Salinity 73 



Caspian Sea contains more calcium than the ocean and is less saline, 

 but insects have not migrated into it in unusual numbers. Calcium 

 salts in the ocean are important in maintaining the alkali reserve 

 and thus prevent the activities of organisms from producing local 

 acidity (Bruce, 1928) . In sea water sodium, magnesium, and 

 potassium as sulphates or chlorides do not affect carbon dioxide 

 pressure materially, but phosphates may do so to some extent, and 

 carbonates have very marked ejffects (Shelf ord, 1929) . 



Respiration in sea water is much easier than in fresh water 

 (Prenant, 1929) . The oxygen requirements of fresh-water animals 

 are greater, probably because they have to expend more energy in 

 maintaining osmotic equilibrium with the surrounding medium. 

 Changes in salt concentration in external or internal media change 

 the rate of respiration in many marine animals (Schlieper, 1929a; 

 Borsuk & Kreps, 1929; Beadle, 1931). In marine animals which 

 cannot adjust themselves to fresh water, respiration may decrease or 

 cease altogether in lowered salinities (Shoup, 1932) . Carbonates 

 help respiration by taking up carbon dioxide (Harms, 1929) . 



Aquatic animals are generally better able to stand slow than rapid 

 changes in salinity (Schlieper, 1933) . Marine animals show tolera- 

 tion for varying degrees of dilution according to the concentration 

 of the medium in which they have lived previously (Federighi, 

 1931). Such adaptations may exert profound changes in the whole 

 configuration of an animal. The brine shrimp Artemia salina (L.) 

 has different forms which are correlated with various salinities. 

 Certain littoral snails are smaller in water of greater salinity and 

 larger in lower salinities (Metcalf, 1930) . Teleost fishes, in which 

 body fluids are not isotonic with the surrounding medium, change 

 weight and the salinity of their body fluids when the salinity of the 

 surrounding medium changes. Most marine fishes if placed sud- 

 denly in fresh water die, probably on account of loss of essential 

 salts (Sumner, 1906; Chassion, 1930) or increased difficulty in res- 

 piration (Schlieper, 1929), but a few species are able to survive. 



