58 



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



with few vertebrates; it is apparently a large form of CI. harenguspallasi, which 

 later moved eastward again across the Arctic basin and reached the White 

 Sea. 



The Pacific cod Gadus morhua macrocephalus, according to Svetovidov 

 (1948), is a descendant of the Greenland cod G.m. ogac which came through 

 the Arctic along the American side of the polar basin. The Pacific navaga (Ele- 

 ginus gracilis), a descendant of the Arctic E. navaga, but of a larger size, pene- 

 trated into the Pacific along the Siberian side of the polar basin. At present 



Fig. 14. Distribution of genus Mesidothea (Gurjanova, 

 1934). 1 Mesidothea entomon; 2 M. sibirica; 3 M. sabini; 

 4 M. sabini v. robusta. 



both the original forms G. morhua ogac and E. navaga live mostly in the White 

 and Kara Seas, in the Hudson Strait and Hudson's Bay. Svetovidov, noting 

 that the Arctic basin cod mentioned above and representatives of the genera 

 Boreogadus and Arctogadus favour greatly diluted waters of much lowered 

 salinity during their spawning, sees in this their longing for warmer river 

 waters. However, it now seems that desire for less saline waters and for spawn- 

 ing at the coldest time of the year is evidence of prolonged existence in waters 

 of much lower salinity of the polar basin during the Ice Age. 



The genus Mesidothea (Fig. 14) can also serve as a good illustration of a 

 group of closely related species, adapted to various degrees of salinity (step- 

 by-step distribution). M. sabini sabini lives in waters of normal salinity. 

 Other forms, M. sabini robusta and M. sibirica, live mainly in the outer part 



