Acid-base and electrolyte regulation, and haemolymph gas transport in crayfish, Astacus astacus, exposed to soft, acid water with and without aluminium

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Intermoult crayfish (Astacus astacus) were exposed to acid (pH 4), soft water ([Ca++]=100 μmol·l−1) in the absence and presence of aluminium (25 μmol·l−1) for variable time periods (up to 21 days) in order to assess the consequences for acid-base and electrolyte balance and haemolymph gas transport. Haemolymph osmolality and concentration of major ions decreased drastically and to a similar extent in acid and acid-aluminium water. Muscle tissue ion concentrations were, however, regulated at an almost constant level. A severe metabolic acidosis was gradually developed, attaining a haemolymph metabolic acid load of 6–7 mequiv·l−1 after 12–21 days. The acidosis was partially compensated by ventilatory means, with the postbranchial haemolymph PCO2 decreasing earlier in acidaluminium-exposed than in acid-exposed specimens. Hyperventilation seemed to be a direct acid-base regulatory response, since the rise in postbranchial PCO2 had only minimal influence on haemolymph O2 transport. The Bohr effect of Astacus astacus haemocyanin was low (δlog P50/GdpH=-0.24), and the mean P50 only increased from 15 to 19 mmHg after 21 days of acid exposure. The decrease in O2 affinity with decreasing pH was accompanied by a decrease in the cooperativity of O2 binding. The haemolymph haemocyanin concentration was not affected by acid and acid-aluminium exposure, but decreased after 21 days due to starvation. Muscle tissue aluminium concentrations were unaffected, whereas gill tissue concentrations increased in acid-aluminium exposed crayfish, most likely due to accumulation of aluminium on the gill surface. Mortality was low, and an internal hypoxia and lactacidosis was not developed in either of the experimental groups. This suggests that the gas transfer qualities of the chitincovered gills of crayfish are much less sensitive to acid and acid-aluminium stress than the gills of teleost fish.