levels in 2, 25, and 36°/ooS throughout the study; but the ion 

 levels increased from ten hours after transfer in 5°/ooS and after 

 the first day in 10°/ooS, both becoming hyperosmotic by the end of 

 the test period. At 32°C test temperature (Fig. 77c) the tendency 

 was to maintain ion concentration in all salinities well above the 

 levels at 25°C. As a result, the ion concentrations became hyper- 

 osmotic to levels in the control animals from the second or third 

 day and continued so thereafter. In 18°C the trend was reversed 

 (Fig. 77a) and the ion levels in the respective salinities were 

 generally lower than in 25° and 32°C. Except in 36°/ooS the ion 

 levels were hyposmotic from the third day onward. 



The ion concentration pattern of animals acclimated and tested 

 at 32°C (Fig. 78c) was similar to those acclimated and tested in 

 25°C (Fig. 77b). Hyposmotic and hyperosmotic ion levels in the 

 different salinities remained unchanged. However, when the test 

 temperatures were lowered to 25°C (Fig. 78b), the potassium ion 

 levels decreased gradually in 25 and 36°/ooS and increased in 5, 

 10, and 15°/ooS. Some other important differences appeared be- 

 tween these two test temperatures. Tlie potassium ion concentra- 

 tion levels were lowered in all salinities. At 25°C the ionic 

 levels were hyperosmotic to the level of the control shrimp in 

 all salinities except 2°/oo. On the other hand, at 18°C the 

 ionic concentrations were hyposmotic except in 36°/ooS. 



Acclimation to 18°C improved the potassium regulation. The 

 shrimp tested at 18°C (Fig. 79a) maintained hyperosmotic ion 

 levels in 25 and 36°/ooS from the beginning as they also did at 

 25° and 32°C (Figs. 79b and 79c) in 5, 10, and 15°/ooS. In 

 2°/ooS the potassium concentration was hyperosmotic after the 

 first day. It should be added that the potassium concentration 

 level of the control shrimp at 18°C was lower than in the other 

 control temperatures, 25° and 32 °C. 



149 



