Physiological Responses to Acid Stress in Crayfish (Orconectes): Haemolymph Ions, Acid–Base Status, and Exchanges with the Environment

Exposure of Orconectes propinquus for 5 d to pH = 4.0 (H2SO4) in decarbonated soft water ([Ca2+] = 0.20 mequiv∙L?1) caused a severe metabolic acidosis and a moderate depression of [Na+] and [Cl−] in the haemolymph. Lactate did not accumulate. Acidosis was caused by a large uptake of acidic equivalents from the environmental water, of which more than 95% was stored outside the extracellular compartment after 5 d. Carapace buffering was probably involved, because haemolymph [Ca2+] rose substantially and Ca2+was lost to the environment. Similar net effluxes of K+indicated that acidic equivalents also penetrated the intracellular compartment. [Formula: see text] was also lost during acid exposure. Haemolymph [Na+] fell more than [Cl?] because of greater net losses to the water. Unidirectional flux analyses with radiotracers demonstrated that negative net Na+and Cl−balance resulted from partial inhibition of influx components; effluxes were little affected. All flux effects were reversed during 5 d of recovery at pH = 7.5. Haemolymph ionic responses in Orconectes rusticus differed in showing a smaller, equimolar reduction of [Na+] and [Cl−] and a much larger elevation of [Ca2+]. At a mechanistic level, the responses of crayfish to acid stress appear very different from those of teleost fish.