that in 15°/ooS at 18°C. Normally the shrimp were quiet in 18°C with 

 good survival rates in the test salinities; but the survival de- 

 creased with a temperature rise and there was a total mortality at 

 32°C in low salinities. 



The calcium ion has a greater impact on survival than the other 

 ions. At 25°C mortality increased progressively as the calcium levels 

 decreased below 35%. In 5 and 10% calcium there was a total mortality. 

 In 15% calcium there was also total mortality at 32°C. In 25 and 35% 

 calcium levels more animals died at 32° than in 25°C. None of the 

 test calcium levels were lethal at 18°C. In 18°C no deaths occurred 

 in calcium levels above 15%. However, at this temperature abdominal 

 cramps developed in salinity media with 10, 15, and 25% calcium levels. 



Schwenke (1958) had shown in red algae that among the calcium, 

 magnesium, and potassium ions absence of calcium caused the highest 

 degree of cell damage. Damage increased with increasing exposure 

 time. Species of red algae showed variations in tolerance to the 

 absence of calcium. Lack of calcium resulted in rapid loss of 

 potassium in Porphyra species. The presence of calcium, potassium 

 (and probably also magnesium) is required for normal functioning of 

 the cellular processes, including ion transport (Eppley and Cyrus 

 1960). 



It was shown that mortality in shrimp increased in reduced ion 

 concentrations with temperature rise and reached a maximum level at 

 32°C. Calcium is known to exert a stabilizing effect on protein 

 structures and metabolic processes in estuarine invertebrates re- 

 sulting in an overall increase in tolerance, especially to high 

 temperatures. The high mortality which occurred in brown shrimp 

 at 32°C in low calcium media may be attributed to this condition. 

 Schlieper and Kowalski (1956) observed that additional amounts of 

 calcium and magnesium increased the thermal stability of tissues 



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