemolymph flow were subjected to 6h of hypoxic exposure. During this interval, the inhalant O-2 partial pressure was reduced i n steps from 18 to 3 kPa. Measurement of haemolymph flow through all a rteries leaving the heart allowed direct calculation of cardiac output , stroke volume and the distribution of cardiac output for both non-st ressed and hypoxic animals. Resting levels of cardiac output were low compared with previously reported values for this and other species of decapod crustaceans. During exposure to the most severe level of hypo xia tested, haemolymph flow through the anterior arteries decreased wh ile flow through the posterior aorta and sternal artery increased by 5 5% and 27% respectively. Cardiac output increased from a control value of 9.8+/-1.6 to 11.9+/-1.2 m/kg(-1)min(-1) despite a decrease in hear t-beat frequency. Scaphognathite beat frequency increased from 82.1+/- 4.3 min(-1) to more than 120 min(-1) after 90 min of hypoxic exposure and remained at this level for the duration of the exposure period. Th e decrease in haemolymph flow, via the anterior arteries, to the anter o-dorsal region of the animal concurrent with an increase in flow to t he posterior and antero-ventral regions, via the posterior aorta and s ternal artery, implicates an active mechanism for redistribution of ha emolymph flow during hypoxic exposure. The high rate of scaphognathite pumping, presumably to maximise O-2 uptake during experimental hypoxi a, was probably made possible by an increased blood supply to these or gans, which are perfused by downstream branches of the sternal artery.