580 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



flowed southward, carrying with it Arctic organisms which populated the 

 freshened or fresh-water inlets which extended far to the south of the Arctic Sea. 



E. F. Gurjanova in 1933 and P. Pirozhnikov in 1937 introduced a new 

 approach to this problem. Since the Caspian forms of crustaceans are closest 

 of all to those of the Kara Sea and are often almost indistinguishable from 

 them, Gurjanova suggested that this must be just where the Caspian immi- 

 grants came from. Pirozhnikov transferred the ideas expressed by Hogbom 

 on the elastic glacier effect to the Ob-Yenisei plain. The main argument 

 against this point of view rests in our ignorance of the distribution of the 

 original Caspian species in the Arctic basin in the post-glacial era. It is quite 

 possible that at that time they also inhabited the European part of the Arctic 

 basin and that later, when the temperature rose, they were pushed eastwards 

 beyond Novaya Zemlya. 



L. Berg's hypothesis (1928) is just as plausible; according to it the pene- 

 tration of the northern organisms into the Caspian Sea from the Baltic took 

 place through the extensive Rybnoe Lake, which in the post-glacial era over- 

 flowed the shores of the Baltic Sea and of Lakes Ladoga and Onega (and 

 also Beloozero and Shesna which were connected with the Caspian Sea) 

 and deposited the striated clays discovered by S. Jakovlev on the watershed 

 between Lakes Onega and Beloozero.. In Pirozhnikov's opinion this hypo- 

 thesis is contradicted by the absence now of Stenodus and Pseudalibrotus in 

 the Baltic Sea; however, as was noted by A. Derzhavin (1939), Stenodus 

 leucichthys is found in the Baltic basin, and Pseudalibrotus could have lived 

 in the Baltic Sea under the severe conditions of the Ice Age and could have 

 disappeared with the rise in temperature. 



Finally A. Podlesniy (1941) admits the possibility that Stenodus leucichthys 

 and salmon penetrated into the basin of the Caspian Sea from the Northern 

 Dvina through the Kol'sko-Vychegodsk confluence of the North and South 

 Kel'tma rivers. He suggests that both forms of the salmon family had pene- 

 trated to the south more than once even in the post-glacial era. 



The intrusion of Caspian fauna into fresh waters 



Apart from the fact of the original marine groups being, in the history of the 

 Caspian fauna, the forms best able to endure a considerable fall of salinity, 

 they evolved a number of new forms which could move even farther ; these, 

 pressed on by phases of increase of salinity which set in after phases of freshen- 

 ing, penetrated into fresh waters. Here again we see mainly the same two 

 groups — crustaceans and fish — best fitted, owing to their more or less im- 

 penetrable integuments, to retain the hypertony of their perivisceral fluid in 

 relation to environment. 



Table 240 

 Isopoda 1 ; Amphipoda 35; Cumacea 10; Mysidacea 6; Decapoda 1. Total 53 



Ya. Birstein (1935) has pointed out that 44 species — 53 according to A. 

 Derzhavin {Table 240) — of Caspian crustaceans have immigrated into the 

 river Volga. 



