Responses to Hypersaline Exposure in the Euryhaline Crayfish Pacifastacus Leniusculus: II. Modulation of Haemocyanin Oxygen Binding In Vitro and In Vivo

The effect of 48 h of hypersaline exposure (25, 50 and 75% SW) on haemocyanin oxygenation properties in the euryhaline crayfish Pacifastacus leniusculus was investigated in vitro and in vivo. In vitro significant increases in affinity and cooperativity were measured, although the magnitude of the Bohr shift was unaffected. In vitro dialysis of haemolymph against physiological salines of variable ionic composition proved that these changes were only partly attributable to altered levels of haemolymph ions, implicating the existence of modulators other than H⁺ and inorganic ions, the possible identities of which are discussed. Significant depressions of both pre- and postbranchial oxygen tensions (⁠ and ⁠) were observed, but O₂ delivery was maintained by utilization of the venous reserve and by an increase in haemocyanin O₂ affinity. This occurred despite a concomitant acidosis whose effect on O₂ affinity was directly opposed by the ‘salt’ effect. Under hypersaline conditions, haemocyanin played an increasingly important role in O₂ delivery in vivo. Despite a reduction in the concentration of combined O₂ at complete saturation of the pigment (⁠⁠), indicating lowered haemocyanin concentration, compensatory changes in O₂-binding and cardiac output precluded an impairment to O₂ transfer. Equilibration at the tissues (⁠⁠) in FW was less effective than at the gills (⁠⁠) but progressively improved with hypersaline exposure reversing this trend. Although effects of increased salinity on O₂ equilibrium characteristics were qualitatively similar in vivo and in vitro, some interesting quantitative differences are discussed.