Magnesium concentration 



Magnesium ion concentration in general increased with the sa- 

 linity of the medium. In 2, 5, 10, and 15°/ooS the increase was 

 generally uneven and lacked a sequence while in 25 and 36°/ooS it 

 was disproportionately high. 



The ion concentration levels in brown shrimp acclimated and 

 tested at 25°C (Fig. 70) were hyposmotic in 2 and 5°/ooS, hyper- 

 osmotic in 25 and 36°/ooS, and isosmotic in 10°/oo and 15°/ooS. 

 The ion concentration in 25°/ooS increased gradually to over 200% 

 above the control level and to more than 700% in 36°/ooS without 

 reaching steady-state levels. The response sequence remained 

 nearly the same at 32°C (Fig. 71). One difference was that the 

 animals lost a little magnesium in 10°/ooS and gained a little in 

 5°/ooS. Steady-state levels did not appear at 32°C in either 25 

 or 36°/ooS. At 18°C (Fig. 72) magnesium ion level in 36°/ooS re- 

 mained the same as in 32°C. In other salinities the ion concen- 

 tration increased to hyperosmotic levels. 



The mean control magnesium concentration in shrimp acclimated 

 and tested in 32°C (Fig. 73) was 5.5 mEq/L, which was lower than 

 the 8.2 mEq/L level of the animals acclimated and tested in 25°C. 

 Among the test salinities, ionic concentration was hyposmotic only 

 in 2°/ooS. The ionic variations were not significantly different 

 from each other in the low-salinity range. Magnesium content in- 

 creased about four times in 25°/ooS; it increased about ten times 

 in 36°/ooS and was still rising by the end of the study. After 

 the first day ionic stabilization started in 25°/ooS. The animals 

 in 25 °C (Fig. 74) controlled the magnesium influx very effectively 

 and better than in 32°C on the first day after transfer. But the 

 concentrations leaped to levels higher than in the previous group 

 from the second day onward. No major changes appeared in the low- 

 salinity range except that the animals in 5°/ooS were hyposmotic. 



139 



