| Summary: | 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 CO 2 was recorded; pH was maintained except in 75% SW where a haemolymph acidosis developed. To permit calculation of CO 2 tension (P CO CO2 ), carbon dioxide solubility coefficient ( CO 2 ) and the apparent first dissociation constant of carbonic acid (p K' 1 ) were experimentally determined in vitro . CO 2 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. P CO CO2 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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