Limiting Effects of Salinity 97 



are able to tolerate higher salinities and to absorb water by virtue 

 of the fact that the osmotic pressure of their tissue fluids is excep- 

 tionally high. Halophytes sometimes exhibit internal osmotic pres- 

 sures as great as 35 or even 40 atmospheres. Such plants inhabit the 

 margins of the ocean and of salt lakes and can grow in saline soils 

 with salt contents considerably greater than sea water. Although the 

 exact relationship is not understood, it is of interest to note that 

 halophytes, living in a physiologically dry habitat, exhibit succulence, 

 heavy cutin, and other xeromorphic characteristics familiar in the 

 plants of the physically dry desert. 



In fresh water the tissue fluids of invertebrate animals have a con- 

 siderable osmotic superiority because of the typically low salt content 

 of ponds and lakes (Krogh, 1939). The chief problem here is a 

 matter of getting rid of the excess water which enters through the 

 membranes. In marine invertebrates the osmotic pressure of the body 

 fluids is ordinarily only slightly, if any, higher than that of the sur- 

 rounding sea water. When invertebrates living in estuaries move into 

 fresher water, they must exert more osmotic work to eliminate the 

 larger amount of water that tends to enter (Baldwin, 1948). The 

 teleost fish must actively regulate water transfer both in the sea and 

 in inland waters to prevent excessive loss of water in the former and 

 excessive intake of water in the latter, A few animals can tolerate 

 exceptionally high salinities as is exemplified by a fish that inhabits 

 Japanese rock pools with salinity of 60%o and insect larvae that live in 

 water of 42 to 62%o at Dry Tortugas Island (Pearse, 1950). 



Limiting Effects of Salinity 



The extent to which the various species can endure changes in 

 osmotic pressure influences their activity and range. Organisms with 

 a low tolerance for diflFerences in salinity are known as stenohaline 

 forms. Species that are limited exclusively to the open ocean or to 

 fresh water are thus classed as stenohaline. Others may be limited 

 to a narrow range of salinity of an intermediate value, but this is 

 relatively rare. Plants and animals that are able to tolerate a wide 

 range of salinities are termed eimjhaline. Thus organisms inhabiting 

 an estuary and fish that migrate back and forth from fresh to salt 

 water are euryhaline. Such fish as the salmon and the eel are able 

 to regulate their water balance in either a hypertonic or hypotonic 

 medium. 



It should be noted that no sharp line or numerical limit distinguishes 



