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Fig. 3. (A) The amount of Na⁺ and Cl^- ingested by a marine teleost fish assuming a drinking rate of 2 ml kg^-1h^-1 (Marshall and Grosell, 2005) and the amount of Na⁺ and Cl^- present in fluids passing through the esophagus, the anterior (Ant), the mid, the posterior (Post) and the rectal segment (Rect) of the intestine. The amount of Na⁺ and Cl^- passing through the esophagus was calculated from concentrations of Na⁺ and Cl^- in stomach fluids from starved fish (Kirsch and Meister, 1982; Parmelee and Renfro, 1983; Smith, 1930; Wilson et al., 1996), assuming that no water absorption occur across the esophagus (Hirano and Mayer-Gostan, 1976; Parmelee and Renfro, 1983). Amounts of Na⁺ and Cl^- present in the intestinal segments were calculated from concentrations of Na⁺ and Cl^- found in unfed toadfish intestinal fluids (Taylor and Grosell, 2006a) and a fractional water absorption of 20% in each intestinal segment, yielding a total fractional water absorption of 80% (Marshall and Grosell, 2005). Note that the concentrations of Cl^- exceed corresponding Na⁺ concentrations in all gastro-intestinal segments but that the absolute concentration difference between the two ions diminishes as fluids are passing along the intestine. (B) Net Na⁺ and Cl^- uptake rates (mmol kg^-1 h^-1) occurring across different segments of the gastro-intestinal tract calculated from the different amounts of Na⁺ and Cl^- presented in A. Note the equal molar Na⁺ and Cl^- absorption in the esophagus and the substantial excess Cl^- absorption in the anterior intestine. See text for further details.

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