52 



THE BIOLOGY OF MARINE ANIMALS 



TABLE 2.6 

 Osmotic Concentrations of the Blood of some Marine Fish 



(Various sources) 



Table 2.7. There is some extrarenal excretion of urea and of salts (Na + , 

 K+ and CI - ) as in teleosts. The urea is diffused throughout the body, and 

 appears to be osmotically and functionally neutral, as far as the tissues are 

 concerned. In addition to this substance the blood contains relatively 

 large amounts of trimethylamine oxide, in concentrations of 0-5-0-9% 



TABLE 2.7 

 Urea and Chloride Content of the Blood of Some Cartilaginous Fishes 



(66-120 mM) (Table 7.3). Like urea, it is resorbed in the kidney tubules 

 and conserved by the fish, and is responsible for some 6-12% of the 

 osmotic pressure of the blood (44, 59, 60, 126). 



A few elasmobranchs have reinvaded estuaries and fresh waters from 

 the sea. These animals have retained the same urea mechanisms as salt- 

 water species, but the urea, and to a lesser extent the chloride, content of 



