Search

The responses of estuarine invertebrates to hypoxic conditions are well established. However, many studies have investigated hypoxia as an isolated condition despite its frequent co-occurrence with hypercapnia (elevated CO2 ). Although many studies suggest deleterious effects, hypercapnia has been observed to improve blue crab walking performance in hypoxia. To investigate the physiological effects of combined hypercapnic hypoxia, we measured Po2 , pH, [l-lactate], Pco2 , and total O2 in pre- and postbranchial hemolymph sampled from blue crabs during walking exercise. Crabs walked at 8 m min-1 on an aquatic treadmill in normoxic (100% air saturation), moderately hypoxic (50%), and severely hypoxic (20%) seawater with and without the addition of hypercapnia (about 2% CO2 ). Respiration was almost completely aerobic in normoxic conditions, with little buildup of lactate. During exercise under severe hypoxia, lactate increased from 1.4 to 11.0 mM, indicating a heavy reliance on anaerobic respiration. The O2 saturation of arterial hemocyanin was 47% in severe hypoxia after 120 min, significantly lower than in normoxia (80%). However, the addition of hypercapnia significantly increased the percentage saturation of arterial hemocyanin in severe hypoxia to 92% after 120 min of exercise, equivalent to normoxic levels. Hypercapnia in severe hypoxia also caused a marked increase in hemolymph Pco2 (around 1.1 kPa), but caused only a minor decrease in pH of 0.1 units. We suggest that the improved O2 saturation at the gills results from a specific effect of molecular CO2 on hemocyanin oxygen binding affinity, which works independently of and counter to the effects of decreased pH.

The respiratory function of the hemocyanin (Hcy)-containing blood in the aquatic spider crab, Libinia emarginata, and the terrestrial ghost crab, Ocypode guadrata, was studied in animals exposed to normoxic and hypoxic conditions a t 25°C. In L. emarginata low concentrations of Hcy, although 90% O2, saturated at the gill, necessitate a high blood and water convection strategy. O. quadrata has blood with a higher concentration of hemocyanin which is 96% O2 saturated at the gill, obviating a very high rate of blood circulation. During progressive hypoxia convection initially increases on both sides of the gill in L. emarginata while in O. quadrata cardiac output decreases. Blood pH increases with decreasing ambient P, below 60 torr in L. emarginata, inducing a greater hemocyanin O2 affinity.

The CO2-specific sensitivity of hemocyanin oxygen affinity and oxygen binding site cooperativity were investigated in five species of crabs. The hypothesis that CO2-specific effects on hemocyanin are related to the molecular CO2 concentrations that occur naturally in crab hemolymph was tested in crabs that experience a variety of hemolymph CO2 pressures. Crab species were chosen from subtidal, intertidal, and semiterrestrial habitats. Hemolymph samples from each species were added to physiological salines buffered with 50 mM HEPES. O2 equilibrium curves were determined spectrophotometrically by gassing the hemolymph-saline mixtures with different P at constant P and temperature. We found no CO2-specific effects on oxygen equilibrium properties of hemocyanin in any species under the conditions investigated. It is concluded that molecular CO2 and bicarbonate ions are not important in directly modulating the oxygen equilibrium properties of crustacean hemocyanins.

A thermodilution technique is used to measure the flow of hemolymph through the heart of crabs. The technique consists of injecting a small volume of cold filtered sea water into the ventricle and monitoring temperature changes with time in an artery immediately downstream from the heart using a surgically implanted thermocouple. An experimental evaluation of the method affirms that it is a direct measure of the bulk flow of hemolymph through the heart. Values of cardiac output determined in this manner fall within the range of Fick estimates of hemolymph flow made on animals treated similarly. Since measurement of cardiac output by thermodilution can be achieved in a matter of seconds, simultaneous monitoring of heart rate permits the calculation of cardiac stroke volume. In two species of crabs examined, much of the variation in cardiac output is accounted for by changes in stroke volume rather than changes in heart rate.