Haemolymph iono- and osmoregulation and acid-base balance were recorded after 48 h exposure at 15 °C to a range of increasing ambient salinities (0, 25, 50 and 75% sea water) in the euryhaline crayfish Pacifastacus leniusculus (Dana). Except for K+, concentrations of all measured inorganic ions and osmolality were significantly elevated in 50 and 75% SW. When compared with ambient changes there was evidence of a transition from hyperto hypoionic regulation above 44% SW. Ca2+ was regulated for a constant blood-medium difference. A progressive reduction in total CO2 was recorded; pH was maintained except in 75% SW where a haemolymph acidosis developed. To permit calculation of CO2 tension (PCOCO2), carbon dioxide solubility coefficient (αCO2) and the apparent first dissociation constant of carbonic acid (p K'1) were experimentally determined in vitro. αCO2 decreased progressively with acclimation salinity but was unaffected by circulating protein. pK'1 decreased as a function both of physiological pH and increasing haemolymph ionic strength. PCOCO2 calculated using these empirical constants, progressively decreased with high-salinity acclimation. The resulting ‘hypocapnic alkalosis’ was partially offset by a metabolic acidosis, whose correlation with extracellular anisosmotic and intracellular isosmotic regulation is discussed.

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