The ionic levels were controlled more effectively in 25 and 

 36°/ooS at IS^C (Fig. 75) than at 25° or 32°C. The magnesium ion 

 content increased about twice that of the control in 25°/ooS and 

 about four times the control in 36°/ooS. Moreover, in both media 

 new steady states appeared from the fourth day onward. The changes 

 in low salinities were no different from those tested at 25°C. 



The magnesium ion content in animals acclimated to 18°C (Fig. 

 76a, b,c) was similar to those acclimated to 32°C. Hyposmotic ion 

 levels were present in 2, 5, and 10°/ooS at 18°C (Fig, 76a); vari- 

 ations in the ion levels between these salinities were not sig- 

 nificant. In 25 and 36°/ooS the ion concentrations were hyper- 

 osmotic. However, these hyperosmotic levels were well below the 

 levels of animals acclimated to 25° and 32°C. At test tempera- 

 tures 25° and 32°C (Figs. 76b and 76c, respectively) magnesium 

 concentration in 36°/ooS dropped below the level of 25°/ooS. The 

 salinity response curves became more widely separated from each 

 other with the increase in test temperature from 18° to 32°C, thus 

 indicating a possible salinity and temperature interaction effect 

 on the magnesium content. 



Potassium concentrati on 



The potassium concentration increased with the external sa- 

 linities but not proportionately. With the exception of 2 and 

 ^(s J2^°/ooS, hyposmotic and hyperosmotic ion levels in the test sa- 

 linities were not maintained consistently when there was a change 

 in water temperature. 



Shrimp acclimated and tested at 25°C exhibited hyposmotic 

 potassium ion concentrations in 2, 5, and 10°/ooS and hyperosmotic 

 concentrations in 25 and 36°/ooS with respect to the ion levels in 

 15°/ooS (Fig. 77b). The ions were held at more or less consistent 



145 



