SODIUM, CHLORIDE AND WATER BALANCE IN EURYHALINE TELEOST APHANIUS DISPAR (RUPPELL) (CYPRINODONTIDAE)

1. Sodium, chloride and water balance were measured in fresh water, 100% sea water and 200% sea water. 2. It was found that the total amount of sodium and chloride decreases with the decrease of the salinity in the medium; minimum water content was found in 100% sea water. 3. The sodium flux is high in sea water and reduced in fresh water. Chloride flux reached only 50% of that of sodium. 4. Maximum water flux was found in 100% sea water, about 66% of body weight. It was only 50% of body weight in fresh water and in 200% sea water. 5. Drinking rates are correlated with the salinity of the media; the drinking rates rise with a rise in salinity. The amount of drinking water was found to be only a small percentage of total water flux. 6. It was concluded that the most important osmoregulatory mechanism in A. dispar is the change of sodium and chloride content in its body when there are changes of salinity in the medium. © 1969 Springer-Verlag.