of Mytilus edulis, while additional potassium decreased it. At 

 reduced ionic concentration levels, nearly 40% of the test animals 

 survived in 40% potassium while in 0% magnesium and 25% calcium 

 levels none survived beyond ten hours at 32°C. Besides improving 

 the thermal tolerance, calcium content plays an important role in 

 the permeability as was shown in the estuarine turbellarian Gunda 

 ulvae. These invertebrates can tolerate temporary exposure to 

 fresh water only in the presence of sufficient calcium. In fresh 

 and low saline waters they suffer from extensive water uptake and 

 salt loss unless both media have a supranormal calcium content 

 (Pantin 1931a, b; Weil and Pantin 1931). 



The high mortality in low potassium media was probably due 

 to the combined influence of the ionic deficiency and high tem- 

 perature than by either of them singly. Mortality was highest of all 

 at 32°C. Abdominal cramps developed mainly in 18°C. Although the 

 incidence of cramping did not follow any trend that was related 

 to the potassium levels, the highest number of cases was found in 

 the lowest level of 10%. It was shown from the behavioral studies 

 that abdominal cramps also occurred at 18°C mainly in 2 and 5°/ooS. 

 In these salinities the potassium levels were generally lower than 

 in 10, 15, 25, and 36°/ooS regardless of acclimation temperature. 

 All this evidence points to the fact that the abdominal cramps are 

 likely to occur mostly in combinations of low temperature and low 

 potassium levels. Literature on abdominal cramps in brown shrimp 

 is absent to the best of our knowledge. 



From the oxygen consumption studies, metabolic rates were shown 

 to be influenced primarily by temperature changes and to a lesser 

 extent by reduced levels of calcium, magnesium, and potassium. Also 

 temperature changes altered the respiratory rates induced by the 

 test solutions. The respiratory rates in 25% calcium were consis- 

 tently higher in 18°, 25°, and 32°C; but the rates in 30% potas- 

 sium and 0% magnesium media exhibited opposite trends to each other 



294 



