60 



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



On the other hand, many species of this autochthonous fauna having 

 acquired a capacity for a wide vertical propagation, and being marked with 

 considerable eurybiotic capacities, have moved far beyond the boundaries of 

 the Arctic basin. Thus some species of the genus Onisimus along the slopes 

 of the Greenland Sea penetrate through the trenches far to the south to the 

 Skagerak and Kattegat, and travel along the Asian coast into the Bering and 

 Okhotsk Seas. 



Most of the forms included in the group of Arctic boreal species are des- 

 cendants of the eurybiotic part of the autochthonous Arctic fauna. In the 



Fig. 15. Routes of exchange between the faunas of the Arctic Basin and the northern 

 parts of the Atlantic and the Pacific (Gurjanova). 1 Atlantic fauna ; 2 Pacific fauna ; 



3 Arctic deep-water fauna. 



Atlantic Ocean they come southward to the North Sea, and in the Pacific to 

 the Sea of Japan. At the same time they go down into the depths and become 

 smaller in size. At present there is constant exchange between the Arctic basin 

 fauna and the Pacific and Atlantic ones via the straits. The main routes of this 

 exchange are given in Fig. 15. 



The Arctic basin is now being rapidly populated by the more thermophilic 

 forms from the Atlantic Ocean. G. Gorbunov (1939) points out a very interest- 

 ing phenomenon of ' the presence, as a rule, of a particularly high Arctic fauna 

 along the continental shores of the Siberian Seas. As one moves northwards, 

 thermophilic forms are more and more mixed with it and gradually the high 

 Arctic forms disappear ; finally on the slopes of the continental shelf the high 

 Arctic forms are represented only by some single species, while the main 

 mass consists of the low Arctic and Arctic boreal forms, and even some near- 

 boreal forms make their appearance. This is explained by the Arctic basin at 



