THE CASPIAN SEA 569 



A very large number of endemic forms (about 60 per cent) are also char- 

 acteristic of the Caspian Sea. 



The very vigorous development of new species brought in from the Azov 

 and Black Sea helps one to understand the biological properties and pro- 

 ductivity of the Caspian population. On the other hand, at different periods of 

 the Tertiary and Quaternary epochs some individual representatives of the 

 Caspian autochthonous fauna left the Caspian basin through the river systems, 

 rapidly settled in a vast territory, and in some cases acquired a cosmopolitan 

 nature. Caspian fauna, especially its fish and crustaceans, readily migrate 

 into the fluviatile systems, penetrating far upstream. 



Brackish-water character of Caspian autochthonous fauna 



The small salinity range tolerated by the brackish-water relict Caspian fauna 

 is its remarkable peculiarity. In contrast to the Sea of Azov immigrants this 

 fauna is incapable — as has been shown experimentally (A. Karpevitch, 

 G. Belyaev and Ya. Birstein, 1946; N. Romanova, 1956) — of enduring high 

 salinity (Fig. 271). Karpevitch has proved experimentally from forms of the 

 two faunas that in contrast to the immigrants from the west (euryhaline marine 

 forms) the brackish-water forms have a considerable stenohalinity. The dis- 

 tribution of these forms throughout the Caspian Sea is determined by these 

 characteristics. On the other hand, it has been shown by the experiments of 

 G. Belyaev and Ya. Birstein (1946) that a salinity of about 15% is lethal for 

 the Caspian brackish-water mysids, while for some species of Gammaridae 

 it becomes lethal at about 20 to 25% . The most saline areas of the Caspian 

 Sea (20 to 25% ) are densely populated by euryhaline marine immigrants from 

 the west — Mugil auratus and M. saliens, Syngnathus nigrolineatus caspius, 

 Cardium edule, Pomatoschistus caucasicus. Among the Caspian relicts only 

 the herring Caspiolosa caspia salina, and the crustacean Dikerogammarus 

 aralensis are associated with them. The first is found only within the areas of 

 high salinity, the second throughout the whole Caspian Sea ; it is particularly 

 abundant in the Aral Sea. 



N. Romanova (1956) has studied experimentally the survival, in various 

 conditions of salinity, of the highest mass species of crustaceans of the Caspian 

 Sea. She has divided them into three groups : 



(/) Species distributed throughout the Caspian Sea which enter the rivers 



of the Caspian basin (saline limits to 13% ). 

 (II) Species distributed throughout the Caspian Sea, which do not enter 



fresh water (salinity range 2 to 13% ). 

 (Ill) Species characteristic only of the Central and Southern Caspian (salinity 



range 8 to 20% o ). 



The low tolerance of most brackish-water Caspian crustaceans of a 

 rise in salinity was confirmed by the experiments of Romanova and 

 Karpevitch. The majority of these crustaceans die at a salinity between 14 

 and 20% o . 



