Fig. 1. Kinetic relationships for Na⁺ (filled symbols) and Cl^- (open symbols) influx rates in stingrays acclimated to either ion-poor Rio Negro water (soft water, triangles, N=7) or ion-rich hard water (circles, N=6). Values are means ± 1 S.E.M. There were no significant effects of acclimation water on the relationships. The lines shown represent the Michaelis—Menten equations derived from the mean values of K_m (in µmoll^-1) and J_max (in µmol kg^-1 h^-1) for all fish in both treatments (N=13) as shown.