THE CASPIAN SEA 581 



No fewer than 1 8 species of Caspian fish of marine origin have penetrated 

 into rivers. Among the other groups only a few forms of the Caspian autoch- 

 thonous fauna succeeded in penetrating into fresh waters: Cordylophora 

 caspia and possibly Polypodium hydriforme among the coelenterates ; Dreissena 

 polymorpha among the bivalves; some species of Theodoxus and Melanopsis 

 among the gastropods ; and among the polychaetes Hypania invalida and Hypa- 

 niola kowalewskyi. Hence crustaceans and fish occupy the first place ; there are 

 only seven species of molluscs, three of coelenterates and two of polychaetes. 



Two theories have been suggested to explain the occurrence of Caspian 

 crustaceans in the fresh water of the Pontic-Caspian basin. According to one 

 hypothesis they are typical relicts, i.e. they continue to live where they were 

 left by the receding sea (A. Derzhavin, 1912, 1924, 1939; A. Behning, 1924; 

 S. Zernov, 1934 and others); or they have migrated up the rivers beyond the 

 limits of Caspian transgressions. According to the second theory these forms 

 are active immigrants from the Caspian Sea into the rivers (A. Skorikov, 1903 ; 

 V. Zykov, 1903; L. Berg, 1908; V. Beklemishev, 1923; Ya. Birstein, 1935). 



In principle there seems no difference between the two theories. The dis- 

 crepancy centres mainly on the problem of the place where the euryhalinity of 

 crustaceans living at different salinities was developed : whether it occurred 

 in the Sea itself or in its inlets, which covered the lower course of the present- 

 day Volga and other Caspian rivers. No objections were raised against the 

 capabiUty of Peracarida to move by some means or other against the current 

 and settle down. The freshening of a considerable part of the Sea and the 

 development of the euryhaline forms in the Sea itself seems to us more 

 plausible. This freshening may have occurred during the melting of the 

 ice when a considerable amount of melt-water flowed into the Caspian 

 Sea. In his last work A. Derzhavin (1939) also relates the appearance of 

 mysids. in the lower reaches of the Volga to the inter-glacial era, marked by 

 the Baku transgression of the Caspian Sea which was caused by the inflow 

 of glacier waters. 



The migration of the marine animals from the Sea into the rivers proceeded 

 no doubt as a result of a subsequent increase of salinity in the Sea, i.e. in this 

 case the phenomenon known as 'saline pulsations' took place. When the 

 freshening of the Sea is followed by a rise in salinity, a definite part of its 

 fauna is unable to adapt itself to this greater salinity and therefore gathers in 

 places of lowest salinity — mouths and estuaries of rivers, for example. This 

 process consists both of extinction and of active and passive transference, 

 differing in degree for various biological forms. Into the complex, multiform 

 phenomenon of the change-over of marine organisms to life in fresh water 

 there are interwoven both moments of relict state and moments of passive 

 and active immigration. Furthermore the same species may be a relict in one 

 part of its habitat and an immigrant in another. 



The marine Peracarida of the Volga (except for its very lowest reaches) are 

 probably immigrants from the Caspian, or a freshened inlet of it where they 

 had settled. Some species enlarged their habitat by passive immigration, 

 attaching themselves to boats and living ensconced in the encrustations on the 

 hulls. The absence from Caspian rivers of sedentary marine forms of molluscs 



