Salinity 75 



(1923) observed the effects of salinity on the development of a 

 tropical land crab, Cardisoma armatum Herklots. He took eggs 

 from a single female and found that in sea water they hatched into 

 healthy larvae; in fresh water some eggs burst the shell, but no 

 larvae lived; in a mixture of half sea water and half fresh water 

 some eggs hatched and produced apparently normal larvae. In 

 Siam, Alexander (1932) studied the adults of a fresh-water crab 

 (Paratelphusd sp.) . Individuals stood direct transfer from fresh 

 water to sea water, but died in solutions of sugar which were much 

 below the sea in density. He concluded that electrolytes in the me- 

 dium were essential. 



Many aquatic animals are able to discriminate slight changes in 

 salinity and respond in such ways that they usually survive. Oysters 

 are sensitive to slight variations and cease to feed when salinities 

 fall below certain limits, which are characteristic for each species 

 (Amemiya, 1928; Nelson, 1928) . Fishes are quite sensitive to slight 

 changes in the salt content of the water in which they live (John- 

 stone, 1908; Shelford & Powers, 1915; Wells, 1915). The euryhalin 

 killifish or top-minnow Fundulus heteroclitus (L.) is able to distin- 

 guish toxic from non-toxic salts, but "variations in temperature or 

 in stream flow profoundly influence the reactions and are more 

 powerful factors in the behavior of the fish than the ^presence or 

 absence of salinity' " (Chidester, 1922) . Aquatic animals which are 

 placed in solutions that are hypo- or hyper-tonic to the medium in 

 which they have lived may die from various causes. In their ability 

 to resist the effects of changes which may be induced by immersion 

 in fluids which differ in osmotic pressure from those within their 

 bodies animals fall into three groups: (1) those which have external 

 membranes which offer little or no resistance to the passage of water 

 and solutes; (2) those which have semipermeable external mem- 

 branes that permit water to pass freely but restrict or do not allow 

 solutes to pass; and (3) those which have essentially impermeable 

 external membranes (Pike 6C Scott, 1915; Hukuda, 1932). Exam- 



