Only a freshwater type of osmotic and ionic regulation was observed in indigenous carp and tubenose goby settled in the Rybinsk Reservoir within the range of endurable salinity. Carp tolerates direct transfer from freshwater into distilled water. The alien species tubenose goby does not survive 8 fold gradual dilution of the freshwater of Rybinsk Reservoir with distilled water and dies because of excessive loss of sodium by the organism. This indicates a higher water mineralization in the geographical range of inhabitance of the original parent population of tubenose goby. Within such an environment, fish cannot evolve adaptive abilities to survive in water of low mineralization. Within the tolerant salinity range, the water content in carp and tubenose goby is supported at the stable level, giving evidence of standard physiological functioning of the osmotic regulation. Hypohydremia is observed when the organism is exposed to water of critical salinity, indicating problems with osmotic regulation. Therefore, the tolerant range of salinity in carp and tubenose goby varies from 8 to 12 g/L NaCl, and the critical ranges are 8-12 g/L and 12-16 g/L NaCl. Tubenose goby tolerates higher salinity, within the ranges of both tolerant and critical salinity exceeding by 4 g/L those usual for carp. Such abilities of tubenose goby could have evolved only at high water salinity, indicating the Caspian Sea as the area of origin of the parent population of this species.
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