Fig. 2. The relationships between Na⁺ efflux (J_out^Na) and external Na⁺ concentration ([Na⁺]_ext) in stingrays acclimated to either ion-poor Rio Negro water (soft water, triangles and solid line, N=7) or ion-rich hard water (circles and dashed line, N=6). Values are means ± 1 S.E.M. By ANOVA, the relationships (considering only the four lowest Na⁺ concentrations) are significantly different, with a specific significant difference (^*P0.05) around 400 µmol l^-1. The thicker line shows the uptake kinetic relationship for J_in^Na from Fig. 1, and the arrows mark the intersection points of balance where J_in^Na=J_out^Na. Axes are drawn to the same scale as Fig. 3 to facilitate comparison.