Salinity 71 



The toleration of animals for variations in salinity, osmotic pres- 

 sure, gravity, temperature, water, light, food, gases, molar agents, 

 and ionization will be briefly considered in the following pages. 



Salinity 



Many animals that had their origin in the ocean can endure salini- 

 ties below that of sea water, and numerous animals that first came 

 into existence on land have taken up life in the sea. Such typically 

 marine animals as the corals that live in the tropics readily tolerate 

 a reduction of 20% in the salinity of the waters in which they live 

 (Vaughan, 1919). The brine shrimp Artemia salina (L.) may live 

 in water which ranges from fresh to that which is much saltier than 

 the ocean, and its eggs will also develop in a wide range of salinities 

 (Sciacchitano, 1927) . Several marine algae live when transferred 

 directly from the ocean to fresh water (Chater, 1927). Various 

 worms, moUusks, and king crabs readily withstand considerable 

 dilutions of sea water (Pearse, 1928) . The fresh-water Rumanian 

 lakes contain a mixture of marine and fresh-water types (Borcea, 

 1931), and the former have apparently slowly migrated in from the 

 ocean. 



Finley (1930) studied the survival of fifty species of protozoans 

 when transferred directly from fresh water to sea water and found 

 that several species were not injured by such treatment. Tetramitus 

 salinus (Entz) lives in salinities of 11% to 15%; Rhizophora salina 

 Kirby, in 34.8% where the pH is 9.48 (Kirby, 1932) . The pul- 

 monate snail Physa heterostropha (Say) remains active in a me- 

 dium to which sea water is gradually added until the mixture con- 

 tains 25% of sea water, but beomes inactive at 35 to 40%, and dies 

 in higher salinities (Richards, 1929a) . No insects live continually 

 in salinities higher than 2.5%, except dipterous larvae and a larval 

 caddis-fly on the coast of New Zealand (Buxton, 1926), though 

 various bugs live on the surface of the open sea and on the brine in 

 salt vats (Hutchinson, 1931). At Tortugas dragon-fly nymphs sur- 



