The mechanisms of extracellular pH regulation were studied in crayfish Astacus leptodactylus under conditions that were either favourable or unfavourable for ionoregulation. Animals in intermoult or premoult stages were kept in normoxic artificial waters at 13°C. In intermoult, acid—base balance (ABB) and ionoregulatory disturbances were induced by increasing the ambient partial pressure of CO2 (PwCOCO2), by decreasing the concentration of NaCl in the water ([NaCl]w) or by associating both changes. In premoult we took advantage of the spontaneously occurring endogenous problems of ionoregulation which are linked to shell shedding.

In intermoult, an increase of PwCOCO2 alone induced a hypercapnic acidosis compensated by metabolic means, whereas in association with a decrease of [NaCl]w (which induced a decrease of [NaCl] in the haemolymph) it led to a ventilatory compensation. In intermoult a decrease of [NaCl]w alone induced a metabolic acidosis that was compensated by metabolic means, whereas in premoult it was compensated by ventilatory adjustments.

It is concluded that when water breathers are facing experimentally induced or spontaneous ionoregulatory problems, compensation for superimposed ABB disturbances can be made by ventilatory adjustments instead of by metabolic means.

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