GENERAL CHARACTERISTICS OF THE NORTHERN SEAS 



55 



mouths and their further penetration into continental waters is easy to imagine 

 From this angle we can easily understand the genesis of the so-called ice- 

 sea relicts: Mysis, Mesidothea, Pontoporeia, Limnocalanus, Eurythemora, 

 Gammaracanthus, Pallasea, Pseudalibrotus, Myoxophalus, Lota, and a 

 number of the species of the families Salmonidae, Coregonidae and Osmendae 

 in the areas of river mouths of the Arctic basin, forming the dominant group 

 both as regards number of species and biomass. The crustacean Mysis relicta 

 (Fig. 11) may be cited as an example. In a number of crustaceans which 



Fig. 9. Limits of greatest sea-trans- 

 gression in Quaternary Era (Zachs, 

 1945, 1948). 



Fig. 10. Limits of the greatest sea-regres- 

 sion in the Quaternary Era (Zachs, 1495, 

 1948). 



completely migrated into fresh water (M. relicta) the original forms, inhabiting 

 the brackish waters of the river-mouth zones, are known (M. oculata). 



From this point of view the biology and distribution of the above-mentioned 

 fish are of interest. In the high Arctic sub-region of the Arctic basin the Gadi- 

 dae, a typical marine family, has five representatives: burbot, Arctic cod, 

 navaga and two species of the genus Arctogadus (Fig. 12). The other 50 

 species are not inhabitants of the Arctic region. Of the five Arctic species of 

 Gadidae, one (burbot) has completely migrated into fresh waters, the others 

 are more or less connected with it during their spawning period. These five 

 species probably survived the Quaternary Era somewhere in the Arctic basin 

 itself and the phases of its loss of salinity are reflected in their biology. 



The salmon family (including Coregonidae and Osmeridae), the most 

 typical of the Arctic basin and so closely connected with fresh water in its 

 distribution and biology, is still more significant (Fig. 13). There is no doubt 

 that most species of this family (which includes more than 80 species) sur- 

 vived the Ice Age in the Arctic basin itself. The specific richness of Salmonidae 



