The effects of pH and salt content on sodium balance inDaphina magna andAcantholeberis curvirostris (Crustacea: Cladocera)

Abstract

1. Sodium balance has been compared in two cladocerans,Daphnia magna, which commonly frequents alkaline waters of moderate to high salt content, andAcantholeberis curvirostris, which is found in acid peaty waters of low salt content. 2. Differences in the affinity for sodium ions exist in different populations ofD. magna. In some populations the uptake mechanisms are half saturated at concentrations of 0.2 mM Na/l but in one population this value (K) was only 0.05 mM Na/l. The sodium uptake system ofA. curvirostris has a higher affinity for sodium ions than that of any population ofD. magna, K being as low as 0.016 mM Na/l. 3. A. curvirostris retains sodium better in acid waters than doesD. magna. In the former the rate of sodium loss at pH 3 is lower than at pH 7 while in the latter the rate of loss at pH 3 is four times that at pH 7. 4. Sodium uptake is depressed in acids waters in both species but the depression is more pronounced inD. magna. 5. The concentration of calcium ions in the medium has little effect on the rate of sodium loss in either species.