Water 47 



have occurred and certain marine fish are not independent of fresh water; some, 

 like salmon, return to fresh water to spawn (anadromous). Conversely others, 

 like the eels, breed in the ocean and reach maturity as adults in fresh water 

 (catadromous). This summary of the history of Hshes is based on paleontologi- 

 cal evidence, which has been strongly supported by observations on osmoregu- 

 lation."^' --■^ Much of the following account is from Smith; more recent 

 work fully supports his hypotheses. The kidney arose in freshwater provcrte- 

 brates as an organ which gets rid of water, and in no fish is the kidney of 

 primary importance in excretion of nitrogenous wastes. 



Fresh- Water Fish. In fresh water the blood of fishes has an osmolar con- 

 centration of 130-170 mM, and the urine is copious but dilute. The gar pike, 

 for example, in water of Ao=0.03 has a blood concentration of Ai=0.57 and 

 a urine concentration of Au=0.08. Volume of urine is high (200-400 ml./ 

 kg./day). --■* Fresh-water fish are reported to drink little water, although 

 measurements of the accumulation in the gut of colloidal material susix^nded 

 in the medium indicate that goldfish do drink some water. ^" The skin is 

 relatively impermeable to water, *'- but much water enters through gill and 

 oral membranes. 



Some fresh-water fish can withstand transfer to dilute sea water. Figure 

 21 ""^ shows that when a carp is gradually introduced into water of increasing 

 salinity the blood concentration increases only slightly at first, then in concen- 

 trations above Au— 0.6 it adjusts to the medium. ''- On direct transfer to the 

 salt medium the blood does not adjust readily when the concentration of the 

 medium is greater than Ao=:0.68, and the condition of the fish is bad in any 

 concentrations higher than this. 



The inward stream of water through the gill and oral membranes provides 

 a water load, and the glomeruli of the kidneys normally filter a considerable 

 volume while the tubules reabsorb most of the salts, leaving a dilute urine. 

 However, the urine is not as dilute as the external medium, hence salt loss must 

 be compensated. Part of the salt comes from food. A second part of the needed 

 salt is absorbed by special secretory cells located on the gills, as shown by the 

 use of a chamber which separates the water bathing the anterior part from that 

 bathing the posterior part of the fish. Krogh i-*i" removed some of the salts 

 from fresh-water fish of numerous species by keeping them in running dis- 

 tilled water. Salt loss was gradual, but occurred more rapidly if any skin injury 

 had occurred. Much of the salt was lost by way of the kidneys, but with a 

 divided chamber it was shown that some goes out through the gills. Krogh 

 then put the fish into very dilute salt solutions and they absorbed chloride 

 against a gradient. A catfish (Ameiurus), for example, which had previously 

 lost considerable salt, took up salt from 1 mM NaCl or NaBr. The roach 

 (Xeticiscus riitilns) in a medium of CI" concentration of 0.042 mM began to 

 absorb chloride after considerable initial loss, and one fish took up chloride 

 from as dilute a medium as 0.02 mM. The goldfish (Carnsshis miratiis) could 

 reduce the CI" concentration from 1 mM to 0.02 mM. Ordinary chlorinated 

 tap water contains about 0.3 mM of CI" per liter, and pond water usually 

 contains less. Acerina cernua, the ruffle, and the perch, Perca fhivlatilis, con- 

 tinued to lose salt in all dilute solutions tested and died after 10 to 14 days 

 without food, although there was some evidence of CI ^ absorption from con- 

 centrations of 1 mM and higher. These fish must depend largely upon food 

 to replace lost salt. 



