change in relation to the flow magnitude of River Ural; below 7°/ooS 

 ion rations change, possibly by the removal of certain ions from the 

 solution (Vinetskaya 1959) . 



Usually there is a higher ratio of carbonate and sulfate ions 

 to chloride and of calcium ions to sodium in estuarine waters than 

 in seawater. Extreme evaporation may lower these ratios (Clarke 

 1924) . Ionic changes may also occur depending upon local soil type 

 of the river, the vegetation and fauna, and industrial or other prod- 

 ucts discharged into the waters. 



Changes in ionic composition of the ambient medium have been 

 shown to modify salinity and temperature tolerance. High potassium 

 content reduced the salinity tolerance in the fresh water amphipod 

 Dikerogammarus haemobaphus (Birshtein and Beliaev 1946). On the 

 contrary, addition of potassium improved tolerance to higher sa- 

 linities in mysids Mesomysis kowalevskyi (Karpevich 1958) . The 

 estuarine turbellarian Gunda ulvae suffered extensive water uptake 

 and salt loss in fresh water and in brackish water of low salt con- 

 tent unless both media had a supranormal calcium content (Pantin 

 1931a, b; Weil and Pantin 1931) . In the mollusk Mytilus edulis 

 addition of calcium and magnesium increased thermostability while 

 addition of potassium decreased it (Schlieper and Kawalski 1956) . 

 Addition of potassium and calcium increased cold resistance in the 

 oligochaete Enchytraeus albidus while addition of calcium and mag- 

 nesium decreased heat tolerance (Kahler 1970). 



Although our laboratory findings show that young brown shrimp 

 can tolerate a salinity range of 8.5°/oo to 34.0°/oo and that by 

 acclimation the range can be expanded from 3.4°/oo to 42.5°/oo, in 

 nature they are mainly confined to less saline waters in which the 

 ion ratios are likely to deviate from normal as shown above. In 

 Louisiana waters Wengert (1972) found that brown shrimp of 11 to 

 100 mm long occur most abundantly in salinities between 0.99°/oo 

 and 3.00°/oo. Postlarval brown shrimp (9 to 20 mm) are found in 



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