Extracellular acid–base balance in decapod crustaceans

influence extracellular acid–base status in brachyuran decapod crustaceans (Truchot, 1973; Truchot, 1981; Truchot, 1992; Mangum et al., 1976; Henry and Cameron, 1982; Wheatly, 1985). Although the underlying mechanisms and the physiological consequences remain unclear, the basis for the relationship between acid–base status and ion regulation can be attributed to the catalysed hydration of CO2 by carbonic anhydrase to give carbonic acid (H2CO3) and, subsequently, HCO3− and H+ (Wheatly and Henry, 1992). HCO3− and H+ not only affect acid–base equilibria, but also act as counterions in the transfer of Cl− and Na+ across plasma membranes via electroneutral ion transporters between the extracellular space and either the ambient water or the intracellular compartment. In aquatic crustaceans, gas and ion exchange between the animal and its environment occur predominantly over the gills because of the relative impermeability of the general body surface (Lignon, 1986; Lignon and Péqueux, 1990). In gill epithelia, HCO3− is usually exchanged for Cl−, and H+ for Na+. These ion exchanges are driven by a basolateral Na+/K+ATPase (Towle and Kays, 1986; Taylor and Taylor, 1992; Towle, 1997), and possibly, in the case of Cl−/HCO3− exchange, an apically located H+-ATPase (Onken and Putzenlechner, 1995). In aquatic decapod crustaceans, the coupled branchial transfer of acid/base equivalents to electroneutral ion exchange is the principal mechanism of acid–base regulation (Wheatly and Henry, 1992). Evidence comes from a number of studies, in which acid–base and ion fluxes are modified during acid–base disturbances. For example, in the strongly euryhaline crab Callinectes sapidus, compensation for a hypercapnic acidosis was accompanied by apparent H+ excretion due to the uptake of HCO3− and associated efflux of Cl− (Cameron and Iwama, 1987). In the shore crab Carcinus maenas and the crayfish Pacifasticus leniusculus, the lowering of HCO3− in the haemolymph during recovery from hyperoxia or hypercapnia was attributed to the excretion of base equivalents into the external medium (Truchot, 1979; Wheatly, 1989). Moreover, crayfish placed in Cl− free medium experienced a haemolymph alkalosis and a reduction in the efflux of base equivalents (Dejours et al., 1982). Collectively these experiments indicate that HCO3− or anion exchange is important to the restoration of acid–base status in decapod crustaceans after acid–base imbalance. In addition, it is thought that anion exchangers play an important role in the uptake of Cl− in aquatic crustaceans exposed to dilute seawater (Péqueux, 1995), providing a potential link between acid–base balance and ion-uptake mechanisms. Consequently, compromises between acid–base compensation and the requirements of ionic and osmotic homeostasis must occur 1003 The Journal of Experimental Biology 204, 1003–1011 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB2963