THE CASPIAN SEA 579 



and Aral Seas on the one hand, and for the Baltic Sea on the other, many 

 members of their faunas could have been included in their reciprocal accli- 

 matization stocks, and some of them have already been used by Nature itself 

 (Dreissensia, Cordylophora and others in the Baltic Sea, and the Arctic immi- 

 grants in the Baltic and Caspian Seas). The utilization of acclimatization 

 stocks, especially in the Caspian and Aral Seas, offers man the prospect of 

 many possibilities for the reconstruction of the fauna of these Seas under 

 conditions of forthcoming changes in their salinity. 



Arctic immigrants 



The fourth component of the Caspian fauna — the Arctic immigrants from the 

 Arctic basin — is in all respects just as remarkable. At present the following 

 are included in this group of forms : (1) Limnocalanus grimaldi, (2) Mesidothea 

 entomon spp. glacialis, (3) Pseudalibrotus caspius, (4) Ps.platyceras, (5) Ponto- 

 poreia affinis microphthalma, (6) Gammarcanthus loricatus caspius, (7) My sis 

 caspia, (8) M. microphthalma, (9) M. macrolepsis, (10) M. amblyops, (1 1) Steno- 

 dus leucichthys, (12) Salmo trutta. The seal Phoca caspia, the polychaete 

 Manayunkia caspia and, according to Dogel and Bykhovsky, some fish para- 

 sites of the genera Corynosoma, Crepidostomum, Bunocotyle and others 

 should most probably be included in this group. 



There is no doubt at present that these organisms penetrated into the Cas- 

 pian Sea from the north after the latter became isolated from the Black Sea. 

 These Arctic immigrants are very thinly represented in the Black Sea. They 

 have deviated very slightly from their original forms. The ten main Arctic 

 immigrants comprise two groups of animals — crustaceans and fish, i.e. the 

 two groups best able to endure the freshening of the water. As we shall see 

 below this indicates a fresh-water route for their migration from the north ; 

 this has already been suggested by O. Grimm (1888), K. Kessler (1877), 

 R. Gr'edner and, in a more definite form, by V. Sovinsky (1902). 



As early as 1916 Sv. Ekman pointed out the closer family relationship of the 

 Caspian forms of the Arctic community with their relatives from the Arctic 

 Ocean, compared with those of the Baltic Sea. This led Ekman to assume the 

 probable former existence of a direct link between the Caspian Sea and the 

 Arctic Ocean ; therefore he does not accept the suggestion of the penetration 

 of some forms, for example Limnocalanus grimaldi, by a fresh-water route. 

 Ekman is inclined to relate the moment of the penetration of this crustacean 

 into the Caspian Sea either to the end of the Tertiary period or to one of the 

 inter-glacial eras. The former existence of a direct link between the Caspian Sea 

 and the Arctic Ocean had been suggested before by G. O. Sarz. 



However, in spite of these difficulties the view that the Arctic community 

 penetrated into the Caspian Sea in the post-glacial era through river and lake 

 systems, as has been suggested by Kessler, must be accepted. Further support 

 for this opinion was given by the Swedish geologist A. Hogbom (1917). 



Attempts to trace the route of the Arctic community into the Caspian Sea 

 through the Humboldt Strait have been abandoned. Hogbom thinks that 

 eastern Europe was flooded by water melting from receding ice, which, on 

 the other hand, prevented its escape to the north, and therefore this water 



